foul exhaust gas like smell in house
We moved into a new construction home late September 2011 in SW Chicago. In early October 2011, we started to smell a gas like smell in a 2nd floor bedroom located in the vicinity above the 2 furnaces, 2 air conditioners, water heater and 3 external exhaust lines. Our roof vent lines and radon line are also on the same (east) side of house. This smell is also evident at the top of our basement stairs. The gas company came out twice and said no natural gas leak. The smell disappeared entirely in the first week of November (when cold weather stayed). The first week of March (4 months later) the foul gas like odor reappeared after strong winds and warmer temperatures. We installed a 6″ fresh air return line from the bottom of the first floor down next to the furnace area in the basement thinking the house was too tight. This did not fix it. Thinking it may be a plumbing vent line problem, one week later we did a peppermint test on the plumbing lines. We detected faint peppermint smells in a few areas right away and stronger peppermint smells in the basement only the following morning but can't capture where the smell originates from. The smell is definitely the strongest in the 2nd floor bedroom above the combustion units and the basement combustion area and top of the basement stairs. We did check all the traps, toilets, ejector pit, etc. too with no success in finding the cause of the smell. I also turned off my hot water heater only for overnight because the smell coming out my exhaust pipes smells like the odor we get. But after doing this test, the smell still was there. There is no foul smell whenever the air conditioner is on or during the cold months between November 1 and March 1. Rain also dissipates the smell.
The smell almost always occurs when we open a window in the house and the temperature is above 65 degrees with no rain. If we open the 2nd floor bedroom window above the exhaust vents the odor in this room is extremely powerful as well as the smell in the basement by the combustion area and top of the basement stairs only. If we open other windows in other parts of the house we smell a faint smell in the first and second floor but still get a strong smell in basement by combustion area and top of the stairs. Again, this smell only occurs when the weather is above about 65 degrees and sunny.
The builder blames the smell on a water based concrete floor sealant and the moisture he says it created in the basement. We disagree along with other people/professionals who have been over to assess this situation. In fact, what we have found on the internet shows that a floor sealant is a good thing to keep moisture away. And the smell is isolated and not all encompassing throughout the house or basement! Our basement smells completely normal when the windows are closed and we have no moisture (no white chalkiness) under the water based sealant in the floor. I have been running a dehumidifier in the basement for the past month and the relative humidity is steady at 40% which is typical of a normal basement. Also as an fyi, I have carbon monoxide detectors throughout the house.
I am only guessing I have a back draft problem and need to know how to fix it.