The default temperature settings for programmable thermostats are not always the best air conditioning temperature from an energy savings perspective. While modern programmable thermostats come with defaults for full cooling mode, away mode, and sleep time mode, you can cut your cooling costs significantly without sacrificing comfort, by tweaking these temperature settings.

Most programmable thermostats provide four daily time periods to coincide with:

  • Your morning active time (from wake up time to when you leave for work)
  • Your away time during the day
  • Your afternoon/evening active time
  • Your sleeping time.

Older programmable thermostats, as well as cheaper modern ones, only let you alternate between two temperature settings: active, and away/sleeping. Others provide three temperature settings: active, away, and asleep. The best ones let you choose an arbitrary temperature for each time period.

For thermostats with three settings, and those that provide an arbitrary temperature for each period, the settings typically default to 78F for active, 82F for asleep, and 85F for away. Thermostats with only two temperatures usually have the away/sleep time set to 82F. This means that by default, there is only a 4F temperature spread between full cooling and energy saving mode, or 7F for thermostats with separate away and asleep modes.

While that 4F to 7F temperature spread will save energy, compared to keeping the house at 78F the whole time, you can increase your savings substantially by expanding the gap between active and away/asleep temperatures, and indeed by pushing all three temperatures slowly upward over a period of days or weeks.

The best air conditioning temperature for while you’re away is around 88F to 90F, provided you set the afternoon cycle to start long enough before your return from work that the house is fully cooled again. (It may take some trial an error for you to figure out how long this is.) For the morning active time, you can try bumping the temperature up a degree a week until things get uncomfortable.

A big part of what makes hot weather uncomfortable is humidity; if the air conditioner is running, even at a higher temperature setting, it will remove humidity and maintain some degree of comfort. A temperature of 90F at 80% relative humidity outside will feel a lot hotter than 90F at 50% relative humidity inside. (In fact, 90F at 50% relative humidity will feel the same as 82F at 80%.)

So let the air conditioner run, but work the temperatures for each setting up day by day as long as you stay comfortable.

The bigger the temperature spread between your active and away settings, the longer it takes to cool your home down. As you bump up the temperature for your away time, you may also need to push the afternoon active time a little earlier to give the home a chance to cool down fully by the time you’re back. This is more of an issue for poorly insulated or poorly sealed homes. A well weatherized home will gain minimal heat during the day and should cool quickly.

Your body adjusts to warmer temperatures over time. I lived in Costa Rica for a year. At the start of that year I found the constant 80F to 90F temperatures and high humidity unbearable. Over time I adjusted, to the point where 95F became quite comfortable, and a mild Toronto day of 77F on my return to Canada was actually quite uncomfortable.

The best air conditioning temperature for sleeping is whatever you can get by on with just a sheet over you. If you have the air conditioner set so low that you need pajamas and a comforter, you have it set too low. Try working your way up from that 82F to 84F, 86F, or even higher. In many climates, by late evening it is cool enough outside that the only work the air conditioner is doing is keeping humidity out of the house; it is often cooler out than in.