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26040 Ynez Road, Temecula, CA United States of America 1-888-QUIETCOOL Mon-Fri 8:00 AM - 5:00 PM PST Contact Us

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Whole House Fan Sizing Formula

We use a general formula of either 2, 2.5, or 3 CFM, or “Cubic Feet per Minute”, per square foot of living space. 2 CFM would provide a good system, 2.5 CFM would provide a better system, and 3 CFM would provide the best system. To calculate how much CFM you need in your home, simply use the calculator found on this page. This math formula will work every time, for any size home, and remember that the more airflow, the better.

Remember, our sizing formulas are backed by the Department of Energy, PG&E, and the California Energy Commission

Sizing Considerations

There are two other considerations to take into account when sizing a whole house fan system:

Location: If located in a coastal region where the climate is cooler, a good system should work great. If located in a desert region where the climate is very hot during the day but cooler during the night, the best system would make the most sense.

Ceiling Height: If the ceilings in the home are taller than 8 feet, be sure to size the system a little bit larger to account for the increased air volume inside of the home.

whole house fan locations

central whole house fan system

CENTRAL (SINGLE-FAN) SYSTEM

A centrally installed system is a single fan system that is installed in a central location in the home, as the name implies. A single system would typically be installed at the midpoint of a single story home, or at the top of the stairs in a two-story home. A single QuietCool system will nicely ventilate an entire home, but lacks the individual room control that a zoned system offers.

ZONED (MULTI-FAN) SYSTEM

A multi-fan zoned system gives the homeowner maximum control over their ventilation and cooling needs. When sizing a zoned system, the combined CFM of all zoned units (not each individual fan) needs to add up to the total CFM requirements. All zoned fans can be turned on simultaneously when the entire home needs to be cooled. A zoned system also allows individual bedroom control, however, and allows for more specific airflow control.

zoned whole house fan system

ZONED (MULTI-FAN) SYSTEM

A multi-fan zoned system gives the homeowner maximum control over their ventilation and cooling needs. When sizing a zoned system, the combined CFM of all zoned units (not each individual fan) needs to add up to the total CFM requirements. All zoned fans can be turned on simultaneously when the entire home needs to be cooled. A zoned system also allows individual bedroom control, however, and allows for more specific airflow control.

attic venting

How to Calculate Attic Venting

Once you determine the correct size of the system you need, based on the sizing principles we just discussed, you need to ensure there is adequate attic venting based on the total CFM of the system.

Attic venting is measured in square feet and is expressed in net free vent area.

As stated by the Department of Energy, PG&E, and the California Energy Commission, each home should have 1 square foot of net free vent area for every 750 CFM in the QuietCool system.

It is very easy to measure the venting in your attic. Simply measure the width and length, in inches, multiply them together; divide by 144, and that is your gross free vent area in square feet. Then, simply deduct 25% off that number to get the net free vent area, which accounts for any air restrictions through the vent. Do this for every vent, add them all up, and you will know exactly how much venting you have or will need to add to equal the venting requirement stated in your local building codes.

Power Venting

So you need to add more venting, right? Sometimes that’s easy! Just add a gable and a few O’Hagins. But other times it can be very difficult. We have a product that can address lack of venting.

Power venting is the process in which an attic fan is installed in the attic. When the QuietCool it powered on, the attic fan is powered on as well to help move the air out of the attic faster!

We actually discovered this by accident while we were testing our fans in our HVI-916 testing booth. When we added an attic fan into our booth while we were testing the air flow of one of our ducted whole house fans, the attic fan boosted the airflow of the whole house fan by over 25%!

We recommend using our attic fans to boost the ventilation when you are 1 to 2 square feet shy of the amount of venting that is needed.

We always recommend adding additional venting when you lack 2 or more square feet of venting.

power venting

Power Venting

So you need to add more venting, right? Sometimes that’s easy! Just add a gable and a few O’Hagins. But other times it can be very difficult. We have a product that can address lack of venting.

Power venting is the process in which an attic fan is installed in the attic. When the QuietCool it powered on, the attic fan is powered on as well to help move the air out of the attic faster!

We actually discovered this by accident while we were testing our fans in our HVI-916 testing booth. When we added an attic fan into our booth while we were testing the air flow of one of our ducted whole house fans, the attic fan boosted the airflow of the whole house fan by over 25%!

We recommend using our attic fans to boost the ventilation when you are 1 to 2 square feet shy of the amount of venting that is needed.

We always recommend adding additional venting when you lack 2 or more square feet of venting.

attic venting

Tips and Tricks on Venting

If you’re still struggling to know if you have enough venting there are simple tests you can run after the whole house fan is installed.

If you turn on your whole house fan and you put your hand over a switch or plug outlet you should be able to feel a very small amount of air coming through. If there is a lot of air coming through, this will tell you that you are probably under vented.

Another way to test for lack of venting is to open your attic access door and then turn on your fan. The open attic access will give your fan more venting to see how much more powerful your whole house fan is when doing so.

Remember that you will always have a little bit of air that will come out of your light sockets, switches, and outlets. This is completely normal. Even if you lack the proper amount of venting, your whole house fan will still operate. It will just lack some performance. We always recommend that customers install the proper amount of venting for the best experience.

Build your Perfect System

Professional Series

Select the three values below to see your suggested system setup.

Next Step:
Figure out how much venting you have

[X] Close
Enter the number of vents in your home for each vent type below and click the calculate button.

Gable

Dormer

Eave

Ridge

Soffit

O'Hagin

Turbine

Your total net free venting area (square feet):

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