Re: selecting a replacement furnace for 2-story condo
Your furnace is not "stupid", but your previous installer may have been.
You need to relocate the furnace. Get estimates from new contractors who come with valid references that you can actually verify. At 15 yrs old the furnace has seen better days, consider replacing it. I've had furnaces last as few as 10 years, and some that are still going at 35 years old - so age is not the only factor.
Also, since you are in a condo, you may be limited to what you can do (check with the HOA - some of them are vicious and quick to sue).