(925) 502-1327 Get a quote
  1. Home
  2. Cooling at night

A summer day with a whole-house fan and an air conditioner.

In the Diablo Valley, a hot afternoon is often followed by a cool night. Used together, a whole-house fan and an air conditioner split the day between them. Here is how a typical hot day goes, hour by hour.

24-hour cooling plan for a hot summer dayA ring with midnight at the top. From 10pm to 6am the whole-house fan runs with the windows open; from 6am to 11am the house is closed up; from 11am to 8pm shades are down and the air conditioner runs only if needed; from 8pm to 10pm the house is opened up once outside air is cooler. 12am6am12pm6pm 24 hoursa hot summer day
Around the clock

Four parts to the day.

  1. 10pm – 6amFlush the heat outWindows open, whole-house fan on. Cool night air pulls through the house and pushes hot air out through the attic.
  2. 6am – 11amClose up and coastFan off, windows shut, blinds down on the east side. The house holds the night's coolness into the day.
  3. 11am – 8pmShade, then AC if neededWest blinds down. The air conditioner runs only when the house drifts past comfortable — and for fewer hours.
  4. 8pm – 10pmOpen up when it's cooler outsideOnce the outside air is cooler than inside, AC off, windows open, fan on.

Illustrative — the right hours change with the day's weather. The rule behind it is simple: run the fan only when the air outside is cooler and clean.

Before you run one

Three rules that matter.

Never on smoke days

A whole-house fan pulls outdoor air straight in. When wildfire smoke is in the valley, keep it off and the windows shut.

Always with windows open

The fan needs somewhere to pull air from. Running it closed up can draw air down gas-appliance flues.

Enough attic venting

The air it pushes into the attic has to get out. We check the vents before we install.

A whole-house fan unit installed between attic joists, flexible duct to the ceiling grille and insulation around it
What we install

Quiet, insulated, sized to the house.

Sized to the house
Matched to the floor area and to the attic venting available.
Insulated dampers
Doors that close tight in winter, so heat doesn't escape into the attic.
Controls
Wall switch or timer; many can be set to shut off by themselves.
With your AC
Not a replacement — a way to run it less.
A bedroom window open onto a quiet Walnut Creek street in the blue light of a summer evening
Evening: open up once it's cooler outside.
A heat pump beside a stucco house at dusk, the last light on the wall above it
Afternoon: the AC runs for fewer hours.
Homes

Best fit

Single-storey and two-storey houses with an accessible attic and enough venting.

Businesses

The same idea at work

Night ventilation and schedules for offices and shops, so cooling isn't running for an empty building.

Cooler evenings, shorter AC hours

Ask about a whole-house fan.

We'll check the attic and tell you honestly if it suits the house.

Get a quote (925) 502-1327
Call Quote