Building codes demand that garages be constructed to prevent garage air
from entering the house (when any connecting door is closed of course).
If you somehow know that "dirty garage air" is entering your house, then
you have a problem with the doors and walls, not the furnace.

Jason

This is interesting. My garage is attached. My furnace is in my basement
which sort of borders the garage but not under it of course. I have a CO
detector in the basement which is mounted on the wall that sort of borders
the garage. If I run a car in the garage with the exhaust facing in,
within 2 minutes that CO detector will start sounding off. The basement
does not smell of fumes, but I am trusting the detector on this one.

So this would be a sign of poor construction? My house is only a few
years old. The return inside the house has a vent that exits the house on
the rear so I assume this is to equalize house pressure. So if any small
holes in my garage, then I assume air will suck in from there as well?

Thanks for any info.

dnoyeB- Hide quoted text -

Sorry, no more info. I reached the limit of my experience with
the Building Code comment. I got some bids on a remodel some years
ago and the code requirements for isolating the attached garage from
the house were a PITA.

One of my later houses had the same arrangement that you describe,
i.e. door between an attached garage and the furnace room. I had a
hefty door with good weather stripping and a well-sealed common wall.

Jason