Gas and Oil Home Heating Furnaces - Power Attic Vent Fans

Greetings from a first-time poster...

I have a home with a Vent-a-matic power attic vent fan installed near the ridge line of the roof. Home was built in 1963, and I wouldn't be surprised if the fan was installed back then. Home is 2-story brick and siding w/basement, located in Toledo, OH.

The plastic dome cover of the fan came apart in a windstorm. I think I found a replacement cover, but in doing some surfing to find it, I ran across an article that indicated that power attic fans can do more harm than good, because a) they can suck air-conditioned cold air out of your living space, and b) they can suck combustion gases into your living space.

I'm interested in any opinions you may have on attic fans. Are they a good idea? If you do have one, how much square footage of other roof vents do you need to prevent the problems (a & b) I mentioned above? And if they're not a good idea, what is a better alternative?

Thanks for any advice!

The rule with attic ventilation is 1 : 300 with a vapor barrier and 1 : 150 without a vapor barrier. So if you had insulation in the attic with a vapor barrier and the attic was 900 sq. ft., you would need 3 sq. ft. of free venting. If it was blown in insulation without a vapor barrier, you would need 6 sq. ft. of free venting. This is when you have an equal amount of square footage of high and low vents ( gable and soffit).

Your concern about attic fans is correct. The problem is not the fan but the over sizing of attic fans. Most attics can only handle about 1500 cfm of ventilation because of the vents. Your fan that you have probably moves about 2000 to 5000 cfm. In which case it could cause a problem with drawing air from the home and causing the problems you mentioned.

Properly sizing the fans is important because of this. You could increase the size of the vents but you're looking at more than doubling the amount. You're much better off putting in the proper size fan. The volume of air in the attic remains constant. So if the vents only allow 1500 cfm and the fan moves 2500 cfm, it will seek to get the air from somewhere, like the home. If the fan couldn't get the air from anywhere else, you would just burn out the fan.

Attid fans are a good idea to help remove hot stagnant attic air in summer. The poorer the insulation, the more desirable they are to prevent heat gain. Its best to cover them in winter to prevent heat loss. If an attic exhaust pulls air from inside your home, then you've got some serious duct problems. Don't quite understand how that same exhaust fan could put combustion gases INTO your home since it would not be running in winter, and if it was running and the furnace was on, then how is that exhausted attic air going to put chimney gases into your house...unless the windows are open I guess. Go with what you've got but cover it in winter