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No thermostat works on my furnace?

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dongemus
dongemus Member Posts: 2
edited February 3 in Thermostats and Controls
My forced air attic furnace stopped working. R has 24v. When touched to W at the thermostat I get heat. But no thermostat will work. I've tried smart thermostats, programmable, and non-programmable and none of them will turn the heat on. Touching the R and W will always turn the heat on. The smart thermostat won't even power on. Why is this happening and is there some type of alternative switch i can use to contact R&W when it reaches 65° and disconnect when it reaches 70 or something like that?
I'm planning on replacing the whole furnace next year and just need to buy some time.

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  • pecmsg
    pecmsg Member Posts: 4,876
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    Are you touching R to w at the furnace or at the t-stat?
  • dongemus
    dongemus Member Posts: 2
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    pecmsg said:
    Are you touching R to w at the furnace or at the t-stat?
    at the thermostat
  • 109A_5
    109A_5 Member Posts: 1,386
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    Hello @dongemus,
    In this situation, to better understand what is going on, I would start measuring system Voltages when the thermostat is calling for heat. Starting at the thermostat, which should have near Zero Volts across its switch, since the switch from R to W is closed.

    Then at the furnace the C terminal to the R terminal, then from the C terminal to the W terminal there should be 24ish Volts in both cases. If so move on through the system.

    It seems like the question is, what is the difference between the thermostat switch closing and the R and W wires touching each other.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • Ironman
    Ironman Member Posts: 7,377
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    Smart thermostats require the common side of the 24v control circuit to power the logic of the stat. In other words, you need at least 3 wires to the stat if it’s heat only, 5 wires if it’s heat and cooling.

    There are some stats like the digital T87 which are power stealing and only need two wires or you can select a stat which is battery powered.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • HVACNUT
    HVACNUT Member Posts: 5,844
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    R and W wires together at the thermostat location, the heat comes on. ANY simple switching thermostat used, no heat.
    Does not compute. There's a variable missing.  
    Teemok
  • Dave Carpentier
    Dave Carpentier Member Posts: 593
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    Is there a presumption you're using when wiring the wires to the stat ?
    Did they use oddball colours ?

    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,945
    edited February 4
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    If you are at your wits end and just want something that will work, this one is as simple at it gets.
    https://www.supplyhouse.com/Honeywell-Home-T822K1018-Mercury-Free-T822-Heat-Only-Vertical-Thermostat

    Or this economy model
    https://www.supplyhouse.com/Honeywell-Home-T812A1010-T812-Heat-Only-Square-Thermostat-w-Pos-Off-Switch

    Nothing digital about it and the accuracy may be off a little but you can compensate by just moving the set point higher if you are cold or lower if you are hot.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    exqheat
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,320
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    Hi @dongemus , It sounds like a basic thermostat like this might do: https://www.supplyhouse.com/White-Rodgers-1C20-101-Single-Stage-Snap-Action-Low-voltage-room-thermostat-with-wall-mount

    Yours, Larry
    EdTheHeaterManexqheat

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