I have three air conditioning units. The largest one cools my main room and has a nice digital thermostat that keeps things within plus or minus one degree Fahrenheit of the temperature I set (the compressor turns on when the temperature gets two degrees above my set temperature and turns off when the temperature gets two degrees below my set temperature).

The other two air conditioning units have an analog thermostat that doesn't even let me set a specific temperature. They are rotating dials with such arbitrary zones set on them as "cool" and "very cool".

Additionally, the hysteresis in the thermostat is very large. I use these units in my two bedrooms and the large hysteresis produces what for me is an uncomfortably large range of temperatures over the course of the night: I will put on the minimum blankets needed to be comfortable when the temperature is at its lowest and still end up getting woken up a few hours later when the room gets too hot, before the A/C's compressor has kicked back in yet.

The unit with a digital thermostat is too big to fit in the hole available in either of the bedrooms, otherwise I would use that one in my bedroom to help me sleep better.

Is there any way to tamper with the A/C units to get the hysteresis held to a tighter range, or to trick the A/C unit's temperature gauge in such a way as to get the temperature held to a tighter range? Or will the only solution for me be to get a smaller A/C unit that has a digital thermostat?

Possibly relevant info: I live in NJ, which is pretty humid. I live on the second floor of a two-story building, and my neighbors below do not air condition. Two of the four walls of my bedroom are exterior walls. The insulation in my building is terrible, especially around the window (I can usually feel air leaking in through where the extended window frame meets the wall). My windows do open but they are not nearly large enough to put in-window A/C units, I'm stuck to using a rectangular hole of a fixed size that is below the windows.

  • 1
    You're probably better off buying a new unit with a digital display. The other option is to rebuild, or bypass the control circuitry in the existing unit. This would involve opening the device and tampering with it, which is not recommended unless you know what you're doing.
    – Tester101
    Jun 2, 2016 at 12:37
  • But if you do have practical experience w/ electric circuits, it might be worth getting a service manual and seeing if there's a trimpot somewhere that controls the hysteresis range. For example, the relay control board on my hot-water-heating system has trimpot settings for over-temp and under-temp range. Jun 2, 2016 at 14:30
  • To your last point: there are low-profile A/C window units which require about 6 inches of window opening; the remainder hangs below the sill. Jun 2, 2016 at 14:31
  • @CarlWitthoft, I unfortunately don't have a service manual for my A/C unit. Any suggestions where to look for a manual, or where I might find the trimpot? I have some practical experience with electric circuits, but not much. I've built circuits in two college electronics classes (including, incidentally, a thermostat) but I'm sure the A/C electronics are much more complicated. As for your second point: my windows are shutter-style windows and I would have about 5-6 feet of empty space I would need to fill if I ran my A/C units through the window. Jun 2, 2016 at 17:09
  • Is there a hysteresis adjustment screw inside the thermostat? Some of the old units had them. Jul 17, 2017 at 16:48

1 Answer 1


Divert some of the A/C output air toward the thermostat sensor. This should cause it to shut off sooner and raise the low end o9f the hysteresis. It works like an anticipator.

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