Attic Fans and Whole house fans - pros and cons

The subject of attic fans and whole house fans is one that elicits a lot of opinions from homeowners who have one.  Those who use them swear that they work great and save electricity though I have not come across an energy auditor (aka Building Scientist) yet who thinks they are a good idea.  The problem with attic fans and whole house fans is the effect that they have on the other systems of the house and the air leakage that they create.

Let’s start with attic fans.  The idea behind an attic fan is to help mechanically ventilate the attic during the summer months in order to keep heat from entering the home.  This in turn is said to improve the life of the shingles and may be a selling point from the roofing company for installing an attic fan.  If the roofing company knows what they are doing they will provide proper natural ventilation to avoid reduced shingle life.  The problem with attic fans is that most attics are not properly designed to allow incoming ventilation through the soffits so when the attic fan is on it causes the attic to be depressurized in respect to the interior of the home.  This depressurization will  (unless your home is very airtight) cause conditioned air from inside the home to be drawn in to the attic.  This in turn will draw dirty outside air in to the home through whatever cracks it can find.  These fans also have an electric motor which will drive up your electric bills.  If you make sure your attic is properly ventilated and insulated you will find that your attic fan is not needed.

What about whole house fans.  When I was growing up we never had air conditioning but we had a whole house fan that we would use every summer.  It did a fairly good job of keeping the house cool and it supposedly didn’t take the same energy as the air conditioner.  I don’t have such an issue with whole house fans as I do with attic fans and I can’t speak to the electric cost issue.  My feeling is that if homeowners had the same cooling expectations of air conditioners that they do with whole house fans the difference in electrical usage would negligible.  The savings come when you compare a whole house fan which people expect to keep the home hopefully below 75 degrees and which they turn on and off and an air conditioner which we know will have no problem keeping the house at a pleasant 68 degrees and can just be left on day and night.  The answer to this is a programmable thermostat which is actually properly programmed. 

Now, I’m not here to argue for air conditioners, I’m just saying the savings difference may not everything they are expected to be and to provide some support for my actual complaint with whole house fans.  The first problem with whole house fans is if they are run without opening windows.  I know this doesn’t happen often but it does some time.  In this case the fan will try to draw air in wherever it can including those same dirty building materials we discussed earlier and also from the hot water heater exhaust which, if the heater is running, will be Carbon Monoxide.  Another problem goes back to the attic ventilation issue.  Most attics are not designed to release the volume of air that these whole house fans create.  If this is your case, you will be running the fan with very little benefit and a lot of electricity usage.  The biggest problem with whole house fans is not during the summer but rather during the winter.  The large whole that you create in to the attic is very difficult to properly seal and insulate and therefore becomes a chimney for hot air to escape from.  This escaping air will cost you money, create the feeling of drafts throughout your home, cause hot and cold rooms due to pressure differences and will aid in dirty unhealthy air to be drawn in to the home.

Make your own conclusions but do so with as many facts as possible.