My fridge broke, so I replaced the analog thermostat with an ESP2688 + relay + DS18S20 temperature sensor.

My server is getting the temperature readings every 10 seconds and is capable of switching the relay to turn the fridge on and off (once the logic is settled, I will bake it into an Arduino).

I've set it so that the server switches the relay on when the temperature is above 5°C (41°F) and off again when it's below 4.5°C (40.1°F).

A full cycle takes about 35 minutes about 5 of them cooling down (the fridge is currently empty, so there is no food "storing any coldness").

I know from my parent's house that the fridge turns on far less often (mine is about 17 years old, theirs about 3).

Is this a problem? Should I increase the temperature range, let's say from 5°C to 4°C? This way a cycle would take about an hour.

enter image description here

The image contains 12 hours of logged temperature data. I can't remember the change I made at around 10pm, I think I relocated the temperature sensor, but at 13:30 (1:30pm) I changed the lower temperature from 4 to 4.5.

Now, what is better for the fridge/compressor? A longer cycle or a shorter one? What is cheaper from the power consumption point of view? I can't place a power meter at the outlet because it's an integrated fridge and quite a hassle to remove it just to get to the outlet.

  • 1
    I added logging temp sensors to several fridges and freezers. They all average to about a 5 degree cycle
    – Tyson
    Commented Nov 24, 2017 at 19:35
  • @Tyson You mean they cycle about 5°C around a specific temperature Like from 2°C to 7°C? Or that the temp you measured was always around 5°C ± X°C? I'm fine with 4-5°C. In Germany 7°C is the recommended max in any place of the fridge (preferably at the bottom in the veggies section), and 2°C the min at the coldest place. How long does a cooling cycle take, you have any data on that?
    – Daniel F
    Commented Nov 24, 2017 at 22:03
  • Amazing work and contribution @Tyson. Yes OA he means the sine(ha) wave is 5 degrees peak to peak, or +/- 2.5 degrees. Fahrenheit I gather? Centigrade seems like a lot. Commented Nov 24, 2017 at 22:07
  • Actually about 6 degrees F, so 3 degrees C. Here's a graph, I choose overnight hours when the door won't be opening: imgur.com/a/KV7bD It's 10 minute data interval.
    – Tyson
    Commented Nov 24, 2017 at 22:16
  • @Tyson Thanks. Do you find the the RH useful? I've got some DHT22 lying around. So that looks like a ~1h cycle maxing out at 6°C with a min of 1°C. These are helpful values for me. Do you have any idea if it is bad or good for the compressor to have a short or long cooling cycle?
    – Daniel F
    Commented Nov 24, 2017 at 22:27

1 Answer 1


I vote for the longer cycles if you can live with the temperature variant. The fewer starts is always going to be better for the equipment. As far as energy consumption, that's well over my pay grade. I might keep it slightly closer to 38*. Keeps the unwanted growth down.

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