I have a gas furnace that seems to work fine; it keeps the house warm when things are running as expected. Sometimes, however, we'll notice the house is getting cold.

The temperature on the thermostat is set to 68., When we check the temperature reading on the thermostat, it could be as low as 64. Normally, the heater would have kicked on well before that, so we change the thermostat from "Heat" to "Off" then back to "Heat" and the heat turns on immediately.

This makes us think that it's something with the thermostat itself, but it's quite the nuisance at night. It did this a couple of times last winter, and has been happening much more this year. We've noticed no problems during the summer with it turning on the AC.

Thermostat Below Temp

  • 2
    Have you tried changing the battery? Dec 15 '17 at 20:04
  • I had not, however, I just changed the batteries before typing this. Dec 15 '17 at 20:50
  • New batteries, same issue. Dec 16 '17 at 1:08
  • 1
    It is possible it is the thermostat however more likely it is a problem with the furnace itself. Most modern furnaces will try to light 3 times and if they fail to light then turning the furnace off and on again will reset the furnace. It could be any number of problems. If your furnace has a diagnostic light look for the code it is flashing before you reset the thermostat. Some of the higher end furnaces will keep a record of the codes.
    – user76730
    Dec 16 '17 at 6:48

You need to preform a test that will tell you if the thermostat is bad or is the problem in the furnace. If you can find a time when the t-stst is calling for heat and the furnace is not responding, at the furnace check to see if you have 24-volts on the white lead coming back from the t-stat. If you do, the problem is in the furnace. If not, the problem is in the thermostat. These thermostats are mass produced with an emphasis on mass part. They do fail.

  • I agree the problem needs to be narrowed down I was thinking of a flakey contact in the stat but a no light problem would cause this issue the control board needs to be checked prior to resetting the stat.+
    – Ed Beal
    May 2 '19 at 21:24

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