Whole house fan or more AC: using Pasadena's cool evenings to your advantage

One of the best things about summer in Pasadena is how quickly the evening air can cool after a hot day, especially in foothill neighborhoods where air drains down from the San Gabriel Mountains. The house, though, does not cool as fast. Walls, ceilings and furniture hold the day's heat, so many homeowners run the AC well past sunset even when it is cooler outside than inside.

A whole house fan is one way to use that cool evening air. Another is to add AC capacity or improve the existing system. They solve different problems. Here is how each works, when each makes sense, and how many homeowners end up using both together. We also cover the attic and gas appliance checks that should happen before any fan goes in, and what a typical installation costs in the San Gabriel Valley, so you can weigh the fan against spending the same money on the AC.

A whole house fan is a large fan mounted in the ceiling, usually in a central hallway, that pulls air from the living space up into the attic. That air then leaves through the attic vents. With windows open, the fan draws cooler outdoor air through the house, replacing warm indoor air in a matter of minutes and flushing hot air out of the attic at the same time.

Modern units are much quieter than the old belt-driven fans many people remember. They use insulated dampers that close when the fan is off, so the ceiling opening does not leak heated or cooled air the rest of the year. Many have multiple speeds and timers. Running cost is low compared with an AC, since the fan only moves air.

A whole house fan cannot cool a house below the outdoor temperature. On nights when it stays in the 80s after a 105 degree day, it will not help much. It should never run on smoke days or high pollen days, because it pulls outdoor air straight through the house. It also needs open windows, which some homeowners are not comfortable with at night, and it does nothing during the heat of the afternoon.

Attic venting matters too. If the attic does not have enough vent area for the air the fan moves, the fan can push hot attic air back into the house or create pressure that pulls air down from places it should not. Homes with atmospherically vented gas appliances such as an older water heater need care, because strong depressurization could interfere with venting if windows are not open enough. A technician checks this before installing one.

Many Pasadena homes do well with both. On a hot day, the AC handles the afternoon. Once the outdoor air drops below the indoor temperature in the evening, turn the AC off, open windows in the rooms you want to cool, and run the fan for an hour or two, or overnight on a low setting. In the morning, close the windows and shades before the day heats up, and let the AC take over later if needed.

Never run the fan and AC at the same time, since the fan would simply pull the cooled air into the attic. A simple routine and a fan with a timer make the switch easy. The result is often fewer hours of AC each day during spring, early summer and fall. A smart thermostat cannot control most fans directly, so a simple wall timer on the fan is the easiest way to make sure it shuts off before morning.

• Turn off the AC before starting the fan
• Open windows in the rooms you want to cool before switching on
• Close windows and shades in the morning to hold in the cool air
• Skip the fan entirely on smoke, ash or heavy pollen days