Attic fans can suck air down flues causing spillage. Spillage can cause CO poisoning. Attic fans can’t always be easily accessed or controlled.
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.
Conclusions
- 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.
First off people should be encouraged to use is the correct solution and not to spend money on band aids. If the attic floor is air sealed and there are copious amounts of insulation then an attic fan is a waste of money. If is it is not air sealed and lacks adequate insulation than the first dollar(s) should be spent there.
With an attic fan you can reduce the temps a couple of degrees but you have to keep in mind that the main method of heat transfer from the roof to the attic floor is via radiant heat. Dropping the temps a couple of degrees does not change that. I will add get the fiberglass insulation out of the attic.
It doesn’t matter how much intake vent space there is if you are moving enough air to lower the attic temp you will have a negative pressure in the attic. If the negative attic pressure is greater than the house pressure you will have air floor from the house to the attic. Of course if you have a perfect air barrier then you can pull air from the house. If you have a large opening like around a flue or plumbing vent you could be pulling a lot of air from the house. The more air you suck out of the attic the greater the negative air pressure and the greater the potential for pulling air from the house.
I see no benefit from using them in the winter to control moisture. Again the attic floor should be sealed and bath fans vented to the exterior so that the interior does not become a source of moisture in the attic. The sun heating the attic will result in lower RH than the cooler exterior air.
If you have ice dams then fix the air leaks and low insulation levels that are the primary culprits. On top of that make sure you have adequate soffit vents and air chutes between the rafters.
Proper natural ventilation should work for most homes. That means using lots of vent chutes and keeping insulation out of soffits and from air flow. These should be starting points for any home.
Lastly attic vents are only needed so far as to prevent humidity levels and to moderate attic temps. Attic venting is not intended to keep attic temps in line with ambient outdoor air temps. Natural ventilation will work when designed and installed properly.
