Whole-House Fans and Night Cooling in the San Gabriel Valley
On the many evenings when outdoor air drops into the 60s, a whole-house fan cools the house for pennies.
By Piggies Air Conditioning HVAC Team · Updated · 5 min read · Your Home's Air & Envelope
The short answer
A whole-house fan sits in the ceiling, pulls cool outdoor air through open windows and pushes hot attic and house air out through the attic vents. In the San Gabriel Valley, where evenings cool sharply much of the year, it can replace hours of air conditioning in spring, fall and on mild summer nights. It needs enough attic venting, an insulated damper, and judgment about when outdoor air is clean.
How it works
Open a few windows, turn the fan on, and the house exchanges its air with outdoor air several times an hour. The hot air mass in the attic, which radiates heat into the ceilings all evening, is flushed at the same time. Run it in the evening until the house matches the outdoor temperature, then close up in the morning and let the air conditioner hold the cool.
Modern units use insulated, motorized dampers and quiet, ducted fans in the attic rather than the loud belt-driven grilles of the past.
Sizing and venting
- Fans are rated in cubic feet per minute. Installers size them to the home's floor area and ceiling height; a complete air change every few minutes is the usual goal.
- Attic exhaust venting must match the fan or the fan pressurizes the attic and moves little air. Expect the installer to add net free vent area if the attic is short.
- The ceiling opening needs an insulated, sealed damper so the attic does not leak heat into the house all summer when the fan is off.
- Gas appliances in the house must not backdraft when the fan runs; open windows provide the makeup air, and the installer should check natural-draft water heaters.
When to keep it off
A good filter on the air conditioner does not help here because the fan bypasses it. On smoke days the strategy flips: close up and run the AC with a high-MERV filter.
- Wildfire smoke or any poor air quality day. The fan brings outside air in by the roomful.
- Santa Ana wind events, which carry dust and dry the house out.
- Peak pollen days for allergy sufferers.
- When outdoor air is warmer than the house, which the fan cannot fix.
Code note
California's energy code treats whole-house fans as a prescriptive measure for new homes in several inland climate zones, which tells you how well the state thinks they fit this climate. For an existing home they are an optional upgrade, usually permitted as an electrical and mechanical alteration.
What to remember
- 1 Evening air exchange plus attic flush, for a fraction of AC run time.
- 2 Attic venting and an insulated damper make or break it.
- 3 Off on smoke and wind days; use the AC and a good filter instead.
This guide is general information for homeowners in Baldwin Park and surrounding communities. Codes, rates and incentive programs change; confirm current terms with the agency or program before relying on a date or amount. For your specific system, book a visit or call us.
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