Many of my clients ask me about the big square box in the center section of the house’s upper floor ceiling behind a set of self-closing louvers. It’s a whole house fan and it can either benefit home owners or make living conditions worse.
Where and when it makes sense to use the whole house fan
* Preferably mild climates with cooler and less humid nights
* Use it only if the temperature and humidity outside is more favorable than the conditions indoors. If your air conditioning system has been operating during the day, and the temperature outside drops lower (but the humidity is still high), don’t turn the whole house fan on. The vent will suck all the humid air into your air conditioned house
* The best time periods to use it would be, of course, early morning and evening
Who shouldn’t use the whole house fan?
If you suffer from allergies, it might not be a good idea to bring all the pollution into the house. The window screens will stop some of it, but whole house fans are very powerful and will make you sneeze more than ever.
Physics of the whole house fan operation!
1. Before you turn the fan on, open as many windows and doors as possible, close fireplace damper and / or glass door, and shut the furnace / water heater room doors (if they are louver type, it won’t matter). The suction of the whole house fan (at least at the highest speed) is so great, that without open windows / doors, it will draw air from the outside through any possible openings like, for example, from the chimney. If it is a fireplace chimney and you don’t remove the ashes… well, it’s going to be a mess. If it is a water heater / furnace chimney, it might blow off the pilots.
2. When everything is ready, you can turn on the whole house fan and usually within the next few seconds you should feel plenty of exterior air flowing around the house. Unless…
3. This part is equally important as opening the doors and windows – both have to be balanced to function properly in order to really benefit from the whole house fan operation. The air drawn by the whole house fan has to be discharged through the attic vents to the exterior. Otherwise, pressure created in the attic area will cause the air that was just pulled out of your house to return with dust and insulation particles through any possible gaps in the attic floor and the whole house fan itself.
Regular attic ventilation might not be enough to support the whole house fan operation; you might need much more to carry all that extra volume of air to the exterior. And the formula to calculate the square footage of the attic vents required to discharge that air is very simple. All you need is the dimensions of all your rooms:
Example: your ceilings are 8′ high and you have four 10′ x 14′ rooms, one 15′ x 17′ room, and a 10′ x 3′ hallway (this is just a sample, calculate all the rooms in your house you think should be covered)
10’x14’x8′ = 1120×4 = 4480
15’x17’x8′ = 2040
10’x3′ = 30
Total – 6550 cubic feet
Now you have to divide 6550 by 750, which equals 8.73 sq feet – this is the area of the required attic vents (all of them combined – soffit, ridge, gable, etc.)
4. If the house you’re moving into has the whole house fan already installed, lets hope that it was properly sized up for its purpose (there might be a tag on the unit itself), which is to replace your house air within just a few minutes.
5. If you’re the one installing the whole house fan, consider 3 or 4 minutes as an optimal time for the house air exchange. To achieve that, you’d need to divide your house total air volume (6550 cubic feet from our sample) by 3 or 4 minutes. That will give us 1810 CFM / 1357.5 CFM (Cubic Feet per Minute).
6. One more important thing: louvers of the typical whole house fan are not air tight and have no insulation applied on their surface; they leak air between the house and attic. I would highly recommend installing a whole house fan cover in attic area to prevent air drafts during the cold season – don’t forget to remove it before you decide to use the fan again.
Whole house fans are noisy and many people never use them, but they might benefit your wallet if you apply all of the above. There are currently other types of house ventilation systems available: they are more expensive (2 – 3 times) but very quiet, more efficient, and provide air circulation for extended periods of time.
Have a cool summer next summer.
By LoggaWiggler from Pixabay