"EXT" wrote:

-snip-
Also check for exhaust fans in kitchen and/or bathroom that may be working
and/or a dryer may also be adding to negative pressure in the house. Any
one of these things coupled with heavy humid air reducing draft or multiple
equipment may create the negative pressure. You need to experiment to
identify the cause. You may need to add some "make-up" air supply to the
area of the furnace/water heater to replace the air being removed.


Good points-- I hadn't thought about negative pressure. If the OP
has central A/c it could very well be causing some negative pressure
there. And it would be worse on hot/humid days.

Jim