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Sizing, Installation, Advantages, & Disadvantages Of A Whole House Fan - Part 2

Part 1

Whole House Fan Attic Ventilation:

What the fan draws into the attic must be exhausted out of the attic. This is one of the biggest mistakes made by installers. If you do not have adequate venting in the attic for the whole house fan you are wasting your money on the installation.

The attic ventilation calculation is: cfm of fan (if the fan is multi-speed, the CFM chosen for the calculation must be for the high speed or the highest CFM). divided by 750.

If the fan chosen has 9,600 CFM as its highest air movement, the ventilation area would be 9,600 ÷ 750 = 10 square feet. This is a minimum number!

And this is where the mistake is made: The 10 square feet from the calculation is based on totally unobstructed ventilation. If your attic vents have screens (which is the case 99% of the time) or louvers, then this number must be doubled to 20 square feet of attic ventilation.

The ventilation can be a combination of roof, soffit, gable or ridge vents

  • If there are any other areas that provide air access from the living quarters to the attic, they must be sealed to prevent the whole house fan from forcing air back into the living quarters. This can occur with bathroom and kitchen fans vent into the attic rather than correctly venting to the outside through a wall, roof or soffit.


The installation of the whole house fan should take advantage of open spaces. In a single story home, a hallway that has provides access to a number of rooms is the best choice. In a multi-story house, the best positioning is above a stairwell or the hallway leading to the stairwell.

The installation of a whole house fan may require changes to the roof structural support as well as electrical additions to power and control the fan. This may put a whole house fan, as a home improvement, out of the capabilities of many DIY home handymen.

Always follow the manufacturer's instructions.

To determine the attic location of your fan, relative to a location in the living quarters:

  • Tape the template provided by the manufacturer on the ceiling in the chosen location.
  • Drill 1/8″ holes in each of the 4 corners of the template and using a coat hangers or pieces of 14 gauge wire poke them through the holes in the ceiling and into the attic. When you go into the attic you will be able to see the area that must be removed to house the ceiling fan.
  • If fan ends up in an area that is not convenient you may wish to move it a somewhat to clear any daunting obstacles that would have to be moved or worked around such as duct work or vent pipes.

    Depending on the size of the fan it may be necessary to remove a section of a ceiling joist in order to accommodate the fan body.

    Detailed instructions for removing a section of ceiling joists.

  • Before you start the installation determine where you will draw your electrical power from (remember that the circuit you choose must be able to handle the additional amperage of the motor for the whole house fan) and where you will locate the fan controls.
  • Additional information on fishing electrical wires.

  • If you live in a colder climate consider that you will want to place a box over the fan to prevent hot air from escaping though the fan opening in winter months. If the fan has louvers, these are not sufficient to provide a barrier to hot air loss. Consider building a box that can be installed in the attic that would sit over the fan, resting on the ceiling joists and providing insulation in the winter months.