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I am having trouble with AC and need to replace it soon ; so I have been listening to it cycle .I knew it only stayed on a short time but I just found it runs less the 2 minutes ; that is with temperatures above 90 F and high humidity ( think Houston TX). So - on for 90 to 120 seconds , then off 10 to 15 minutes .The set temperature is 76 F and an infrared thermometer shows about everything in the house is close to 76 except the aluminum window frames are about 82 F. So the AC starts about 75 to 100 times a day in hot weather. No wonder the "start" capacitor has been replaced about 4 times in 13 years. The unit is 5 tons and the house is 2500 sq.ft. I plan to replace it with a 4 ton unit ; but my question is can I find a thermostat that will decrease the number of start cycles . Then instead of the house temperature cycling from 76.2 down to 75.8 F ( an educated guess) it will cycle from 76.5 down to 75.5 F.

  • maybe it's the position of the thermostat in relation to the airducts. If cold air gets blasted straight at it it's going to measure colder when the AC is on and hotter when it is off. – ratchet freak May 25 at 0:34
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"Swing" is the term you want to look for. Some thermostats allow you to adjust the swing which usually changes when it comes on and turns off. For example, say the swing is set to 2 degrees and the thermostat is set to 72. It will then turn on at 73 and cool to 70 (two degrees cooler than set temp) then stop. Some thermostats might use a number that doesn't directly correlate to degrees, but it works similarly.

If you look at the manual for this thermostat it says:

TEMPERATURE SWING A thermostat works by turning your heating or cooling system on and off whenever the room temperature varies a certain number of degrees from the set-point temperature. This variation is the “swing.” Your system should cycle on about 3 to 6 times per hour. A smaller swing number increases the number of cycles, so room temperature is more constant. A larger swing number decreases the number of cycles, to save energy in most cases.

It seems that it uses a number from 1 to 9 that doesn't map to degrees.

I just looked at the help page for the smart thermostat I use, and they don't use the word "swing", but instead have a variety of "thresholds" that are adjustable. So I guess that is another term you can look for. I highly recommend the Ecobee thermostats, but they are pricy and don't make sense unless you really want high tech.

  • Another term, perhaps more familiar to readers outside the USA, for the "swing" is hysteresis. – Andrew Morton May 24 at 20:11
  • @AndrewMorton as an engineer, I'm familiar with the term, but didn't realize it would be found in thermostat manuals or feature lists. Thanks for pointing that out. – JPhi1618 May 24 at 20:13
  • We used to call the thermostat feature that adjusted cycle length the anticipator. It actually warmed up the thermostat's integrated thermometer either more or less (depending on user adjustments), resulting in shorter or longer (thus greater or fewer per time-segment) cycle times. – Jimmy Fix-it May 24 at 20:51
  • I remember the old round Honeywell , the had a small adjustable power resister My present Carrier does not , and I bought a new Honeywell RTH 2300,It does not have a "swing" adjust. I don't want anything that needs a password. – blacksmith37 May 24 at 21:48
  • Yet another term, one that uses degrees: differential. Thermostats With Adjustable Cycle Rate Differential – hvachowto.com - They used to be more or less unavailable in residential. Now I guess it's a thing. – Mazura May 25 at 2:50

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