Attic Fans Can Cause Headaches for Technicians

Comment

Comment by A. Tamasin Sterner on September 6, 2011 at 5:04pm

Does it say "static"???  It should say "stack".

Comment by Ed Minch on September 6, 2011 at 4:42pm

"Reverse static effect in the summer moves hot air from the attic into the living space."

I'm going to need an explanation of this.

Ed Minch

Comment by Paul Stevens on September 6, 2011 at 9:59am

Thanks,  great food for thaught or even better action.  This is a condition that could be easly overlooked.

Comment by Dale@EnergyWright on September 6, 2011 at 8:39am

Nicely said, Tamasin.  Yet articles promoting attic fan use are everywhere.  They tout energy savings when the opposite is often true.  You can even buy a solar-powered attic fan at Home Depot so you can feel green as you turn blue from CO poisoning.    

 

Reverse static effect in the summer moves hot air from the attic into the living space. This convinces homeowners that the heat from the attic must be removed to keep the second floor cool.  And as you pointed out, power-venting the attic can have dire consequences in the CAZ.

 

The August issue of Fine Homebuilding has a great article about roof/attic venting by Joe Lstiburek.  No fans at all, he recommends a 60/40 ratio of soffit vents to ridge vents to ensure a subtle positive pressure in the attic.  

Comment by David Koski on September 6, 2011 at 8:07am

Excellent thoughts on the attic fan. This is a great heads up for inspectors/auditors. The conclusions are well founded!

What about whole house fans? Is it a best practice to tell a home owner that by turning on their whole house fan they risk CO entering their home?

Thank you Tamasin for your 'always-excellent'' comments on these topics! David

Comment by David Meiland on September 5, 2011 at 8:56pm

A couple of days ago I saw the largest attic fan I've ever encountered--a belt driven unit that you might install in a school gym. No air conditioning, no gas appliances in the house at all, and the owner had cut in some gable vents, but I have no doubt this thing was pulling quite a bit of air from the house, which is probably why they like it.

Comment by A. Tamasin Sterner on September 5, 2011 at 10:35am

Your experience is almost exactly ours.  People are often sold a bunch of phooey about the effectiveness of attic fans.  The only ones we have seen that have not impacted the inside pressures are ones that are installed in attics that have PLENTY of passive vents to supply the air that is exhausted by the attic fan.

Comment by Joshua Lloyd on September 5, 2011 at 10:08am

I have only come across this twice now, however one was just last week.  The power attic ventilator was rated for 1100 CFM.  I noticed it running when I was inspecting the attic, then when I was setting up Worst Case Draft I noticed air movement right at the draft hood and knew what was going on.  So to show the home owner I kicked the water heater on and sure enough the water heater was not drafting at all, turn the fan off and it drafted fine.  The attic ventilator was creating a depressurization of -3pa in the CAZ on it's own. 

 

So needless to say he was not happy that the utility program I do audits for would not provide rebates until the fan was decommissioned.  But I think I was able to explain to him why his attic was cooler now.  It was sucking the conditioned air from his house into the attic, so it was actually increasing his cooling bills not only to cool his home, but the attic.

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