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Whole House Fan Tips

Whole House Fan Tips

Whole house fans are really fantastic devices. They can, in many instances, provide extremely effective cooling with very little energy. Only areas with oppressive humidity levels find these fans not so useful. The effectiveness of a whole house fan is directly related to the relative humidity.

I explain in this linked air conditioning article, the effects of humidity and cooling. Simply put, our bodies cool themselves by evaporation. The process of evaporation actually extracts heat from our body. That is why we perspire when we are hot. The body wants that perspiration to evaporate so that it will become cooler. If it is humid outside, the perspiration can't readily evaporate into the air. The air has just about as much water as it wants! That's why it's humid!!!!!!

Air movement increases the rate of evaporation. That is why clothes dry faster on a windy day. Whole house fans are designed for just that purpose. They create a breeze which blows across our skin, which evaporates perspiration, which makes us feel cooler. It's really amazing when you think about it.


As with anything, make sure that you have the right size. Can you imagine a large dump truck being powered by a lawn mower engine??? Don't make a similar mistake with a whole house fan. These fans all have different capacities. They can push only so much air. Assuming that you have 8 foot ceilings, determine the total square footage of your house. Multiply this number by 3. The result is the minimum cubic feet per minute capacity that the fan must produce.


What ever goes in must come out. If the fan wants to push 8,000 CFM, it better have a place to easily push it to. Your attic or roof vents must be adequate, otherwise the fan will be restricted. The blade will spin, but it won't be pushing as much air as it should be. You must have 1 square foot of free open exhaust area for every 750 CFM capacity of the fan.

Supply Air

The fan must be able to easily replace the air it is pushing. That means that you must open enough windows and/or doors to give it an ample supply of air.

Balancing Air Flow

Your personal level of comfort is a function of the rate of evaporation. You can maximize this by increasing the rate of air flow through the rooms you occupy during the day and night. For example, at night, you should only have windows open in the bedrooms and hallways feeding these rooms. This will maximize air flow through those rooms. The fan will not suck air from the other unoccupied rooms. During waking hours, reverse this scenario. Close the bedroom windows and open the living, kitchen, family room, etc. windows instead. After all, these are the rooms you have now decided to occupy.

The other rooms will become stuffy. However, these fans are so powerful, that if you had only one window open in the house, it might replace the air in that room with cooler outside air in as little as 15 seconds!


If you decide to install one of these fans - and I might add that it is a good idea - you must follow the instructions to the letter! One of the most critical steps is making sure that the hole you cut in the ceiling is square! If the hole is not square, the louver will quite possibly bind. That is a major problem!!! Checking the hole for square is simple. Let's say, for example, that the fan needs a hole 31 inches by 31 inches. The hole will be considered square if the two diagonals are the same measurement. A diagonal is the line which goes from one corner to the opposite corner. There are two diagonals in every square or rectangle. In this example, the diagonals would measure 3 feet 7 and 27/32 inches.

Also, if you have to cut ceiling joists, etc., DON'T do it unless you know for a fact that you are not harming the structural integrity of your house. If in doubt, contact an experienced carpenter or a structural engineer. Absolutely do not cut a prefabricated truss!!! If you must, contact a registered structural engineer and obtain his or her opinion. Trusses are not designed to be cut!!!!!


Regularly clean the fan blades and motor. These will get dusty in a hurry. Excessive dust buildup on the motor can lead to overheating which will shorten its life. Be sure that the power supply (circuit breaker) is OFF. These fans are extremely dangerous!! They can cut off fingers or hands. They spin rapidly and, because of their size, they have tremendous tip speeds. The outer tips or edges of the blades are spinning extremely fast. Don't take a chance. Someone may accidentally turn the fan on, a child may do it, anyone. ABSOLUTELY KILL THE POWER!!!!!!


If you live in a cold climate, you will not use the fan in the winter. The louvers don't always fit tightly. They can leak tremendous quantities of air. The best way to cope with this is to put a lid on the fan. I usually did this fairly easily. I always made a box that fit around the fan. I would usually make this box about 16 to 18 inches tall. This box would keep the attic insulation away from the fan blades. I would then make a simple lid for the box. This lid should have insulation stapled to the top of it.


Timers are a must. Many people like using these fans at night. However, if the fan runs all night, it can, in some cases, actually get cold in the house. A timer takes care of this problem. You can set the timer to operate the fan for several hours. The fan turns off while you are asleep (ZZZZZZZ).

Variable Speed Fans

Some fans are available with variable speed motors. This can be a benefit. If you need rapid cooling, turn the fan on high. Once you are comfortable, adjust the fan speed so as to maintain your level of comfort. Give these fans serious consideration.


Many fans have rubberized drive belts. The adjustment of these belts is critical. If the belt becomes loose, the fan may not spin fast enough. The instructions which come with the fan usually tell you how to check this adjustment. Keep this booklet. Put it in a zip-lock freezer bag. Attach this bag to the outside of the wood tunnel in the attic. It surely won't get lost if you do this.

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