It is possible that snow is clogging your Flue vent pipe on the roof and is causing fumes to back into your house.
You will have to go on the roof and check to see if the flue vent is stopped up.
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no. the flue is clear and they tested the air flow going up the chimney and said it looked ok
I smell it every time the heat goes on. and it did just again now
could the heat exchanger have a problem?
the furnace is fairly new....
If the flue is clear, You more than likely have a cracked heat exchanger.
how can I check this?
But I would advise you to first thouroughly inspect the flue vent pipe to make sure there are no leaks.
they checked it
You will have to call a professional out to check to see if the heat exchanger is cracked. There are a couple of tell tell signs that you can look for. First watch the flame inside the furnace and see if it flutters when the fan motor comes on.
If it does, the heat exchanger is cracked. But if it was cracked, they would have detected CO2 (Carbon Monoxide) in the air.
I also have a co monitor in the house which reads zero
so, if CO2 was zero, but I still smell exhaust, what could this be?
You may have another issue at your house. Have you had your house checked for mold?
I would advise you to have that checked because mold can cause throat and eye burning sensations.
Or is it possible that some kind of chemical has wasted in your vents and duct system?
ok, but the heat has been running for over a month before I began to smell this, and it started suddenly after the snow storm
what does that mean "has wasted in the vents"
how do I check for mold?
Are your registers or vents in the ceiling or in the floors?
house is a ranch
Possibly some kind of cleaning agent may have been poured in mistakenly into the registers.
Also, do you have cats?
I'm not aware of any thing being pour in any register. I do not have any pets
There are no problems with your furnace that I can tell. I would advise you to have some one come out and check other appliances such as water heater or Dryer.
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would something being wrong with those other appliances cause you to smell this only after the heat went on, or the presence of mold?
The heating system spreads anything that is in the air throughout the house. If mold is present in one central location, the heat system will transport it to other areas of the house. This is why you smell it when you turn the heat system on. There is something in the duct supply that is causing you discomfort.
When the heat is turned on, it is blown into the house.
But it does not sound like it is the furnace. If it was the furnace, there would definitely be a CO2 reading.