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.
Best Answer
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.