Slightly elevated CO reading that went down -- explanation?

Yesterday evening I inspected a house that had a horizontal gas, forced air furnace in the attic that appeared to be manufactured in 1987 or 1988 (Singer, serial # 788749P01), which had a carbon monoxide reading that was mostly around 4ppm, topped out briefly at 8ppm, and went down to 3ppm as it ran more. Ambient CO outside the house at eye level varied from 2-4ppm, ending at 2ppm when the furnace was at 3ppm. The house was only about 100 yards east of an elevated freeway, and the wind was westerly, and variable.

Note: I’ve had ambient readings as high as 13ppm east of freeways during rush hour.

The fact that the CO reading went down suggests the furnace is not dangerous. I’m wondering what the explanation is. The furnace had a missing filter, and undoubtedly the supply air side of the heat exchanger got a lot of dust, mites, etc., and the furnace p-robably had not been started in months. Could the slightly elevated CO be caused by this burning dust and debris in the heat exchanger?

Was it a natural draft type? They sometimes take a bit of heat to really start drafting well.

That’s a tough one.

My approach would be something like this:

Most likely a systematic method would have to be used in order to isolate readings …maybe;-)

Taking measurements like:

Outside around the house at multiple locations.
Inside the house at various locations with FAU off
Kitchen location FAU Off By stove or similar appliance.
Next to Water heater (natural gas) on
Next to Water Heater when Off
Chimney?? What are the breaches like if any?
Air infiltration into house?? With all appliances off
Next to FAU when off
Source of combustion air (at vents in attic)?? FAU off
FAU on , Next to FAU
Inside the house at Kitchen with stove on FAU on


Are you going to do all of that? It’s a challenge and may not be a good idea since it’s beyond the scope of the inspection.

I had my CO detector on in the car in traffic… Some city intersections at stop lights read up to 5 PPM while others 1-2 PPM

So many variables.

I have a feeling that the expressway / highway could be a major source of that CO.:smiley:

My thought exactly, Patrick.

The high readings were in the kitchen relatively close to the (new closed chamber) water heater, but its exhaust vent appeared OK. The house was vacant, with all appliances off.

In any case, it stands to reason that any leak in a heat exchanger would get worse and CO would accumulate as the furnace runs longer and the metal expands with heat, so it’s highly unlikely there’s a problem with the furnace itself.

This is the danger of doing testing / procedures outside the scope and AOR of a home inspector. You are now squarely in the middle of a process and may not have the training or certifications needed. (or any credibility / license to even be performing these functions) I do not know if you have made references to this dilemma in your report or are just curious. I am not attempting to slam anyone but this is a very dangerous and slippery slope to find oneself on. Do your clients now expect a professional answer to this situation? We are talking about safety of life. Any answer you give to them makes you at minimum partially culpable for the consequences of their decisions regarding the gas furnace. If a gas furnace is 17-18 years old with little routine or annual maintenance I would recommend thorough cleaning / servicing by a licensed HVAC contractor regardless of the performance. The HVAC equipment is probably the most ignored or neglected piece (and expensive) of equipment in the average home. No one thinks of it until it quits working.

Dangers of Carbon Monoxide

                     According to the U.S. Consumer Product Safety Commission                           (CPSC), an **acceptable level of CO is a 15 parts per                           millions (PPM) average over a time span of eight hours                           or a 22 PPM average for an hour.** If you have 1,000                           PPM for over thirty minutes, it puts you at a high                           level of danger in the form of a collapse into a coma                           or permanent brain damage. Carbon monoxide is a colorless,                           odorless gas that results from incomplete combustion                           of fossil fuels such gas, propane, oil, wood, coal,                           and gasoline.

I’m not sure I understand the concern with the CO levels reported in the oringinal post. The reported CO levels would not indicate any elevated risk within the home.

Thanks, guys.

I put a strong recommendation for carbon monoxide detectors in *all *sleeping rooms and commonly used rooms when there is an older (over 10 years or so) furnace, and a recommendation to have carbon monoxide detectors on each level with any gas furnace. When a situation is uncertain, I do not make a “yay” or “nay” call. I tell people straightaway that I’m a general practitioner, and not a specialist.

So I understand the “slippery slope,” but still prefer to do the test. I recently inspected a house in which the CO level got up to 26ppm, and the clients and their agent were very pleased that the inspection might have saved lives.

People are very satisfied with the care and thoroughness of my inspections. I think in that sense the best defense is a good offense. :smiley:

Write it up for a HVAC Technician to inspect.

If you have that high off a reading in the plenum the levels in the flue pipe will be even more elevated and way above the ANSI standards for furnaces of 400 ppm free air requirements.

Plus there are numerous other sources of CO info available.