Live in Southern Florida, so we use AC nearly year round. We live in a block house, concrete tile roof, fairly new, well insulated. We do not use a programmable thermostat as we are home most of the time, and when not it is not predictable times.

However... recently had the AC replaced, but it had not completely failed. We got the house down to 75 before they started about 8:30am, and they finished at 6pm and the house was only about 79 degrees. Gave me an idea...

From 1am to 10am (+/-) will set it to 75 and over-cool a bit while the outside air is about 20-25 degrees cooler than daytime. The house has a large heat capacity, and cools and warms slowly. That ought to hold us under 78 until mid-late afternoon, so the unit would not run (or not much) at all during the hottest air temperature time.

In other words, try using it to shift the biggest cooling load into the most efficient time, in the wee hours, and during the least efficient time try not to run it (hardly) at all.

Does this sort of cold-soaking work reasonably? Any guidance as to whether it will be an improvement, or are we making it worse by the cooling below 78, rather than just keeping it at 78 all day.

  • I visited Florida a few years ago to see if I would like to retire there. I checked on newer houses in retirement communities and the construction of those houses. I was told that the R-value of the walls was 19. The construction was concrete block outside with fur strips, 3/4" styrofoam ,1/2" drywall inside. I told them that they must be from a dream world, because that type construction does not yield R-19.They said that they were the engineers and that I was mistaken. When I pulled out the books they walked away. So be careful when you say fairly new and well insulated in the same breath.
    – d.george
    Oct 17, 2017 at 9:57

4 Answers 4


It's all about thermal mass

You say your house is concrete block and insulated, now where's the insulation? If the insulation is outside the block, then you are using a technique used in passive-solar construction called thermal mass. **, Trying to have a large thermal mass in the house which resists change in temperature. If the insulation is inside the concrete, different deal, then it is your friend in the morning and your enemy at night.

If it can store/ resist temperature change for 12 hours, then this is worth doing.

Otherwise its just yet another excuse people make up when they want to crank the A/C.

The other thing is, your thermal mass may be different than most. But if this worked for most people, the power company would be telling everyone to do it. They want to sell you power, sure, but the hardest thing for them is producing peak power. Anything they can do to move power consumption from the peak to the valleys (night/morning) they're all about.

Based on what you say, I don't think a 3 degree swing will matter much. The stored energy is degree difference x mass, and a small temperature difference doesn't give you much storage.

There are better ways to do this

First, the best way to avoid interchanging with 120 degree air is interchange with 50 degree groundwater. If you have groundwater in your area that is near enough the surface to be worth pumping. You have to buy a condenser made for this, of course. It works particulary well with heat pumps, because groundwater has a usable temperature all year, even in cold places. You make yourself 2-5 watts of heat for 1 watt of electricity, the other 1-4 watts come from a very large RTG called planet Earth.

Second, you do the same thing using dirt itself, but the coolant loop must be much, much larger, because it needs to interchange with a lot of earth.

Third, if your house isn't built with a thermal mass, you can get your own thermal mass with a huge insulated tank of water. At night, you use cheap evening electricity to chill the water, by day you use it to interchange heat, as above. Nowhere is it written the tank of water can't have a concrete skirt, a shallow end and a diving board... But it does need to be well insulated.

  • I think what I was looking for was confirmation that it MIGHT work, as opposed to the math just not being practical. Relative to ground/water possibilities, it is attractive but just too difficult to do where we are (postage stamp lot, salt water, excessive city rules). The accidental experiment without AC was what made me think to try this. Telling if it works, with so much variation in heat load (cloud, wind, temps) makes it hard to tell over the short term if it works, I think it will take some months for year v year comparison to know for sure.
    – Linwood
    Oct 17, 2017 at 13:08

While I have moved (still in hot Florida) I now have a better situation to actually test this. I put an IoTaWatt on my power, so I have minute by minute power draw on the A/C, as well as I can computer control my thermostat.

I have been running variations of two situations:

1) A steady 74F thermostat setting all day, and

2) A cold-soak setting that starts at 70 or 71 at 1am, then begins raising to 77F at mid-day, and drops back to 75 in the evening.

I have run a variety of times, so as to try to drive the majority of the A/C load into times when the outside air is cooler.

It is very easy to avoid the mid day, it only takes a few degrees increase in the thermostat (though it does become noticeable as it heats). Here's what the data look like:

enter image description here

The filled green (air handler) and yellow stacked (AC Compressor) are watts. The red line is OAT and the blue inside, with the yellow the thermostat setting.

You can see in the wee hours it keeps the temperature quite low, and coasts up. In this case from about 8:30am to 5pm it didn't run the A/C at all, with the inside temp peaking around 77-78. But then... it turns on and takes quite a while to get the temperature back under control.

I have also tried this delaying the rise in set point until later in the morning (as the OAT is pretty cool still), which can slightly delay the evening run time, but it is not enough.

As a typical comparison, this day required 19.01 kWh for the managed day, and 15.59 kWh for a steady 74F setting. The OAT both days was about the same (and same 90F high each day). On advantage this time of year is weather here is very repeatable. I compared with attic temperature, the managed day reached 105.9 and the day I set to 74F it was 106.8 (I use the attic as a rough indicator of radiant heat load); while different it would imply a bigger demand on the lower kWh day.

I've gone through 3-4 iterations of a day each way, with different configurations. In every case, the steady setting came out better in terms of power usage, some larger, with this example being pretty average.

Now that's not to say there is not a magic arrangement, and it might vary depending on efficiency of the outside unit (mine is one year old 16 Seer lennox).

But as that famous TV series said, at least for me: Myth Busted!

PS. The explanatory solution is certainly correct from @Harper - Reinstate Monica is correct, but I offer this as a specific answer from actual data.

  • Are you able to get a day/night or time-of-use metering setup? Here in NZ, we pay about half as much per kWh for energy between 11pm and 7am. That might swing the balance depending on other loads. Mar 31, 2020 at 6:04
  • No, electric rates where I am are flat. You're right though, peak hour rates would make the math quite different.
    – Linwood
    Mar 31, 2020 at 11:37
  • It's a choice for the end user here. You can also just buy a flat 24-hour rate. Mar 31, 2020 at 19:40

I would not turn the A/C off as you indicated since it will have to run much more to cool the house down to your required temperature. Remember this; everything in the house is the same approximate temperature. When you had the old unit replaced and the temperature did not rise rapidly that was because everything in the house, walls, furniture, rugs and flooring, everything was able to absorb the heat that penetrated the walls of the house. So when you try to again cool the house everything has to give up the heat that was absorbed while the A/C was off. The air temperature will feel cooler in a short time but the solid objects will take a longer time to get back to the temperature you normally keep the house. Also, this does not address the increased humidity that happens when the house A/C would be turned off. humidity is harder to remove from the house than is the heat. I do not think that there would be any savings with your idea. One last thing, I live north of Pittsburgh, Pa. and the humidity in my house is hard to control. I run a dehumidifier any time my A/C is turned on which is usually all summer. A dehumidifier would probably be a great device to have since it will remove a lot of what makes your home uncomfortable and would allow you to raise the thermostat a few degrees higher. Mine cost $200.00 at Sams Club and is a 70 pint model that I set to 45%. Hope this helps.

  • You misunderstand. I do not plan to turn it off, I plan to turn it colder during the night time, so in the morning it is at 75 (let's say) but at about 10am the set point is raised to 78. Since the room temperature is 75, it must heat up before it runs again, and am counting on that taking several hours during the hottest part of the day when cooling is least efficient. 78 is our "target" but being colder in the morning is not uncomfortable - the question is whether it is more, or less, efficient.
    – Linwood
    Oct 16, 2017 at 22:47
  • This will provide no reduction in kWh, but if you want to, then give it a try. Read your electric meter at the same time every 24 h to see if there is any change. Oct 17, 2017 at 1:43
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    @JimStewart it must help somewhat. An A/C unit works a lot more efficiently interchanging into 70F air than into 100F air. Oct 17, 2017 at 1:48

We can't say, only guess; it's complicated and depends on too many variables for broad speculation. It makes sense that a more efficient transfer with cooler air would improve efficiency. Likewise, setting your thermostat higher during peak air temps obviously reduces consumption. Additionally, some meters bill you more during peak hours, so even if the net energy consumption was a wash, you'd still save money with your strategy.

The complications come from evaluating the heat flow in the house over 24 hours. If you have a lot of thermal mass, like an indoor pool, and you don't mind objects becoming warming to the touch at different times of the day, it makes a lot of sense. Other factors are wind, sun exposure, humidity's effects on human temp perception, size of house, cost of power, efficiency of the AC, etc. Too much to "back of the envelope" it for you.

I would say: try it. Lookup past degree days and your bills to see which pattern yields a more efficient conversion. It's very possible that one can save money. I would love to hear what you find.

  • aside: I once rented a house with a crappy central AC that could barely keep up. If i wanted to cool down the house, say for company coming over, i could hose down the condenser for 5 or 10 and the house would drop several degrees in that time...
    – dandavis
    Oct 17, 2017 at 5:20

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