The furnace working ok for several days after you opened the basement window 3" is good information. But .. sounds like there is still negative pressure in the basement, even with that window open 3". 2 possibilities come to my mind .. 1. the return duct in the basement, is somehow pulling a lot of air out of the basement. 2. there is a "chimney effect" flowing air between the basement and the attic, possibly through walls And it is clear that something changed, to cause this negative pressure in the basement, since the furnace did fine for 7 years. Possible negative pressure in the space where the furnace is ... is the main reason why furnaces are often installed as "closed combustion" ... using 2 pipes to the outside, one to supply combustion air, the other to conduct exhaust gases to the outside, and the combustion area being sealed. If this problem persists, it's possible your furnace can be converted to "closed combustion" without a lot of expense. If that's not cheap and easy to do, you might consider a new high-efficiency furnace. Your existing furnace is probably 80% efficient max, since it's a non-condensing type. And it has 7 years of use on it ... not a lot, but it's not new. A new condensing furnace will increase efficiency to the high 90's, noticeably reducing your gas bill. Get a good USA made, known-brand furnace, avoid the exotic brands with lots of proprietary parts. Consider Trane, Carrier, etc. A new two-pipe furnace, properly installed, won't be affected by strong negative pressures in the basement. It might even qualify for a tax credit.