Gas and Oil Home Heating Furnaces - negative air flow thru water heater flue

just ran across what seems like a potential hazard home owners should check for with attic/whole house fans. I noticed a hot smell in basement yesterday. Upon further investigation found water heater exhaust was not going up the flue but was being pushed back into basement by air coming down thru flue. When i would turn forced air heater on (has it's own flue blower) air would reverse back up thru flue. After checking that there were no obstructions in flue (very clean) i was puzzled and nervous. I had my wife turn off attic fan while I was at water heater (with forced air heater already off) and noticed air flow went back up thru flue. Then today went up and took off top vent cover making absolutely sure there was no blockage there (there wasn't) i cracked windows in basement this time with attic fan on and air flow went up thru flue. Then it occured to me that since we got a dog last year we would close rooms off in the house to keep dog out of those rooms. I opened up all rooms in the house and a few more windows and was able to close the basement windows and still had good air flow out flue. I'm pretty sure lack of sufficient air thru the house (not enough windows open) caused this. With the weather so nice right now and people shutting off a/c and turning on attic/whole house fans make sure you have enough windows open. If anyone knows of another cause for this please post.

That will do it every time. Had the same issue with my basement exaust fan in an old house.

If you're going to run it either shut the basement door or open a window or two down there.