On 1/7/2012 2:39 PM, L. H. wrote:

Subject: Myth #1

I’ve read your info stating that this myth is wrong….carbon monoxide released into the home if heat exchanger has a crack. That is what I want to believe, but I read several other sites that agree that carbon monoxide does get into the home with a cracked heat exchanger. We recently had a furnace inspector tell us that our heat exchanger is cracked and that we would need a new furnace. It seemed he was pressing a bit too hard.

We called another authorized inspector and he told us that he would rebuild the heat exchanger, that carbon monoxide goes out the exhaust pipe….consistent with you. Please give us more backup on this issue. Thank you.

D. and L. H.


Dear D. and L.,

Your second inspector is correct. However, it is usually more cost effective in the long run to replace the furnace versus replace the heat exchanger.

My argument with the industry is with the scare tactics they use, mis-information they circulate and dis-service they do to customers.

1. The heat exchanger is an empty metal shell sitting inside a bigger metal shell (the outside of the furnace) with a blower pushing a lot of air through it. The air passes from the blower, over the heat exchanger and out through your duct work. The same pressure that pushes air out of the duct vent (register) also forces air INTO any cracks or holes in a heat exchanger. The first tell-tale sign that there is a heat exchanger issue is that the main burner flame wavers (is blown around and changes shape) inside the heat exchanger when the blower comes on. This indicates air moving INTO the heat exchanger.

2. Heat exchanger cracks need to be dealt with. In most cases it makes more sense to replace the furnace. Sometimes heat exchangers are not available. Usually a new furnace will be more efficient and will carry at least a 10-year, more probably a 20-year exchanger warranty.

3. My issue is that service contractors put folks against a rock and a hard spot and will force them to shut off working equipment in the middle of the winter. Many times the cracks or holes that are discovered have been in the equipment for years, yet the furnace ran correctly and presented no hazard. Families are inconvenienced and their property is put at risk of pipe freeze ups because of a false sense of urgency. The family budget is destroyed by the need for an immediate furnace purchase. At the same time the ability for the owners to get competitive bids is almost eliminated. Folks are stuck with the prices and equipment offered by the contractors who are at hand and have equipment in stock.

My suggestion would be to replace the furnace. I would get at least 4 bids and make sure the contractors give you prices for their “standard” equipment (similar to the furnace being replaced), their “upgrade” model and their top of the line model. Also get prices on extended parts and labor warranties. Most new furnaces come with control boards that cost $400 to $1,000 to replace when they fail. The electronics usually come with a one-year to 5-year warranty.

If you want to delay the replacement, get a carbon monoxide detector and put it in the bedroom area. Note that carbon monoxide detectors are not 100% reliable, but they will give you a good margin of protection until the furnace is replaced. The waiting period should be days. It will give you enough time to get competitive bids, but I wouldn’t wait for an entire season. What appears as a small crack or split today, may become a large gaping split tomorrow.




Subject: Question About Flue Gas CO Level


We have a hydronic hot water system (radiant heat) in our home and recently had an HVAC company come out to service it. They took a CO reading directly in the exhaust flue of the boiler with a probe (measures O2 and CO) and alerted me that the furnace needs replacing because of high CO levels. I am assuming that there is always some level of CO in the flue. What would be an acceptable CO level in the flue stack?





(76x) 4xx-9xxx



Get a second opinion. Check Angie’s list for a reputable contractor.

High CO level in the flue gasses means “keep looking for a problem”. Most of the time high CO is due to a correctable problem like dirt, rusted passages, blockage from soot or other foreign matter contained in the products of combustion. It can also be caused by incorrect adjustments or bad automatic gas valves. If it is an oil system, then burner adjustment, poor pump performance, partially clogged nozzle, blocked fuel filter or contaminated fuel may be issues.

I’m not familiar with acceptable CO levels for oil burners, however for gas systems, anything over 10 ppm (parts per million) would indicate the need for a closer look at the system. Anything over 100 ppm usually indicates an issue that needs to be addressed. Natural gas burns clean so any amount of CO over 9 or 10 ppm is suspect.

In most cases, the issues can be handled without replacing the equipment. However, if the equipment is old, 15 years or more, then it may be economically more practical to replace the system versus try to fix the problem.

A qualified HVAC contractor familiar with boilers should be able to help you in that area.



On 9/12/2011 12:21 AM, Jxxx Gxxxxxxx wrote:
Subject: CO2 alarm

I was experment in gwith natrual gas two years ago becouse I was thinking of natrual gas stoves adn how thay are not vented out side.
I had bought a house and just lost my job money was not tight until going into the second year of unenployment. I have a gas fire place upstairs
I opend the frount of the fire place and blocked off the flue to see if it would set off the co2 or the fire alarms in the house and it did not the gas fire place is on a thermastate
so i would set it to 70 deg. F. Using the princible that warm air travels to cold I let it heat the basement that way since it was only being used for Laundry
and stuff like that. I know co2 is heavy than air but gets mixed in with air but the co2 detectors down stairs did not even go off.
I think the furnace (natrual gas) is harmless. I have herd stories on the news of a family get together barbacue and the weather turning bad and barbacuing in the garage with th edoors closed and everone
in th egarage died from co2. but the barbacue was propane.
And outher people using propane heaters in a tent while deer hunting and died of co2 again Propane or butane.
I can not find any news about deaths caused by co2 (natraul gas) in a home.
When I started looking about Mythe of natrual gas on the web today I found your artical. Question in a cracked heat exchanger the return air is at a higher PSI
wich would eather blow out the combustion or add to the combustion air am I thinking along the wright thought process.
By the way i really like your web page..
Please le me know If my thought process is messed up in any way thank you.

Sent: Mon, September 12, 2011 2:15:01 PM
Subject: Re: Co2 myths


You are “mostly correct”, but be very careful.

Natural gas burns very cleanly when the flame has sufficient fresh air. However, if it “re-burns” the air it has already used, it becomes toxic and contains high levels of CO. This is why they warn to NOT use your kitchen stove or oven for heat. When the products of combustion get re-burned, high levels of carbon monoxide are emitted. It only takes a few minutes to build to lethal levels once the oxygen in the area is partially used up.

CO detectors require around 30 minutes of exposure to high concentrations of carbon monoxide (CO) before they will alarm. This is designed into the detectors to prevent nuisance/false alarms and is part of the UL and NFPA standards. You can’t use a standard CO detector to determine if equipment is emitting carbon monoxide. The detector may not respond for 4 hours.

By the way, if you are painting or using chemical floor cleaners or having your air-conditioner or refrigerator serviced, remove your CO detectors from the wall and place them in air-tight, zip-lock baggies. The fumes and refrigerant gas will ruin the sensors and make them inoperable. If the detectors are over 5 years old, replace them.

CO is only 3% lighter than air, so if it is mixing with warm air, it will rise. Mixed with cold air, it will hug the floor.

Never run any gas appliance without adequate fresh air. That includes your kitchen oven and stove-top burners. Any flame, natural or LP gas, consumes oxygen. When there is plenty of oxygen available, everything works OK and there is no danger. Once the oxygen gets used up, or the appliance starts to re-burn air it has already consumed, the flame will produce very high levels of carbon monoxide very quickly. If you’re baking a turkey or cake, crack a window open for fresh air.

There definitely have been fatalities in residential homes due to carbon monoxide poisoning. They are usually mis-diagnosed by the media and safety officials, and in almost every case, the problem was the way the equipment was being used, and not the equipment itself. For example, a blocked flue (due to a bird’s nest) causes the products of combustion (used and burnt air) to fill up the flue, then drift out of the draft diverter on top of the gas water heater that is connected to the same flue. As those products of combustion drift into the basement or utility room they get pulled into the return-air stream around the filter and through leaks in the duct work. That’s how CO actually gets into the home (not through cracks in a heat exchanger.)

The web site and my book try to make the point that there are more things to consider besides just the equipment.

Thanks for visiting,



11/7/2011 2:25 PM

From: zxxxxxxx@rxxxxxs.com

Subject: Great site and question



I like your site and will buy your book. I have a question for you.
I have 17 year old furnace , maintained annually, last inspection showed no problems. There was no issues
with heat exchanger or CO leakage. Then this so called consult tells me that due to age of my furnace
CO could be leaking …etc. This does not seem logical. Do you have any facts to dispute these statements
as it looks like he is just using sales pitch to sell furnaces.


 I get a visit from so called “Certified Energy Management Consultant ” and told the following –

The composition of the heat exchanger will degrade over time. As it begins to crack, carbon monoxide is leaking and becomes part of the air flow in your house. Once a level of 100 parts/ million is detected, the gas has to be shut off to the furnace and it is “red tagged”. Any contractors insurance plans does not cover the heat exchanger of any furnace they don’t carry. Manufacturers warranty their own heat exchangers for a limited time and we will replace them, but at a labour cost of approximately $1500-$2000. You can call Rheem and see what their warranty is, but at this age of a furnace, it’s hardly worth repairing. But that would be up to you. 


Hope this helps. 

Certified Energy Management Consultant

f I could tell you how long your furnace would last, I’d buy a lottery ticket for the both of us, and we wouldn’t have to worry about cost. 🙂 

Seriously, at that age, the composition could change over the course of the season and potentially leave you with out heat….or it could last. 

I just don’t understand how anyone could say, emphatically, that something will last “X” number of years. My personal opinion. 

So, I’d prefer my clientele to benefit for piece of mind and not have to worry, and receive rebate money while it’s available. 


Great questions. Look forward to your response. 

Certified Energy Management Consultant


Sorry Zxxxxr, I’ve been out of town and missed your email.

Heat exchangers develop cracks over time due to thermal expansion and contraction. If there are excess moisture issues, they may also rust to the point of being paper thin. Moden day furnaces rarely introduce carbon monoxide into the home’s air stream even with a crack or a number of cracks. Typically the force of the blower pushes air into the the heat exchanger and causes problems with ignition and combustion efficiency.

At 17 years old, I’d say you’ve gotten good value from your equipment and I agree with your consultant that you should consider replacing the furnace. I seriously doubt that carbon monoxide would become an issue simply due to the furnace’s age, but consider this:

As backward as many communities are, there are currently ordinances on the books that dictate that furnaces be red-tagged and disconnected if they have heat exchanger cracks. So, in the interest of saving yourself some money, you run an extra two years and the furnace finally develops a crack. Now you have to go into panic mode, get estimates, and have the system replaced.

If you replace the equipment “on your terms”, at a time of your choosing, you are much better off. You can negotiate better prices, have more time to review and select equipment, and can schedule an installation around your family’s schedule.

The point of my book is to highlight the mis-information surrounding furnaces and carbon monoxide and to let technicians and homeowners know there are more things that need to be checked besides the furnace.

Good luck,



On 11/9/2011 7:16 AM

Re: Myth #1 – A furnace with a cracked heat exchanger will definitely produce carbon monoxide and poses an immediate danger.


Dear Gus, 

I recently called an HVAC to my house because my furnace wasn’t blowing cold air.  I attached a picture above taken by the service technician that told me that my family is in immediate danger.  He shut down my system and gave me an estimate for about $3900 to replace my furnace and A-Coil.  After he left I did some checking online and was able to diagnose my problem myself.  I replaced my pressure switch and my furnace is working again.  When the technician followed up with me he was alarmed that I had turned my furnace back on.  He encouraged me to go on the internet and Google “cracked heat exchanger” which I did and now I’m concerned.  Your article “Myth #1” matches more closely to what I was told by a friend who has been in the industry for 30 years.  My furnace is old (14 years) but I have CO detectors in my house.  Should I call the gas company out to measure the CO levels?  I’m just wondering if I can make through the winter before purchasing a new furnace. 

Thank you  



11/9/2011 11:51 AM

Hi C….,

Resign yourself to the fact that at some point in the near future you will have to replace the furnace, or at least the heat exchanger.

Typically a small crack like the one in your picture does not represent a problem and will not introduce CO into the home. Generally the crack allows air from the blower to be pushed INTO the heat exchanger which can upset combustion efficiency, blow the burner flame around, cause delayed ignition, create problems with ignition sensing on start-up or cause the flame to roll out the front of the furnace.

If you have working CO detectors that are less than five years old, you will be warned if CO becomes an issue. Detectors that are over 5 years old should be replaced.

Your gas utility might come out and measure for carbon monoxide, but my guess is that they would also red-tag and shut down your furnace. At least that’s what they do in the mid-west and most areas of Canada.

My suggestion would be to start getting competitive bids on a replacement furnace and potentially the air-conditioner if it is also 14 years old. In the long run, you’ll be money ahead.

Best of luck,


On 8/1/2011 3:57 PM, Mike Beeton wrote: Hi
I only have one thing to say. Standard eff furnace,Mr squirell
builds a nest in the chimney,furnace has a cracked heat exchanger,
products of combustion come into the living space. Hope those people
didn`t spend the 40 bucks on your book instead of a co detector, cause
if they did they are dead.

8/2/11 7:07 AM


You obviously have NO idea how a furnace works. You might as well be Mr. “squirell” or squirrel. (I can tell spelling and English are not your strong suit.) I’ll just bet you have the equipment and knowledge to check for the proper draft through a heat exchanger or water heater and can detect reduced volume through a flue when it is partially blocked.

And I’ll bet you already know that if you TOTALLY block the flue, the products of combustion will roll out the draft diverter of a “standard efficiency” (assuming you mean atmospheric burner) furnace. If the furnace is equipped with roll-out sensors, then in theory, they will trip. All draft-induced furnaces have roll-out sensors, most atmospheric-burner furnaces do not. But you already knew that, right?

But, I’ll bet you didn’t know that If the furnace blower is operating there is absolutely 0 (zero – that mean nada, nothing, zilch) CO or any other gas going to come out of a heat exchanger into the indoor air stream.

Geez louise, pay attention.

The furnace blower is positively pressurizing the duct, moving air at over 700 feet per minute through the furnace. The first thing the air hits is the air-conditioning coil, which causes static pressure inside the furnace cabinet. So, you have .2″ WC pressure surrounding the outside of the heat exchanger cells, and 0″ WC on the inside of the heat exchanger. Which way will the air go? The products from the flue get into the house by being sucked in through cracks in the the return-air system or the furnace is already inside the home in a closet or utility room.

Whether the heat exchanger is cracked or intact makes no difference. (That’s the point of my book. HVAC guys condemn furnaces because of a small hole or hairline crack and NEVER check the flues. In 60-70% of the cases there’s a gas-fired water heater teed into the same flue. With a partially blocked flue and a draft-induced furnace and gas water heater teed into it, where do the products of combustion go?)

If the indoor blower creates enough pressure to push air through an a-coil and out a register (creates static pressure in the duct work), which way do you think it will push air if there’s a hole in the heat exchanger. What, you think it “sucks” as it pushes air around the heat exchanger cells?

By the way, there are 100’s of THOUSANDS of CO detectors being used by home owners at this very moment that won’t alarm when high levels of CO are present. They’ve been contaminated with paint or other petro-chemical fumes or refrigerant gasses or they are just old. Electrically they are intact, but chemically they are dead. Based on past random testing, around 1 in 5 detectors would not alarm at lethal CO levels. The CO detector manufacturers say replace the detectors every five years. Beyond that time, the sensors die a natural death due to age.

So, a CO detector is no substitute for a technician that knows what he is doing.

I feel sorry for the customers that get you, though — their survival chances will not improve.


PS: I know these are big words and science is a foreign concept to you, but don’t trust me. Take an old furnace and figure it out. Make sure there is an a-coil on it, drill a bunch of 1/2″ holes in the heat exchanger, and watch which way the wind blows.

4/19/13 11:47AM  From: Tim Exxxxxx
Subject:  CO with induced draft motors and cracked heat exchangers

Gus, thank you for the info on your site.  Very informative and eye opening.

I am a commercial and residential HVAC contractor.  As I am sure you have heard many times this goes against all I have been taught about cracks in heat exchangers.

I would ask that you please note that contractors have been misled for years in training classes.  Most of us are very honest family men and are not trying to use fear to sell.  I was told early on that we are required to shut off the gas when we find a crack.

That being said I do appreciate your site and the info provided and have passed it on.

One question:  If the unit had an inducer motor (a pkg unit for example) would that static pressure cause CO to move into the air stream?


Tim Exxxxxx
Owner & President
Sxxx xxxxxxx Heating & Air Conditioning, LLC


I’m glad you found my web site and recognized the little white lie that has been passed on from the days of the “steel riveted heat exchanger.”

Package units are a different animal concerning the behavior of a heat exchanger crack or hole.  In most cases, the situation will be similar, but it depends on the size of the duct work and the discharge opening of the package unit.

Most package units I’ve worked on (Lennox, Carrier, Aaon, Trane, etc) have draft inducers that pull air through the heat exchanger.  The negative pressure keeps the products of combustion from getting into the indoor air stream in most cases.

I don’t think I’ve worked on package unit that had a combustion blower that pushed air into a sealed combustion chamber, so I’ve got no experience with a positively pressurized heat exchanger.  I can tell you, however, that a gas conversion burner or a “powered gas burner” (think oil-burner for gas with a combustion blower) will create a positive pressure in a heat exchanger. 

The following is theory on my part that I have not had many chances to test, so view it with a critical eye.

The static pressure at the heat exchanger on a package unit will depend on whether there is duct work attached or the unit simply feeds a distribution box beneath the roof curb or through a wall.  If there is little duct resistance (friction loss) and the opening from the package unit is large enough, most of the air flow will be velocity pressure and very little will be static pressure.

You can observe this by using your manometer at a number of locations around the cabinet on the discharge end of the unit.  (I’ve drilled a few holes in the bulk head separating the discharge area from the condenser fan area and found that the static pressure varies from above 1″WC down to -.5″WC.  That means in some places air would be sucked into an opening that would, in theory, have positive pressure.)

My guess is that the phenomenon has to do with air turbulence inside the package unit and is similar to the effect of a top take off being cut into a supply trunk too close to a turn or transition.  The dead spot created by the turbulence in the duct prevents air from leaving the trunk and entering the branch run.  This results in a floor register that delivers very little air flow into the user space.

If that turbulence is next to the heat exchanger in the package unit, then static pressure may not be uniformly felt around the exchanger.  Having said that, a big hole or crack may actually experience negative pressure and at that point the products of combustion may be “sucked” out of the heat exchanger and mixed in the indoor air stream.

I’ve gone through this long description because I’ve had cases where I could see the big holes in the bends on a tubular heat exchanger but the flame was not rolling out of that burner tube.  The flame would be distorted, but not roll out.  The CO readings where high (600 to 1,000 ppm) at the draft inducer discharge, but I could not measure any CO in the discharge air stream in the building.  Background CO was 20ppm, so I knew something was not right, but with LP fork lifts running the previous day, I could not rule out trapped CO due to poor air changes.

Here’s the “theory” part – it may be possible that the turbulence inside a package unit could create a situation where the products of combustion could be pulled into the indoor air stream even thought the unit has a draft inducer. 

I haven’t had many chances to actually test the hypothesis so I keep it to myself.  In any case, the holes in the heat exchanger were enough to get the package unit replaced.

In regards to shutting off gas to a furnace with a crack, I agree.  It is actually written into many municipal ordinances and is part of the AGA standard.  Their theory is that if there is a mechanical defect of any kind, it is a safety issue.  (I was at AGA testing automatic gas valves as a manufacturer representative when those “discussions” were going on.)

My argument is not that we have to shut the gas off, my problem is that we put families out in the cold when there is no demonstrated danger.  A crack  or hole in a heat exchanger does not necessarily mean an immediate danger is present.  Granted the heat exchanger or furnace needs to be replaced.  However, when you shut off and disable the furnace, you now force the family “into the cold”.  They have gather up their kids and make arrangements to sleep somewhere else.  Plus, they have limited options when it comes to getting competitive bids on replacement equipment.

I’ve stood in service meetings with 25 or more “family men” who told me in no uncertain terms that they would shut down a furnace with a heat exchanger crack, no matter how severe, even if NO measurable CO was detected in the house.  To them the potential liability “forced” them to do it.  Most said they wouldn’t even get a CO meter out of the truck if they could see the crack with a mirror.  All they needed to see was a flame disturbance, or the crack.  And each man said that they tell the homeowner that this is a safety issue and has to be addressed immediately.  That sounds like scare tactics to me.

The worst part is not the scare tactics, it is that we miss the real potential dangers by focusing only on the equipment. 

I’ve been involved in a number of CO incidents, three of them involved deaths.  In two of the three death cases, when the furnaces were checked, they were operating within accepted parameters with CO at the flue less than 20ppm.  The third unit was putting out over 100ppm at the flue, but had no other problems and no heat exchanger cracks.

In all three cases, the equipment manufacturers and contractors lost the wrongful death law suits.  Fortunately the contractor’s insurance carriers protected the service companies.  I don’t know how the manufacturers handled the loss, but the families of the victims were awarded millions.

Each of these cases had flue problems that were never highlighted until after the litigation.  One case had a flue cap crushed by the roofers who had replaced the roof two years before the deceased owners had purchased the home.  The reduced flow up the flue caused the furnace to spill back at the draft diverter when the furnace ran for long periods.

The second case was caused by a partially crushed flue liner that was pulled through an offset chimney.  It would exhibit a positive draft, but could not carry sufficient volume during long run times which caused the flue to spill back down the water heater draft diverter that was teed into the same flue connector. 

The third was a two story class B flue with an 80% furnace and a 40 gallon water heater.  The flue was sized correctly, the flue connectors were sized and installed correctly and the flue cap was intact.  The problem was that the flue could not establish a positive draft during extremely cold weather and would spill back down the water heater draft diverter.

All of these problems were not detected by the service technicians who had performed the fall clean and checks.  All three cases were made worse because the return air ducting was “loose” and could pull CO laden air from the furnace area into the return-air stream.  None of the flue problems were tested during the discovery phase of litigation, however all three furnaces were sent to testing labs to “prove” that they were the CO producing culprits.  All three furnaces were given a clean bill of health by the labs, yet the manufacturers lost the law suits.

The lesson here is that your technicians need to look around and understand their craft and always question what they see.  People die because they make assumptions about the condition of some “things” while targeting hot button issues in other “things”.  Gas appliances should be viewed in the context of operating systems where safe operation is dependent on a number of “things” beside simply the furnace or boiler or water heater.

A correctly operating furnace can kill if there are issues with the flue or combustion air.  It is not the fault of the appliance, and it is our job to make sure that doesn’t happen.

Stay safe and question everything.



8/22/13 4:52PM  From:  Bryan Cxxxxxx
Subject:  Cracked Heat Exchanger


               I have a question for Gus.  About a month ago I had a local company come in and seal my rim joists. In order to do so they had to perform a test where they turn on all the fans in the house and then turn on the heat. We have a window ac unit that keeps the house comfortable. The house has a central cold air return. I block it so the cold air won’t go into the basement. I forgot to remove them. After a few minuets the air smelled funny like a burning smell. The instrument the they were using to measure the air flow up the flue also detected carbon monoxide. The level was at 350. in the room it was 20 when  the gas company came 15 minuets later.  I was told that the heat ex-changer was cracked. upon inspection we were told. ” checked heat ex-changer with flame going and noticed 3rd section flame starting to flutter and turn yellow. Took a closer look and found 3/4 inch crack starting to form on back of heat ex -changer. ” It is an old Whirlpool vertical unit that was put in when the house was built in 1985. !00,000 btu in 80,000 btu out put. 

 What do we do. Re test the Co levels? 


                                                                                     Bxxxx Cxxxxx


8/23/13  6:19 AM


You’ve definitely got a heat exchanger crack.  I’m surprised that simply blocking the return would result in any amount of carbon monoxide entering the house.  In every case I’ve seen, the blower pushes are into the heat exchanger.  That’s what causes the flame to waiver back and forth and the flame tips to turn yellow.

With the return-air blocked, the blower would have a difficult time moving enough air across the heat exchanger to keep it within its normal heat-rise rating.  (This rating is usually stamped on the same tag with the model and serial number.)  That’s probably where the burned smell was coming from.  The heat exchanger was running very hot and the duct work around it would have been getting warmer than normal.

It is possible that the blocked return air opening caused a slightly negative pressure around the furnace duct work and air filter that was felt by the flue.  As burned gasses were pulled back down the flue, they may have been pulled into the front of the furnace and re-burned.  When that happens, a flame will product large amounts of carbon monoxide quickly.  A reading of 350ppm in a natural gas flue indicates a problem.  It may be due to incorrect gas pressures, incorrect air shutter settings, or partially blocked burner openings or orifices.  Generally heat exchanger cracks will not cause CO output to go up that much.

If your technician says they found a crack and could see a flame disturbance, then you’re going to have to replace the heat exchanger or install a new furnace.  There’s no way to get around that.  Almost every municipality or gas supplier has a requirement that furnaces with cracked heat exchangers be shut down and red-tagged.

Check with a couple of Whirlpool dealers and ask if your model furnace had a lifetime heat exchanger warranty.  I’m pretty sure the heat exchanger is no longer available, but Whirlpool may offer a discount toward a new furnace.

Best of luck,



9/13/13 9:06PM  From:  Stephanie Bxxxx
Subject:  Carbon Monoxide Question


I have a basement in my home where the furnace, water heater, washer, and dryer are located. The utility room is about 5×12 feet. We have a bedroom with what I believe is an intake vent and an outlet vent right beside the utility room. We also have a family room down there. My son’s bedroom is at the top of the stairs. The stairs actually land into his bedroom. My question is how can I make this safe? I used to be the one who would open the bedroom window in my kids rooms at night, but after reading your site I won’t anymore. I read that lack of ventilation can cause the carbon monoxide so if I open the window that is closest (about 12 feet away) to the utility room which only has a sliding door would that prevent Carbon monoxide build up? 

Thank you

Sxxxxxxxx Bxxx

9/15/13 6:26AM

Hi Stephanie,

You’re between a rock and hard spot, and definitely have a problem.  I’ve attached a “Kindle formatted” version of my book as a pdf file.  You need to read it.  (I’m still working on the book, but it does have information concerning combustion air, transfer grills, and fresh-air duct sizing.)

You either need fresh air ducts brought into the utility room, or transfer grills that will supply enough fresh air for combustion.  Just opening a window nearby may help, but depending on the wind, it could also cause problems with the flue.  (And truthfully, would you keep the window open if it were 10 degrees outside and windy?)

Assuming you have a 75,000 btu furnace, 35,000 btu water heater and a 30,000 btu gas dryer, your total connected input is 140,000 btu/hr.

If you look in the attached book under “Combustion Air Requirements”, it shows that 140,000 btuhs requires 7,000 cubic feet of free and open space for combustion air.  That is 50 cubic feet for every 1,000 btuhs of fuel consumption which is required by the National Fuel Gas Code.  When all three appliances are running, you technically need at least 1,000 square feet of open floor space if the basement has a 7 foot ceiling height.  Obviously your situation is far short of the suggested free air minimum.

Without knowing exactly how your house, appliances, and flue are configured, I can’t accurately help you figure out a good solution.  I would suggest you block the door to the utility room open so that it doesn’t accidentally get closed.  Then I would install a carbon monoxide detector in the utility room, another in the basement bedroom, and a third by the upstairs bedrooms.

I realize most manufacturers don’t want their detectors placed near gas appliances.  However, in your case, you want an early warning anytime there is a chance that your flue is back drafting.

Then, I would find a reliable contractor who can survey your home and figure out whether you need ducted fresh air from the outside, or simply transfers grills.  I would hesitate to use your existing HVAC contractor, they obviously missed warning you of potential combustion air issues.  A 5 x 12 room with a furnace and water heater and no transfer grills should have raised red flags.

Best of luck,




10/22/13 8:51AM  From:  Jason
Subject:  Questions on Heat Exchanger

I am being told by one company that I have a cracked exchanger (via camera), and another that I don’t (also via camera).  No C.O. issues – two detectors in house and also had one of the hand held calibrated “sensitive” ones brought in.  Problem is that my furnace keeps kicking out the upper limit switch (which has been replaced) when the furnace runs for awhile warming up.  They keep adding to the cold air returns and want to do more.


After reading a lot on your site about cracks and C.O. – Since a small hole/crack may not be responsible for C.O., could it be responsible for making things too hot and having the upper limit switch kick out?



10/22/13 11:38AM


Heat exchanger holes/cracks have nothing to do with limit switch trips or overheat problems.

Somebody should have quoted you a temperature rise in degrees F. That number will tell you if the problem is overall airflow (quantity of air in CFM moving through the furnace) or if there is a localized air restriction/diversion.

The minimum and maximum temperature rise are stamped on the ID plate inside the furnace.

My hunch is that the temperature rise is probably caused by a dirty/blocked air-conditioning a-coil or there is a problem with the blower motor not pushing enough air.  That’s assuming the system operated correctly for a period of years.  If this is a new installation or there was remodeling or other structural changes, then more parameters need to be checked.

An easy test to see whether inadequate return-air is part of the problem is to run the blower (with burners off) and attempt to remove the blower access panel.  If there is a lot of “negative pressure” (the blower is sucking the door against the furnace with great force) then the return is blocked or too small.  There will always be a reasonable amount of suction pulling against the blower door.  However, when the return air is restricted, you will hear the blower greatly change sound, RPM and air flow when the blower door panel is pulled away from the furnace cabinet.

A reverse of the above test is to slide cardboard into the filter rack and intentionally block return air flow.  If there is no appreciable change in temperature rise or blower sound, then there is a return-air restriction somewhere.

If the problem is return-air ducting, then replacing the furnace will not fix the problem, and you’ll have issues with the new system.

Let me know how it works out.

Best of luck,


10/23/13 9:58 AM

Wow, very impressed with your response – thank you very much!!  The below is probably more than you wanted to hear back, but since this has become such a mystery, thought I’d share.


Here is what I know:


I live in an apox. 1,000 sf. ranch house on a crawl – three bedrooms.

I have a Bryant Plus 80 Natural Gas Furnace and A.C. – 56,000 BTU Down-flow and is aprox. 12 years old, but has had the following replaced….

*  March 2010 – New Glow Coil

*  June 2012 – New 1/3hp 1075 rpm blower installed (has been double checked that this is the proper blower for unit)

*  June 2012 – New Capacitor installed

*  Aug. 2013 – New Limit Switch installed

*  Sept. 2013 – All new 6″ insulated flex heat ducts (9) installed in crawl; main trunk insul. with R-6; all new 8″ insulated flex cold air return ducts (5) installed in attic; new hard pipe 14″ main cold air duct insul. with R-6, which runs from furnace in gar. to box in attic, where the 8″ lines are run to.

*  Oct. 2013 – A.C. coil cleaned

*  Oct. 2013 – Living Room 8″ cold air changed to a 10″ and ran direct to furnace;  hole that L.R. duct used to run to at box, was changed to a 10″ and run directly to furnace.  So I now have 4-8″ cold air ducts (3 beds and one Dinning Room) that run to box and then the 14″ takes to furnace.  Have 1-10″ from box to furnace.  Have 1-10″ duct from L.R. to furnace.  Vent sizes are 12×24 and larger.

*  Temp. Rise Rating is 40-60 and last test was at 53

*  A.C. A-coil has been cleaned and seems good

*  Blower door ck. seems to be ok

*  C.O. tested in stack, and it does not have an increased output


When the furnace has a call for heat more than a deg. or two, the limit switch kicks in and turns off flame until it is cool enough and then allows flame to come on again.  This cycle continues.


Because of all of the above, the next thing they have thought about is the circuit board.  Because of the expense and it only being another possibility – they have opted to replace – I can only hope this is the right thing to do.




10/24/13 11:20AM


It sounds like you have more than adequate return-air ducts as long as the flex ducts are not kinked or making sharp turns.

Return air duct sizing is normally based on 400 cfm per ton of air-conditioning.  If your house as a 3-ton a/c, then you’ll need to move 1,200 cfm through the system for the a/c to operate correctly.  For the typical 1,000 sq ft home (I’ve engineered and installed 1,000’s of these kinds of systems) you should wind up with some combination of return-air grills that are equivalent or greater than 3 – 30×8 grill openings.

In higher quality installations, that usually translates to one 16×8 return in each of 3 bedrooms, one 30×8 in the living room or common hallway and another 16×8 near the garage or mud room door. Each 30×8 moves 1-ton (400 cfm) of air, each 16×8 can move around 120cfm without making a lot of noise.  I normally figure them at 100 cfm.

Supply registers (floor registers, low sidewall registers, etc) should be figured at 100 cfm each, meaning you’ll need 12 supply registers for a 3-ton system (1,200 cfm total system air.)  These registers are normally fed by 6″ round metal ducts.  Depending on how the supply trunk is arranged, the amount of air coming from the supply registers will vary considerably, however, the overall air flow will keep the structure at the desired set point temperature.  For example, you’ll have a standard register and duct feeding a small bathroom that only requires a 4″ or 5″ duct. The easiest solution is to close the supply register damper to reduce air flow.

If your system only has a single return-air grill it would need to be at 16×25 or equivalent.

Hope that helps,




11/10/13 6:07 AM  From: David Gxxxxxx

Have to disagree with ya there Gus. 

Here in Iowa, we don’t have many house fans up this way. 

Here is a few examples, of what I have found.  Yes you can be right in some ways, but I have had one furnace that wasn’t putting out CO with a couple 3 inch spits in the HE.

But at the same time, this furnace wasn’t putting out much in the exhaust.  However I have pulled a cover off a HE on a heil furnace and it smelled like exhaust off your car and my meter went over 400 ppm.  Pumping CO right into the house. 

Another Intertherm 6 years old pumping 300 plus into a house and the lady was in bed sick as a dog, and before I got out of the house her husbands boss called and they had him in the hospital for CO poisoning.  He spent 8 days in there. 

Another one on a friends house, Goodman clamshell, had splits in all four and they were splits 8 inches long and 1 inch wide, Unbelievable.  Not a lot of CO but enough to make them sick.  Complaint was a howling noise and NO rollout.  I couldn’t believe the no rollout but sure enough not.  Duct sizing was it’s problem. 

Another one I had just cleaned and the homeowner was playing with my CO detector and got 15 ppm off his furnace while I was still working on it.  None in the house, other than the furnace plenum.  Fan blowing. 

I would take down your site on this before someone sues you into the ground.

11/10/13 7:18 PM

Hi David,

I’d be the last guy to say I know it all, but I have worked on more than a few gas furnaces over the years. And, I’ve seen some situations like the ones you describe.

In this case, disagreements are how we learn, so bare with me.

It’s been my experience that the CO that’s measured is coming out of the front of the furnace and then pulled into the return-air stream. It either gets sucked in around the filter, where the return-drop meets the furnace, open return-air headers or panning, and sometimes right through the cabinet because of abandoned or unused openings between the vestibule and the blower compartment.

An 80% gas furnace (upflow, downflow, horizontal) with an a/c coil on the discharge is basically a pressurized cabinet with another box (the heat exchanger) inside of it. When the furnace blower kicks on, it pressurizes the inside of the furnace and pushes air around the heat exchanger, out the discharge, through the a/c coil, and out the registers.

The force of that air from the blower is over 100 times more powerful than the draft inducer or natural draft air moving through the inside of the heat exchanger. The static pressure inside the furnace exists in all directions and puts force on every outside surface of the heat exchanger. Agreed?

Because of the pressure on the outside of the heat exchanger shell, any hole or crack will allow blower air pressure INTO the heat exchanger. In other words, air travels from the blower side to the fire side of the heat exchanger, not the other way around.

That’s why most heat exchanger cracks, especially the big cracks, cause the burner flames to waver or roll out the front of the furnace. It’s why they put rollout switches on them.

Before you decide I’m completely off my rocker, why don’t you read through this thread on the HVAC-Talk web site? I responded to someone who was trashing my web site, but didn’t have the courtesy to contact me directly.

Take the time to read through the entire thread and see what other HVAC guys are saying.
http://hvac-talk.com/vbb/showthread.php?1425811-Anyone-else-think-this-site-is-bogus/page3. It might be an eye opener.

By the way, there are links there to ACHR News articles, one of which is titled “Cracked Heat Exchangers Don’t Cause CO.”

If you’ve run into situations where what I’m saying is wrong, let’s investigate it. If I’m wrong, I’ll most certainly change the information I’m presenting. But, that will take some convincing, some facts, and some pictures.



PS: If you don’t have time or are not inclined to read through the forum thread, here’s one of the responses that might give you something to think about.

Member: Chuck – professional member
I agree completely with Gus, there are so many CO poisonings that have absolutely nothing to do with cracked heat exchangers that cracks are almost a non-issue, almost. Most of the guys that think cracks are so dangerous, and red tag every one they can find, are walking past more dangerous problems on a daily basis and have no clue.

Qwerty, nobody here has said it is ok to keep using a furnace that has a crack. The bottom line is, we have tools and instruments that will tell us if a furnace is safe or not regardless of a crack. The homeowner always gets advised when a crack is found and proper recommendations are made. That rooftop you showed would not have passed any safety or combustion test and thus would have been red tagged. That is not what this thread or the OP is about, it is about those small cracks that are not affecting the combustion at all.

Anytime a crack starts leaking to the point that it is causing rollout, etc.. they can be easily diagnosed with a combustion analyzer. I believe this thread is about the cracks that are not leaking.

Anybody work on the famous dura curve lennox heat exchangers? They are all cracked, at least all that I’ve found. I have replaced the HX and 2 years later there is that same crack. Normal operating parameters such as temp rise, static pressure, etc. On this particular model, the crack is very tight and does not leak, period. They are not an immediate hazard and do not warrant shutting down the furnace.

Post 41 by “second opinion” is excellent and very true.

11/11/13 1:46AM

Ok gus, this is what I’m thinking…. and yes what your saying makes sense, but for some reason doesn’t always work out that way.

Example Frigidaire 3 years old…..center tubes at the ends rusted out… heat exchanger made bellering noise. no roll out… but yet 3 inch cracks 1/4 inch wide. No CO but the vent pipe had like 15 ppm in the exhaust…. so dilluted I”m guessing with air…

That is why you can’t get a reading sometimes within a plenum, because not much anyway in the exhaust.


I don’t know but I’ve seen some with high CO in the plenum and no apparent easily seen breach in the HE before……. can’t explain it either. Somewhere in the seconandary.

I wonder, instead of pressurizing like you’d think, it’s actually sucking it out.

whold house fans, I’ve seen blow out pilots on water heaters…… and I can see those sucking the exhaust right into the house on a water heater….or 80 percent furnace…. I wonder how they keep those legal actually when they can do that.


I think you’d agree that safety is on the mind of every homeowner untill you tell him to make something safe it’s going to take a few bucks….. example cold weather and b vent up the side of the house. Nope it’s ok… Can’t figure out why it won’t vent at zero degrees though..

Or I have one old lady right now, I stuck her finger in a crack and finally she found some 80 year old man to tell her it’s ok,,,,,,,,, so now I’m the bad guy. It was ok till she found out she made too much money to get a free govt furnace.

11/11/13 11:54 AM


Hi David,

I’m glad you didn’t just dismiss what I’ve said out of hand without consideration. That means I’ve got a chance to convince you, and you are open to learning, which is great. Most guys read this stuff and just say it is bogus without giving it any consideration at all.

First, there are NO absolutes. Things do not always work the same way every time. So I recognize that you may have run into situations that are unique and do NOT appear to operate like we are discussing here.

Let’s discuss each of these examples and go over the symptoms.

Example Frigidaire 3 years old…..center tubes at the ends rusted out… heat exchanger made bellering noise. no roll out… but yet 3 inch cracks 1/4 inch wide. No CO but the vent pipe had like 15 ppm in the exhaust…. so dilluted I”m guessing with air…

That is why you can’t get a reading sometimes within a plenum, because not much anyway in the exhaust.

The 3yr old Frigidare probably was not producing more than 5 or 10 ppm CO in the flue. The bellering noise was blower air causing a whistle sound as it pass through those cracks into the heat exchanger. You are correct that the flue products are diluted, but they are diluted as they go up the flue. You don’t read any CO in the plenum because there is no CO in the plenum.

I don’t know but I’ve seen some with high CO in the plenum and no apparent easily seen breach in the HE before……. can’t explain it either. Somewhere in the seconandary.

I wonder, instead of pressurizing like you’d think, it’s actually sucking it out.

You are right about the secondary and I have no doubt about high CO in the plenum. 

Our discussion is about 80% or lower efficiency furnaces. The secondary heat exchangers on 90+ furnaces, which sometimes look more like a/c coils than heat exchangers, can definitely put CO into the indoor air stream. I’ve got a big caution note on my web site about that. There are definitely turbulence between the primary and secondary heat exchangers that may do just what you say, cause it to suck combustion products out of the cracks. (This is an area I’ve never tested, so I’m guessing. But, you are right in that secondary heat exchangers can put CO into the indoor air stream.)

whold house fans, I’ve seen blow out pilots on water heaters…… and I can see those sucking the exhaust right into the house on a water heater….or 80 percent furnace…. I wonder how they keep those legal actually when they can do that.

Whole house fans, or big kitchen exhaust fans can cause flues to back draft, no question. The problem here is consumer education. As an industry, we’ve done a poor job of it. That’s one of the reasons I put my web site together. There is a whole page devoted to fans sucking the air out of the house.

In fact, I told my customers to put a carbon monoxide detector right in the utility or furnace room to warn them if the downdraft occurred. Many families had teenagers who didn’t give a second thought to running an attic fan with only a couple windows open, when their upstairs bedrooms got warm. They had NO idea that they were creating a dangerous situation for everyone in the house. If the downdraft is severe and the water heater re-burns the combustion products that have been sucked back into the utility room because of the attic fan, the water heat will be throwing 400 to 800 ppm CO. Then the attic fan pulls that CO laden air through the rest of the house.
I think you’d agree that safety is on the mind of every homeowner untill you tell him to make something safe it’s going to take a few bucks….. example cold weather and b vent up the side of the house. Nope it’s ok… Can’t figure out why it won’t vent at zero degrees though.. You are right, many homeowner’s don’t want to spend money to be safe. They think their furnaces should run forever, they skip routine maintenance, and then have a problem spending their cash to correct the problems. That’s why building codes and ordinances mandate certain building parameters. Things like the Fuel Gas Code and the International Mechanical Code set minimum standards for safe installation and operation.

B-vent up the outside of a structure is specifically covered in the old GAMA flue tables and in the National Fuel Gas Code. It’s NOT supposed to be done in cold climates and the vent is supposed to be enclosed in an insulated chase in mild climates. (I forget what the climate zone designations are, but they are in the code.) When contractors don’t follow the code, they are not doing anyone any favors. They may save a few bucks on the installation, but create problems like the ones you’ve run into.

Or I have one old lady right now, I stuck her finger in a crack and finally she found some 80 year old man to tell her it’s ok,,,,,,,,, so now I’m the bad guy. It was ok till she found out she made too much money to get a free govt furnace. You don’t want those folks for customers. They create such a headache for you and they don’t understand that you are trying to keep them safe. They think it’s to make a sale instead of a warning that their equipment could be dangerous.

Our conversation has made me think about a few things. If you’re confused about what I’ve got on my web site, then homeowners might be MORE confused. I’ll have to go through it and see if I can make the information more clear.

My goal was to stop guys from condemning every little inconsequential heat exchanger crack and pay attention to the entire job. A little heat exchanger crack won’t do anything, but a return air trunk that’s open near a furnace and water heater can back draft the flue and kill the occupants during really cold weather.

Still think I should take my site down?

Best Regards,



11/25/13 7:47 AM  From:  Harry Mxxxxx
Subject:  Furnace Info

Gus; thank you so much for your website info.  I went downstairs to check the flame on my furnace, and saw that it was just fine.  However, when I last turned on the pilot light, I left the cover off that area.  Of course, that gave me a real good view of the flame, which was good.  My unit was manufactured around 1974 when we built the house.  The unit has two main burners that use a tube with holes in it.  I appreciate your information.

Harry Mxxxxx

Fxxxxxx, GA

PS: Not related to this email, you are invited to my website at: spotlightoncreation.com

PPS:  I saw a couple of typo problems in the first part of the website. In a couple of paragraphs, you spelled the word “through” with a  “t” on the end (throught). No biggie; just thought I would mention it. 

11/25/13 10:29 AM


Hi Harry,

I’m happy to hear that the information on carbonmonoxidemyths.com helped. 

Please note that even though the furnace burners appear normal, you need to thoroughly check the flue and make sure you have adequate combustion air for all the gas appliances in the area – especially during extremely cold weather.  That’s when the house will be most tightly sealed with the least amount of infiltration air from doors being opened.

As we weatherize and seal our homes, we actually start competing with gas appliances for oxygen.  In every situation, the appliances will win because they will simply “burn” what ever is available.  That’s why the flue is the most important safety device in the home.

Be sure to put the cover or flame shield back on the furnace <grin>. 

Thanks for spotting the typo’s.  I corrected those, and found a couple more that I fixed.

Best wishes for the holidays,

11/25/13  11:14 AM

Gus, thank you so much for the new info.  I am honored that you responded so quickly to my email.  Harry


11/29/13 11:42 AM  From: Mark Bxxxxxxxx
Subject:  Help Gus

Hello and you seem to have all the answers about carbon monoxide and furnaces. I want your advice, I have a 27 year-old forced-air furnace with no air condition. I live in Colorado and probably turn the unit on 40 times per year. I just had a tune-up and off-course the tech said I need a new unit because I have a small coil-crack and was extremely high-pressure. 
My Carbon test was .10 ppm with it set to max (seems to be safe). I know it’s old and eventually needs to be replaced, but do I have to replace asap? 
In addition, I have an attached garage and I have a CM alarm. I know this only goes off at higher levels (+70) ppm and has never triggered.
Just wondering if I was being over-sold or its a real emergency?
Thanks for your help,

11/29/13 3:17 PM

Hi Mark,

Sounds to me like you’re being “over sold”.

I’m not sure what the CO reading he gave you means.  If it was really .10 ppm, that’s low background levels you would find outside in clean air. 

Most older furnaces that are operating properly are in the 3 to 10 ppm range when measured in the flue.  You’ll get .10 ppm standing outside in any big city in the US.  (I’ve never tried to measured background CO with a meter in truly fresh-air country, so I don’t know what the background levels would be.)

Look at the furnace burners and watch the flames when the blower comes on.  If you see the flames jump around all-of-a-sudden, or start wafting out the front of the furnace, then you have an immediate issue.  Otherwise I’d call someone for a second opinion.  My guess is you are probably the victim of high-pressure sales tactics, but you need to get a second look from a reputable HVAC contractor to be sure.

Best Wishes for the Holidays,


11/29/13 5:55 PM From:  Mark Bxxxxxxxx

Ty…no flames jumping around or shooting out. He measured the reading from a downstairs (basement) air-vent close to the furnace (it read .10 ppm). 

Thanks for your opinion and I will get more opinions. Do you know any in Denver? 

Happy holidays, 


11/29/13 8:13 PM

Ha,  .10 ppm at a basement register means NO carbon monoxide at all. 

Most test equipment is rated at plus or minus 2% of of reading or 2 PPM (whichever is larger.)  Meaning, most hand-held test equipment can’t accurately read PPM that small unless they’ve just been calibrated and you know if the gear normally reads a little high, or a little low.

Sounds like they’re trying to sell you something.

I’m in the midwest between Kansas City & St. Louis, so I don’t have any information about contractors in the Denver Area.  I checked Angie’s List and found that my membership only includes nearby communities, it doesn’t cover the entire country, so no help there.

If you’re a member of Angie’s list, that’s where I would start.  If not, the membership is about $25/yr and well worth the cost.

You can also try yelp.com.  Here’s a link for the Denver area and heating contractors.

Take some of the reviews “with a grain of salt”, especially the really negative ones.  Once folks have their feelings hurt they will write some pretty mean things.  Sometimes a small one or two man shop gets busy (they become a victim of their own success) and when they drop the ball or don’t get to customers in a timely fashion, they may get overly harsh reviews.

Best of luck,


11/29/13 8:15 PM  From: Mark Bxxxxxxxx

Thank you so much.

Sent from my iPhone

12/9/13  7:42 AM From:  Bill Fxxxxx
Subject:  Lennox “natural draft” furnace question

Thanks for your post on cracked heat exchangers,

 we have a Lennox Natural Draft Furnace probably 30 years 

old, blower and parts of furnace were replaced 18 years

ago, doesn’t say Pulse anywhere on it for example.

Recently had an inspection and was told we had cracked

heat exchangers and should replace them, but not really

worth it on 18 year old exchangers. 

We had ONE 1995 CO2 detector and bought a couple 

others while we shopped systems, nada. We often

just turned the heat off at night because we like it

cold, and now do exactly that.

My question is, as a non forced air system, is our 

system as unlikely to get CO2 into the heating vents

as forced air systems? It is the older 80% system.

We ARE going to replace it, the AC is 30 years old and

while works fine it does spin the meeter.

Our 2nd questions is, who had the BETTER design

for an 80% furnace, single or two stage between 

Carrier and American Standard?

THANKS very much for any thoughts.


12/10/13 12:12 PM

Hi Bill,

I’m a little confused.  In the first paragraph you say you have a Lennox natural draft, 30-year-old furnace, that doesn’t say “Pulse” anywhere on it.  Yet in the fourth paragraph, your question is “as a non-forced air system is our system as unlikely to get CO2 into the heating vents as forced air systems? It is the older 80% system. “

Just so we keep the wording straight —

“forced-air” refers to a furnace that uses a big blower to push air through duct work.
“forced-draft” or “power-draft” or “induced draft” refers to a small blower that pushes or pulls air through a heat exchanger.

Your old Lennox, natural-draft furnace uses a standard blower.  Since it has an air-conditioning coil on the discharge, there will always be static pressure inside the furnace and around the outside of the heat exchanger.  This will always force air INTO the heat exchanger through any cracks that exist.  The problem with cracks is they lead to safety problems like flame-roll out, burned wiring, delayed ignition, etc.

I don’t think you’ll have an issue with carbon monoxide (CO), but I would recommend replacing the furnace.  A 3″ crack could split wide open and then flame-roll out and burned wiring could become a problem.  30 years of service from any furnace is unusual.  You’ve gotten your “monies worth”.  Those furnaces are usually replaced at the 20-year point or sooner.

You may not be able to purchase an 80% furnace in the near future.  Read AHRI’s article and check HARDI.   Your dealers should be familiar with the standards in your area.  The Department of Energy has been pushing hard to eliminate 80% furnaces and force everyone to install 90% or greater efficiency furnaces.

My advice would be to consider a high-efficiency furnace with PVC venting.  These furnaces are much more efficient and have proven as reliable as 80% furnace have been.  Yes, they’re a little more expensive, but there’s a quick ROI based on fuel savings.

Just make sure the installing contractor makes adequate adjustments for your flue to handle the water heater if it is gas-fired and in the same flue.  If you’re existing furnace vents into a masonry chimney, make sure they’ve included a flue-liner that’s sized for the water heater.

As far as a better design, that’s a hard one to answer.  I like both products.  Carrier equipment is usually reliable and efficient.  American Standard equipment is also well built.  Both manufacturers provide good service to their dealers which translates to good service for you.

Concerning single versus two-stage, I would lean toward two-stage equipment.  You’ll find that a two-stage furnace is “more comfortable” as it doesn’t overshoot temperature like a single-stage system can.  Most of the time you’ll be running on low-fire, which will be more efficient and make less noise.  When outdoor temperature drops, then “high-fire” will kick on and you’ll get the full capacity of the furnace.  The downside of two-stage equipment is usually the need to run a new thermostat wire between the furnace and thermostat.  In older homes where all levels (including the basement) have been “finished”, it’s much simpler and less expensive to install a wireless two-stage thermostat.  Honeywell makes reliable wireless wall thermostats that makes two-stage upgrades much easier.

When talking with your dealer, make sure they are offering a furnace where the blower speed changes with heat output.  This is usually called an “ECM blower”.  This is a very efficient, electronically controlled, multi-speed blower that starts “softly” and can provide up to 12 different speeds that helps match blower output to heat output.

Good luck and best wishes for the Holidays,


12/10/13/ 12:36 PM

Thanks, I had a feeling I was botching the language, no expert.

They replaced the coil and parts of that furnace 18 years ago. I’m sure it is a standard blower, which

also went out once. So the Lennox heat exchanges are under a 20 year parts warranty but not

going to invest anymore in that unit, quiet though. We like it cool and often just turn the furnace

off at night, so we are definitely doing that now, AND have 3 CO2 detectors properly placed.

We’ve gotten a number of quotes and have 2 or 3 good choices.

Thanks, very helpful. We are in the NW suburbs of Illinois, where are you. 

We will look to find somebody to donate your CO2 myth book for Xmas.

Happy Holidayze,


12/10/13 2:53 PM

Ha, I wouldn’t worry too much about the language, it’s tough to keep straight even when you’re in the business.

Simple task:  put the condenser on the left side of the house.

The problem:  whose left?  The left side while facing the front of the house, or the left side while standing in the house looking out the front door?

I’m now in the St. Louis, Missouri area, northwest of there around 30 miles.  I have distant relatives up in your area, NW of Chicago.

I also spent a lot of time in upstate New York, in the Rochester and Syracuse areas.  I was up there during a number of major snow storms (getting 3ft of snow in 24 hrs) and saw a lot of problems with flues, blocked venting and a host of other combustion issues.

Have a good Holiday,



2/1/14 1:22 PM  From: Tim Bxxxxx
Subject:  CO Detectors

Great site! Noticed you recommend the Pro Tech 7035, but use the CO Expert 2010 in your video test. You also have a Defender advertisement on the left side of the website. Do they all perform the same and are of equal quality? Thanks very much!

2/2/14 6:20 AM

Hi Tim,

Glad you liked the site.

I like the Pro Tech 7035 above the others.  I used the CO Expert video
clip because the contractor did a good job showing the relative response
times compared to standard detectors.  The CO Experts alarm is in the
$250 range.  It is high quality and reliable, but pricey.

The Defender detector shows up in an ad location on the web page.
Tomorrow it could be a furnace or a brand of grape jelly.  I “lease”
those spots out to help pay the hosting bills.  It is funny, but I don’t
see detector ads, I usually see furnace manufacturers, furnace repair
ads, video games and sometimes general purpose ads.

The Pro Tech has a 5-yr lithium-ion battery and can be installed so that
it is tamper proof.  A tenant or child can not remove the detector from
the wall without removing a mounting screw.  They also use a
tamper-proof screw on the battery compartment.  Plus, it provides good
low-level CO notification.

If you have family with respiratory problems or have newborns or elderly
in your home, also look at the CO Inspector.  I think it’s the best
general purpose low-level CO alarm on the market.  If you travel and
stay in hotels, then it is a must.  Keep it in your brief case or
backpack and set it on the night stand at night. It’s only downside is
its two-year sensor life.  But, it is a heavy-duty, waterproof CO
monitor that can be a life saver.

Best regards and stay safe,