My furnace will come on with heat SOMETIMES and sometimes it WILL NOT?

If I turn thermostat Down and then Up... it come on. Works for a few hours and then may or may not come back on?

This is Thermostate problem or Furnace problem?

Anything I can check?

Thanks,

Lee R

Go to the furnace and locate the wires to the thermostat. When the thermostat is calling for heat and the furnace isn't on, check to see if there is 24 volts between both the red wire and the wire and the blue one. If the red doesn't have 24 volts, you have a problem with the transformer in the furnace. If the red is hot, but not the white, the problem is in the thermostat or the wiring. If the white is hot, but the furnace is not on, there is a problem with the furnace. When it calls for heat, the thermostat switches the power from the red wire to the white one. If your furnace doesn't use those colors, let me know, and maybe I can figure it out. Note, often getting at the wires means removing a cover which opens a switch cutting all power. If so, you have to hold the switch in while checking.

If you don't have a volt meter, you can pick up one good enough for simple tests like this for $5-10. You could also short the R and W terminals together and see if the gas comes on.

My furnace runs like clockwork all day until evening. Temp falls below normal. Click it on and off then it works again. Thermostat is only two years old. Should I try replacing that first?

Changing the thermostat would be the first order of things. If you can manually turn it up and the furnace runs than shouts off when the temperature is satisfied, THEN it will not start again when using only the thermostat without your help, may indicate a faulty thermostat.

Hi I was just reading this post as I just replaced my old round thermostat with a new honeywell programmable one.. I have the same issue that it runs fine during the day and it some point during the night it does not kick on... I have to shut it off and turn it back on.. customer service says the unit is fine.. but it's hooked up like the old one was... so do you think the problem may be a faulty thermostat? Thanks