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
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?
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:
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
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
Thanks for visiting,
11/7/2011 2:25 PM
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
with heat exchanger or CO leakage. Then this so called consult tells me that due to age of my
CO could be leaking ...etc. This does not seem logical. Do you have any facts to dispute these
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
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
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
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.
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
11/9/2011 11:51 AM
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:
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.
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
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
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.