Attic Fans and CAZ Pressure - Testing Fun!

Consider turning attic fans on when checking for the worst case CAZ depressurization. The fan may be changing the pressure dynamics of the home in dangerous ways.

While doing a comprehensive quality control visit, our Pure Energy final inspector noted that both the natural draft water heater and the natural draft draft boiler failed the following 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, the negative pressure in the combustion appliance zone (CAZ) with reference to the outdoors was greater than allowed by BPI Standards; that is, the CAZ exceeded the maximum combustion appliance depressurization limit allowed. This held true both 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 an attic fan installed in the roof. The pressure caused by the operating fan was sucking air out of the attic, the house, and the CAZ. This was placing the CAZ under too much negative pressure. Since every CFM of air that the fan exhausted had to come from somewhere, this negative pressure was causing some of the makeup air to come down the flues. The water heater and the boiler could not vent the flue gases properly, and the fumes were being vented to the inside of the house. This is dangerous and unhealthy.

The reason the fan could suck air from the CAZ was that the attic was not fully separated from the living space and from the basement, as is required by the utility efficiency program for which we are the QC Inspectors and also by BPI.

Not only did this defective attic pressure boundary allow heated air to leak into the attic, but it allowed moist air to leak into the attic, hot summer air to leak into the house, and polluted air to leak into the house. It also put the CAZ under negative pressure and it posed a serious risk of CO and other pollutant poisoning.


Here is what I learned from the inspector’s experience:

▪ Attic fans can suck air down flues and cause IAQ and health problems for occupants.

▪ If the air conditioner is on and the windows are closed, mechanical ventilation makeup air cannot come in from the outside through windows. Since makeup air must come from somewhere, it may come down the flues instead.

▪ Attic fans can run year-round, if the thermostat is poorly set, or if the fan is on a standard switch and occupants forget to shut it off in the winter when the boiler is running.


This is what I recommend to program administrators and technicians:

▪ Consider checking the status of attic fans when doing the worst-case CAZ depressurization setup. An operating attic fan might reduce the baseline CAZ pressure.

▪ Consider turning attic fans on when checking for the worst-case CAZ depressurization. This will help you to determine the true worst case.

▪ Be sure to record the CAZ depressurization with and without the attic fan on.

▪ Explain to customers how attic fans can affect the operation of natural-draft appliances.

▪ Be sure to fully separate the attic from the living space and the CAZ with a tight pressure boundary. 

This article originally appeared in Home Energy Magazine Attic%20Fans.pdf

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Tags: CAZ, Diagnostic, Pressure, Tests


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Comment by Ken Summers on June 12, 2017 at 11:00am
While showing the client the effect from a PAV is good, the bigger issue to us the amount of air that the fan is taking from the home which in turn brings back in hot outside air and in many parts of the country contains humidity. Don't get me wrong, back drafting appliances is always a bad thing how ever in studies of fans a home with just one fan brings in an average of 237 CFM. Using an area with a 91 design outside temp and 36 GD in summer will add a load of 9897 Baust to the home. Our suggestion at CI is to remove the PAV, definetly seal the attic floor and if you need to lower the temp in the attic us a radiant barrier foil or spray. PAVs have been found to be not a great idea.

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