Monday, July 25, 2011:  Updated Monday January 6, 2014

Attic Fans:  Will They Save Me Energy?  

A couple of years ago, I installed a thermostat controlled attic fan. 

I had read that venting out the heat in the attic will reduce cooling costs, (makes sense). But it also takes energy to spin a large fan for 10 hours every day.

If the house isn't properly air sealed, cool conditioned air from the house will be sucked into the attic space and vented outside. In this case, the central air conditioner will use more energy with an attic fan running.

If the attic is far under ventilated, the attic fan will not be able to draw out enough hot air to make a difference but still draw a lot of power.

If the attic has superior ventilation (most homes, including mine don't), an attic fan will not do any good either because the attic vents are already doing the an attic fan's job only passively.

Ever since I air-sealed our home, I have been wanting to quantify the attic fan's performance and usefulness. I got the chance this weekend when my entire family was away and left me home alone. No stoves cooking, no kids leaving outside doors open, no TV's being left on. Just a relatively constant, measurable whole house power consumption. To help in comparing apples to apples, the weather forecast all weekend was sunny and in the low 90's .

I ran one day with the attic fan on and one day with it off.

Notice how the air conditioner has shorter duty cycles on the day the attic fan is running. 

All things being equal and even with R-38 insulation in the attic, the attic fan lowered the central air conditioner's power consumption by 20%. 

Even though my attic fan draws 190 watts when it is running (190 x 10hrs/day = 1.9 kWh), it actually reduced the central A/C usage by 6.6 kWh for a net gain of 4.7 kWh. 

After operating for 2 summer seasons and at an electrical savings of $70/ season, my attic fan just paid for itself last year. 

Update: 6/2013
A fantastic energy saving opportunity came through my neighborhood last year. An insulation company was going around blowing in an additional R-19 of insulation into people's attics for free.  All they were expecting in return was that you forward the Questar Gas rebate check to them.  
They were not very honest with the amount of insulation that was actually being installed.  Instead of 7 inches across the entire attic they were only putting in 2-3 inches in some places and none in others.  A few of my neighbors caught on to their scam and made them come back and do it right.  
Now wanting to be shorted, I set out to observe the entire attic insulation instillation process.  Even though it was June and about a million degrees in the attic, I donned a dust mask and went up there with a headlamp and a tape measure.  I made sure that I got the correct allotment of insulation.  

After a years worth of data collecting, The energy savings from adding more insulation are real and well well worth it.  
But now it begs the question about the attic fan:  Is it still saving energy by removing hot air in the attic or is the additional insulation invalidating any potential savings?