I've been having a problem with exhaust coming back into my house through the wall vent. It seems to be at its worst when it's very cold. I live in southern NH and it's been really bad these past couple weeks, especially with the frigid temps. There is a fan, but when it's very cold, it doesn't seem to help. My house was built 4 years ago and the builder installed a wall exhaust vent rather than a flue. One serviceman who looked at it said that is the backdraft effect and that it's normal for the type of venting sytem I have. I'm afraid this is unhealthy and dangerous to breathe. This morning I woke up to the whole house smelling like exhaust. Does anyone know how I can fix this without spending a fortune?