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I’m struggling to find a solution for my furnace. It’s an older Lennox furnace and runs fine. The problem I’m having is that it keeps burning up my thermostats and once it does it won’t let the termostat kick it off. I’ve tried basic 7 day programmable thermostats and nest 2n gen Smart thermostats. Nest has replaced the circuit board a couple of times for me but they end up burning up after a month or so. I’ll come home from work and my condo is 90 plus degrees because it won’t turn off. The wires I have in my furnace are as follows Y1, W1, G, RH. I’ve had multiple HVAC companies come out and look at my furnace and they all say it’s a thermostat issue and then they try to sell me a new furnace. Any ideas on what could be going on here? Is my transformer tripping out the circuit boards in the thermostats.

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    What model is the furnace? Can you post photos of the wiring configuration at both ends + the wiring diagram on the furnace? – ThreePhaseEel Mar 31 '18 at 17:15
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It looks like your nest is wired without the common, so to draw the power it needs, it triggers frequently your heating circuit, doing so, your furnace relay will read a permanent 'call for heat' and won't stop. So, check if in your wall there is a spare thermostat wire (pick black one if present) and connect it to the 'common' side of feeding transformer, so the nest will be able to get power directly and won't have to trigger frequently the heating circuit just to recharge itself anf finally, your furnace will stay off.

To reduce 'thermostat frying rate' an option (if 24V transformer is bundled with your boiler and you can't replace it easly) is to put a second (external) transformer and connect the stat via relays (you'll need 3 relay for DC with 24V coils) in this case the thermostat will have to operate only on 'auxiliary' relays and worst case is that you'll have to replace often only the inexpensive relays.

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  • Y1-Yellow Cooling On
  • W1 White Heating On
  • G Green Fan
  • RH Red for Heating 24VAC.

You might have some other wires there like Black 24VAC, Red 24VAC Rc 24VAC.

1: Check your transformer secondary voltage to make sure it is 24VAC you want to make sure that it is ONLY 24VAC, your primary might be TOO HIGH and the xformer secondary therefore is high check that too.

Take some pictures of your thermostat connections and wiring, also post the make and model of your AC / Heater / Air handler Unit. Take a photo of your terminal connections at the Air handler and post it. Tale a picture of the wiring diagram located on the inside panel of the air handler.

The possibility exists that your furnace unit is killing your thermostat - but no model numbers and we don't have a wiring diagram you could post that diagram which should be on the inside of your Air Handler Cover Panel.

  • No: green is the fan not the ground! RH is the 24V 'supply', W is 'consensus' for heating, Y is 'consensus' for cooling and G is for the fan. Anyway: check the wiring at furnace side if color code is match, also, if you have an external transformer to provide 24V, have it replaced. – DDS Oct 30 '18 at 8:02
  • @DDS - you are correct I was apparently was mind wandering when I was posting that.. really bad of me.. thanks for commenting - I usually post my connection map . RH is for heat, RC is for cool, it is possible to only have R. I don't have time right now - but I have posted it previously. T-Stat wiring is generally standard for basic units and there can be differences like Blue on Trane.. but once the unit goes beyond basic heat and cool ie heat pumps etc.. the color code and connection map gets less uniform. – Ken Oct 31 '18 at 2:27

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