My Hvac fan won't stop even after the desired heat is reached after the gas burning stops. I changed the furnace limit switch, no difference. Today I noticed there is an auxiliary limit switch on top of the furnace. From what I read so far it is for shutdown in case of overheating.

The fan stops some times after desired heating is reached, not most of the times. Needless to say my electric bill is bloated, my guess is because of the fan running unnecessarily for long times.

I did install a new nest thermostat and ruled out its thermostat issues with Nest help.

My next guess is some capacitors on the board are blown, to replace the board? the board itself is shielded in a box of sheet metal where all the wires are fed into.

The furnace is made by Carrier, model 396JAW036045. 30+Yr old.

Any help is appreciated. Thanks in advance.

  • Hello, and welcome to Home Improvement. I wouldn't guess that a continuously-running fan would greatly boost your electric bill; maybe that's a hint as to the cause. – Daniel Griscom Feb 10 at 0:40
  • Check the other limit switch. Also, did this happen before the Nest was installed? If this is new since the Nest install, check for thermostat control or equipment control of the fan. My Ecobee has this option and I had to switch from thermostat control of the fan to equipment control of the fan. – Jeff Cates Feb 10 at 21:32
  • Thanks, Daniel. Yes, I need to find why I'm going to second tier with electricity which is more expensive. I have led's for all lighting, hvac is the other big consumer besides gas. – Vinnie Feb 11 at 2:34
  • Hi Jeff, i recently bought the house, didn't check if the old ones were working, they are 30+yr old. Since the fan does stop some times when the heat is met, I did consult Nest customer support and we ruled out its nest. I will check the second limit switch of I can reset. – Vinnie Feb 11 at 2:39

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