… continued from CO-Warnings 

Eventually the problem corrects itself, usually. The flue connector gradually warms up and more warm exhaust gas begins pushing its way up the flue. As it does, the warm flue gasses hit the cold flue pipe and condensation occurs. If the flue is a straight up through the roof, then the condensation drop back down into the flue tee or bottom of the flue. Ever wonder why some flue tees seem to rust or corrode more than others? Ever wonder why some jobs eat flue liners?

The point here is that conditions are not always what they seem. When a technician shows up in the middle of the afternoon and checks a system, the system may perform correctly. The flue is not ice cold and there are people opening doors which allows air pressure to equalize and fresh air to enter the house.

However, as the outdoor temperature drops and traffic in and out of the house is minimal, a whole different set of circumstances may exist. Now any bath fan or kitchen exhaust or drier is working against the flue.

Here’s how remodeling can kill.

Death Trap Utility Room

This basement utility room is long and narrow and measures 6ft x 20ft with a 7ft lay-in ceiling. Inside the room is a 75,000 btuh gas furnace, a 40,000 btuh gas water heater and a 35,000 btuh gas drier. The vent for the drier is “hard-piped” with a 4″ galvanized for 26 feet to an outside wall.

There are two doors to the utility room. The door on the left is well sealed and accesses the garage. The door on the right accesses the family room in the basement. Normally the door to the family room is left open and the utility room is used as a hallway to get to the garage.

Here’s what happens. The homeowner does a load of laundry. When the washer is finished, he takes the clothes from the washer, places them in the drier and starts the drier. Then he places another load in the washer and starts it.

After 30 to 45 minutes the CO detector in the family room alarms. The homeowner silences the alarm and then goes around the area looking for some kind of smoke or fire, checks in the garage, looks outside, checks the water heater and furnace but finds nothing. No odd smells, no smoke – nothing.

The homeowner had already replaced the CO detector during the summer because it was tripping with nuisance alarms. Now, during cold winter nights the new detector starts doing the same thing.

No one is sick; no one feels ill. The homeowner and his friends begin to think that he purchased from a bad run of carbon monoxide detectors and simply leaves the detector unplugged. 

Here is where it gets interesting.

I came over with my CO meter after he told me two different CO detectors would needlessly go into alarm with no apparent cause. He knew it had something to do with doing laundry, but couldn’t figure out if the problem was the drier or the water heater or the furnace. 

The water heater showed 4ppm CO when operating. The furnace varied between 4 and 6ppm. The flue always had a positive draft and was installed correctly. The drier vent was clear and unobstructed and the entire laundry room was clean.

I checked the flue vent with the drier running and it still showed a positive draft. I checked the flue with both the water heater and furnace running and draft was still OK.

Then I called the homeowner into the utility room and asked him to close the door. We talked for about 15 minutes and then I checked the flue draft again. This time the flue was down drafting at a high rate, well over .02″WC. It would float a flat Kleenex off the top of the water heater. When the water heater burners were on, the top of the water heater got warm.

After another 15 minutes the CO meter showed 30ppm in the room. The next time the water heater kicked on, its flue gasses were over 100ppm and climbing and part of that was spilling into the room. When I kicked the furnace on and checked its flue, the CO meter showed over 150ppm. The room was getting noticeably “stuffy” so we vacated the area.

With the probe of the CO meter slid under the door to the utility room, it read 150ppm. At this point I had the homeowner turn off the furnace and we opened doors to clear the space.

This is an example of the “domino” effect when it comes to equipment operation.

Both the water heater and the furnace were producing more carbon monoxide than they should have. It is obvious that the flue should not back draft under any circumstances. However, it is NOT obvious that there is a major problem with the installation. Each piece of equipment operates within specifications and is entirely safe to use.

However, as soon as the second door the utility room is closed, the conditions are different and both the water heater and the furnace begin generating carbon monoxide. Anyone in the HVAC industry would know that there is no way that the furnace and water heater should have been put into such a small space without the required transfer grills to provide adequate combustion air.

Unfortunately the remodeling contractor didn’t know about transfer grills and the original homeowner just decided to always leave the laundry room door open. The current homeowner never had an issue until he put a new TV in the family room and they began using it.

Because of the noise made when the drier had jeans and work clothes in it, the homeowner or someone in the family began closing the utility room door. This dropped the noise to an acceptable level and no one gave it another thought until the CO detector kept alarming.

Here’s the part where you need to pay attention. The problem only occurred during temperature extremes. It was a problem either during a very hot summer day or late in the evening when it was extremely cold outside. During both periods (hot or cold) all the doors and windows remained tightly closed and no one was going in and out of the house. So, there were no fresh air exchanges.

Here’s the most dangerous part – this is a 100% lethal arrangement. 

The only thing between certain death and survival is basically dumb luck!

Look at the return-air drop next to the furnace. (The return air drop is the rectangular silver box on the left side of the furnace.) At the bottom of that return duct is a supply register with a damper. The damper is mostly closed, but not 100 sealed.

Picture what happens during the summer when the air conditioning is running. The blower causes the return-air to “suck” air back to the furnace,  then pushes it through the A/C coil and out into the house. During very hot days, the air-conditioner runs for long periods of time. While it is running the register at the bottom of the duct is pulling in cold air off the floor.

The problem is, when the door to the utility room is closed, the register in the return duct causes the flue on the water heater to back draft. When the homeowner is doing laundry, even during the summer, there are flue gasses being pulled into the utility room. These gasses are being consumed by the water heater burner and creating a lot more carbon monoxide than normal.

After the drier is finished and buzzes, the homeowner removes the clothes from the drier and moves the stuff from the washer into the drier and repeats the cycle. After a period of time, usually less than an hour, the CO detector would alarm. 


Nothing serious will occur until it gets bitterly cold outside. During any other period, the water heater only runs for 10 or 15 minutes then shuts off. So the total amount of CO it produced gets diluted by fresh air being drawn through the utility room by the drier. The drier typically runs an hour per load, so the air in the room gets “changed out” by air from the flue and what ever air is pulled from the duct work and under the door.

When the weather gets cold, conditions change drastically. Incoming water temperature is a lot colder, the basement is cooler and the water heater runs for a longer period. On top of that, the furnace runs 90% of the time which adds considerable CO to the problem once the utility room has begun to load up with CO from the water heater. So both appliances are running and generating CO and they are dropping the CO into the utility room.

The furnace pulls the CO laden air through the register in the return duct and pushes it into the house. Since it will run 90% of the time when the outdoor temperature is 0 degrees or below, the house begins to fill up with CO. If the homeowner doesn’t have a CO detector, the family may not wake up in the morning.

Depending on how duct work is installed, the return-air duct work on a furnace may have as much infiltration (leakage) that it matches or exceeds the air being pulled through the register in the return duct in the photo above. Typically when CO is produced and gets into the house, it is being pulled in by the furnace blower through leaks in the return-air duct work and circulated in the living space.

The moral of the story: perfectly good equipment produced lethal levels of carbon monoxide. All it took was a minor change in circumstances and a drop in temperature.

I understand that most HVAC technicians would recognize that the utility room in this example was entirely too small to support the operation of the furnace itself, not to mention the water heater and gas drier. However, let me tell you that three different highly rated HVAC contractors worked on this exact system and only one mentioned combustion air requirements. And that technician was “satisfied” when the homeowner said that they never close the utility room door.

Here’s your take-away — the ENTIRE basement does not have enough free air area to safely support the furnace and water heater. This is a 900 square foot house.

The basement is a walk out with one half of it dedicated to a two car garage. The other half is a finished family room, study, half-bath and utility room. When the house was constructed there was insufficient free area space for proper operation. Outside air ducts should have been installed when the house was built.

So, three “qualified” HVAC service contractors and one local “authority having juristiction” passed right over the fact that there was less than 50% of the required free-air space available for the gas-fired equipment. Sadly, this particular housing development probably has hundreds of homes in the same situation. They are all the same size with similar floor plans and most have been remodelled.

How are the other homeowners surviving? Many have remodelled and installed finished stairs to the lower level, eliminating the door between the first floor and the basement. Others have never updated and sealed the door between the garage and the basement. Infiltration air has been their friend.

Read through Myth #1 Heat Exchanger Cracks to find out why a cracked heat exchanger will not introduce CO into the indoor air stream.

Here’s a potential hazard being taught in a sheet metal duct design course.  This illustration proports to show an acceptable gas furnace installation.  What it really shows it how to put the occupants in a dangerous situation.

Flue issues are not confined to forced-air gas furnaces.  They are just as dangerous for gas-fired boilers.  Read this 2007 article “Watching a Killer at Work, Part 2” in Contracting Business about a boiler on a snow-melt system that could have killed the homeowner, and almost got the contractor.  (Part 1 of the article is here.)