Out of Control Attic Fans

Posted by A. Tamasin Sterner on September 06, 2011
Out of Control Attic Fans

Never underestimate lessons learned while out in the field. Just last week, Pure Energy Coach LLC found in two low-income weatherization jobs that attic fans can, put simply, be out of control! The following are notes from the field, as well as recommendations for program administrators and technicians:

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.


A. Tamasin Sterner is president and chief coach of Pure Energy Coach, LLC and The Pure Energy Center, based in Lancaster, Pennsylvania, and Lavina, Montana.

Add a new blog comment!

Enter your comments in the box below:

(Please note that all blog entries and comments are subject to review prior to posting.)


<< Back to blogs

While we will do our best to monitor all comments and blog posts for accuracy and relevancy, Home Energy is not responsible for content posted by our readers or third parties. Home Energy reserves the right to edit or remove comments or blog posts that do not meet our community guidelines.

SPONSORED CONTENT What is Home Performance? Learn about the largest association dedicated to home performance and weatherization contractors. Learn more! Watch Video