This summer i want to be more efficient and save money on my AC usage because I can remember last year it was an disaster. For example, on a typical 100 degree day, the highest the temperature would read on the thermostat is about 85-86 degree around 5pm without any AC, all windows and shades closed. Last summer I set my thermostat to 81 degree and the AC will kick on right about 3pm when temp start to go above 81 degree. And then every 15 to 30 min. or so, the AC and fan will constantly turn on and off. I usually run AC for 4 hours from 3-7, then we open all windows around 8pm when nightfall.

Below are the options I can think of, which one is more efficient and cost effective?

  1. Set the thermostat at 81 degree and keep fan on for four hours.

  2. Set the thermostat at 79 degree and fan auto for 3 hours.

  3. Set the thermostat at 78 degree and fan on for 1 hour then wait another hour and turn AC back on fan on for another 1hour.

My system is Lennox G16 Q3-50 Furnace and I think a 1.5 ton ac outside.

  • A 1.5 ton ac is 18 000 BTU/h. Is that what it says on the condensing unit outside? Ours is 3.5 ton (42 000 BTU/h) for a poorly insulated 2000 sq ft house in Dallas TX. We do have a lot of mature shade trees which we planted when we moved in 39 years ago. On a 100 F day our 25-year-old Carrier unit (12 SEER) will cool the inside to about 78 F. The compressor will remain on continuously for at least 4 hours in the middle of the day. Commented Apr 12, 2017 at 1:08
  • Is this a desert climate with cold nights, hot days, and low humidity? Commented Apr 13, 2017 at 15:31
  • A typical 100 degree August day in S. Cali. will be 68 degree at night. Commented Apr 13, 2017 at 20:32

2 Answers 2


Whatever you do, use the blower fan on auto only. The blower fan is ineffective for circulation compared to ceiling fans or to fans on stands or to box fans. If your ducts are in a hot attic, circulating air will pick up heat through the relatively thin insulation of the ducts. If your ducts are in the crawl space of a pier-and-beam foundation, then maybe using the blower on for circulation might do some good.

I am well into my 70s and can't stand being hot so my recommendation is to set the thermostat at 78 F and leave it there. We keep ours at 74 F and just pay what it costs. If you have standard thickness filters, change the filter once a month or two months at most. Let everyone (including the dog) enjoy life at a civilized temperature and humidity.

Where are you? What was your monthly kWh last summer, say for July?

Edit Our electric power use kWh (so you can judge the validity of my answer on your question) for two retired people who stay home a lot and don't travel much, share one automobile (~one tank of gas per month), electric kitchen range (never eat out), electric clothes dryer, gas water heater.

2016 electric power use kWh Mar 500 Apr 500 May 700 Jun 1000 Jul 1700 Aug 1950 Sep 1500 Oct 1000 Nov 700 Dec 600 2017 Jan 750 (space heating of garage for painting) Feb 500 Mar 500

  • Southern California, Last year it was 650kwh at $.16 per. Commented Apr 12, 2017 at 0:48
  • See edit to answer for our electric power use. Looks like our annual bill for electric power is approx $1500. Commented Apr 12, 2017 at 14:31

Your options are needlessly complicated.

The bulk of operating cost of air conditioning is the compressor, not the fans. Leave fan control on auto. If anything, running the fan continuously will increase the interior temperature, because mechanical energy is being applied to the air, which converts to heat through friction.

Check whether your thermostat records the cumulative duration of air conditioning (like a car odometer); if so, you can use the numbers as a surrogate for electrical cost. You could also experiment to determine how much extra the AC runs for each degree change in set point. However, variables such as exterior temperature and radiant sunlight will, unfortunately, be more significant than a few degrees difference in temperature set point.

Consider other options to lower your cooling costs.

  • Evaporative cooling. This may be an option if your humidity is low.

  • Interior fans. I know I told you, above, not to run the fan, but moving air will make you feel cooler by increasing convective cooling (aka wind chill). If you're sweating, a fan even will turn your body into an evaporative cooler!

  • Thermal mass. Your afternoon air conditioning effort is hampered by the fact that your walls, floors, ceiling, and furnishings all start transferring heat to the interior air as soon as the AC starts. Passive cooling may be achieved by "storing" lots of cold during the night in, e.g. water tanks and using that cold mass to absorb heat during the day.

  • Thanks for the answer. Its a pity, i spend so much time in adding a C wire just to get the Fan to be used independently, now I still have to leave it in auto. Commented Apr 13, 2017 at 20:41

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