Attic Fans Can Cause Headaches for Technicians

Attic fans can suck air down flues causing spillage.  Spillage can cause CO poisoning.  Attic fans can't always be easily accessed or controlled.  Read on:


While doing a comprehensive quality control visit last week, our Pure Energy Final Inspector noted that both the atmospheric draft water heater and atmospheric draft boiler failed these safety tests:

  • Both spilled fumes for longer than one minute (the spillage tests failed)
  • Both had inadequate draft pressure in the flues (the minimum draft pressure wasn't met)

In addition to failing those tests, the negative pressure in the combustion appliance zone (CAZ) with reference to the outdoors was greater than allowed (the CAZ exceeded the maximum CAZ depressurization limit allowed) during the baseline and under the worst case conditions set up.

Upon further investigation, the Final Inspector found that, not only were the typical mechanical ventilation appliances making the CAZ negative, but the attic fan was on as well.

The attic was being ventilated to the outdoors by a typical attic fan installed in the roof.  The pressure caused by the operating fan is sucking air out of the attic... and the house, and the CAZ, causing the CAZ to be under too great a negative pressure.  Since every CFM of air that the fan exhausts has to come from somewhere, this negative pressure causes some of the make-up air to come down the flues rather than from the passive attic vents. The water heater and the boiler could not vent the flue gasses properly, and the fumes actually were being vented to the inside of the house.  This is dangerous and unhealthy.

The reason the fan could suck air from the CAZ is because the attic was not fully separated from the house and from the basement as is required by the program and also from BPI.

So, not only does a leaky pressure boundary allow heated air to leak into the attic; moist air to leak into the attic; hot summer air to leak into the house; polluted air to leak into the house... but it also can impact the CAZ and cause CO and other pollutant poisoning.


  • Attic fans can suck air down flues and cause health and safety problems for people.
  • Closed windows (due to air conditioning operation), can keep mechanical ventilation make-up air from coming from the outside through windows, and make-up air must come from somewhere, and this might be down flues instead.
  • Attic fans sometimes run year round due to poorly set thermostats. Sometimes the fans are on standard switches that people forget to shut off in the winter when the boiler is running.


Recommendations for program administrators and technicians:

  • Consider checking the status of attic fans when doing the worst case CAZ depressurization set-up.  The baseline CAZ pressure might be impacted by an operating attic fan.
  • Considering turning attic fans on when checking for the worst case CAZ depressurization for the true worst case.
  • Be sure to record the CAZ depressurization with and without the attic fan on.
  • Educate customers about the impact of attic fans on atmospheric draft appliances.
  • Be sure to fully separate the attic from the house and the CAZ.


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Comment by A. Tamasin Sterner on September 6, 2011 at 8:04pm
Does it say "static"???  It should say "stack".
Comment by Ed Minch on September 6, 2011 at 7:42pm

"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 12:59pm
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 11:39am

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 11:07am

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 11:56pm
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 1:35pm
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 1:08pm

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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