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I recently moved into an old craftsman house in southern California. The heat has really moved in in the last week, and I'm trying to get a handle on how to keep the house cool at night. The last week or so, the highs have been roughly 90F, cooling down to about 70F by 9:00 PM.

The has both AC and a whole house fan (specifically, this one: https://quietcoolsystems.com/docs/CL-2250.pdf).

During the day, I've had the AC set to 78, with all the windows closed and blinds drawn. Once the temperature drops to about 75 or so, I've turned the AC off, turned on the whole house fan, and opened windows at opposite ends of the house. The house is a single-story rectangle, and the whole house fan inlet in the ceiling is roughly in the center of the rectangle.

The thing that's vexing me is that once I turn the AC off and switch over to the whole house fan, the temperature inside the house continues to creep up for a few hours. People seem to rave about these whole house fans online, but so far I've been underwhelmed.

I've also tried just keeping the AC running, and the AC unit seems to just run ceaselessly well into the night. This seems less than ideal, but I'm a complete newb when it comes to home DIY stuff. I guess this is what they refer to as a "heat load"?

The house is 1200 sq. ft, and the whole house fan moves 2280 cfm of air. This is ~2 cfm / sqft, which I gather is on the low end of the typical recommendation, but doesn't seem unreasonable. As far as insulation goes - I know that all the exterior walls are insulated, because the stucco was recently repaired and the contractor checked/installed insulation where needed. There's insulation on the floor of the attic, but as I have no real experience here, I'm not sure how much is "enough".

Is my expectation that the temperature should not continue to increase into the evening/night-time hours reasonable once I've got a whole house fan running? What should I check and possibly improve/renovate to address this?

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  • Any shading? Trees are good for shading in summer - leaves which fall off for winter.
    – Solar Mike
    Commented Jun 24 at 6:19
  • When you turn on the fan, despite opening windows on both sides, you might not be drawing (much) air into all rooms. A big window close to the fan will supply much more air than a small one further away (I'm assuming the fan extracts, but the same effect holds if it blows in). Also, windows on the south/west side will pull in warmer air than on the shady side.
    – Chris H
    Commented Jun 24 at 6:31
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    Is it possible to do a couple of temperature measurements of the walls (or as close to them as you can) when you switch over from AC to fan? Measure the temperature of the indoor side of your outside walls and also an indoor wall that is near the middle of your house. That will give an indication of how much heat has seeped through the insulation and has to be extracted from the walls by the draft created by the fan.
    – Bart van Ingen Schenau
    Commented Jun 24 at 9:03
  • @SolarMike it's early days but having had solar panels installed on my S-facing roof I'm really impressed with how much they do to keep the attic cool. That's pretty well insulated (250-300mm of glass wool over most of the floor area) but it's still a reduced heat load on the house
    – Chris H
    Commented Jun 24 at 10:08
  • The structure of the house is like a big heat sink. Air temp can change fast, but drywall/wood/insulation will take hours to heat up/cool down. Some of the retained heat in the walls will move to the inside air.
    – crip659
    Commented Jun 24 at 10:30

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A whole house fan provides air flow through the house by creating negative pressure (ie sucking) air outside through one room, and drawing it in from others in a way you control by opening doors and windows to various degrees.

If you want to exchange air in many rooms, open their doors and crack open their windows just slightly. One inch. Even less. Not just the opposite side of the house but all rooms except the one with the fan. If you stand by any window you should feel and hear air being drawn in.

Any room without an open window will not receive flow. If any window is open too much the whole scheme fails. If they are all open too much, the rooms nearest the fan will get all the flow.

Adjust doors and windows to get the effect you want.

Cooling

That's just flow. You want cooling?

  • you have to master the above
  • the sun has to be set.
  • the outside air has to be cooler than inside

All of those.

This works better in a multi storey house where the fan is upstairs and only downstairs windows are open. In your house with one floor the conditions have to be perfect. If the outside air is only 3 degrees below inside and the sun hasn't set, the temperature inside will rise. It may rise a little slower than if you didn't have the fan but the house won't be cooled by it.

Set it up right, wait til dark and at least a 5 degree differential. Close windows and doors in rooms you're not using. The remaining rooms should cool.

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    Re the last paragraph, it's worth cooling the unused rooms as much as possible in the morning before the day warms up - get that heat out of the house so it can't spread to the space you really care about. Inside walls don't provide much insulation
    – Chris H
    Commented Jun 24 at 10:06
  • @ChrisH I disagree. O/P has A/C and the night air isn't very cold so I think the limited effect of the fan cooling should be focused on the rooms in use. Removing residual heat from unused rooms, 1) is a single sprinkle on the icing on the cake given all the other details in the question, and 2) is better left to the A/C.
    – jay613
    Commented Jun 24 at 12:59
  • Free cooling is free cooling. If it's down to 70F by 9pm it will be a good few degrees cooler in the morning. Get some of that into all rooms including any unimportant ones and the A/C has to work less hard during the day
    – Chris H
    Commented Jun 24 at 13:06

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