Whole House Fan Size Calculator

A whole house fan is meant to pull stale heat out of a building. It pulls cool air into the house through open windows and blows hot air out through the attic. Unfortunately, if you gets the fan size wrong it won’t work. The key is to match the volume of air in your house with the ability of the fan.

To do this we rely on some simple geometry. First, calculate how much air needs to be moved. This requires finding the volume of the area you are trying to cool. Multiply the length times the width times the average height. Most folks overlook the height. But if you have tall ceilings, all that additional volume mean more air for the fan to push around. Our tool takes care of the math for you. The calculator handles the math once you plug in your dimensions.

How to Size Your Whole House Fan

Now that you’ve got the volume, what’s the rate of airflow? You can select how fast you want the air replaced by deciding how many times an hour you want to change the air in your entire house. We call this air changes per hour, or ACH. The faster the flush, the more it’s a tradeoff between speed and comfort, but also the louder the fan operates. To keep it quieter, set your target to, say, a dozen ACH; this will be comfortable on milder nights, or if you’re sleeping while it’s running. For a faster flush (which gets the heat out quicker), set the target as high as two-dozen ACH; you’ll need extra attic venting and a bigger fan. See the CFM requirement at each target on the calculator, so you can size both fan and vents, and match performance different than noise levels.

The second reason most projects don’t work is that your attic won’t support the airflow. Even with a strong fan, you’ll have nothing without an opening for the air to exit through. To do this, your attic require enough Net Free Area (NFA) to exhaust hot air. Don’t confuse this with the overall size of your vents. Louvered and screened vents cuts down on available space. For a basic rule, your existing vents must allow at least one square foot of free area for every seven hundred to one thousand CFM of fan power. Too-small vents make it impossible for the fan to work correctly. Inside the home, it generates a vacuum which shuts your doors or prevents the fan from operating properly. Make sure there is enough open area above to handle the amount of air being exhausted.

It gets more complicated when it comes to installation. There is always friction loss in your ductwork and even tighter grills. The tool accounts for that. By default, it assumes 80% friction loss (for normal installations). For narrower airways or higher attics, you can go up from there. That way, the sizing isn’t too low. It will still function according to specifications, both on paper and in real life.

Remember: we’re talking about moving an actualy amount of air, not just numbers. Ideally you’re trying to chill the place out before dawn. So it’s not “a little bit cooler” but actually cool in the mornings. Getting this right means sizing the fan properly: knowing how much air is in your home (how much cubic footage there is?), how big the rooms are, etc., and making sure that the vents aren’t restrictive. If everything jives then the outcome is simple: the air flows, the temp goes down, and the house start cooling off.

You should of checked all dimensions first to make sure you recieve the right airflow for your luxurius home.