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Cooling won't turn on. Need help identify Thermostat Wiring

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heatingrabbit
heatingrabbit Member Posts: 21
in Thermostats and Controls

First time trying to switch on the cooling in my house since we move in. The house contains 2 thermostats controlling two outdoor HVAC Units for different rooms. (Heating for the house is using boiler)

Thermostat A works fine to turn on the cooling (when cooling on is set, outdoor unit starts fine), but the Thermostat B doesn't seem to do anything (outdoor unit won't turn on).

I check the wiring found those two are using completely different wiring.

Thermostat A (working) is using 3 wires G Y R

20250713_121358.jpg

Thermostat B (not working) is using 3 wires B Y R

20250713_121246.jpg

Anyone know based on the wiring, is Thermostat B wired for cooling or not? Thank you

Comments

  • HVACNUT
    HVACNUT Member Posts: 6,885

    Thermostat B serves a heat pump. It does air conditioning, and heat (AC in reverse). Indoor blower motor (fan) is on but the outdoor unit isn't? It could be any number of things. Low refrigerant pressure. High refrigerant pressure. Contactor. Capacitor. Relay. Defrost board. Compressor. Where are you located? It needs an HVAC tech who knows heat pumps.

  • heatingrabbit
    heatingrabbit Member Posts: 21
    https://forum.heatinghelp.com/discussion/comment/1861456#Comment_1861456

    Both indoor fan and outdoor unit are not on.

  • mattmia2
    mattmia2 Member Posts: 13,614

    jumper r to y and see what happens

    HVACNUT
  • HVACNUT
    HVACNUT Member Posts: 6,885
    edited August 2

    Switch off. Circuit breaker off or tripped for the air handler. Fuse in the air handler.

    If the breaker is tripped or the fuse is blown in the air handler, it happened for a reason.

    A heat pump air handler might have breakers in the air handler door if there's electric resistance heaters.

  • heatingrabbit
    heatingrabbit Member Posts: 21
    edited August 2
    https://forum.heatinghelp.com/discussion/comment/1861460#Comment_1861460
    https://forum.heatinghelp.com/discussion/comment/1861462#Comment_1861462

    Just measure the voltage on Thermostat B wires, there is no AC voltage between any of the two wires.

    Checked electric panel all the switches are on.

  • HVACNUT
    HVACNUT Member Posts: 6,885
    https://forum.heatinghelp.com/discussion/comment/1861482#Comment_1861482

    Blown fuse or transformer. But why?

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,283

    You probably won't read any voltage at the stat that will make sense. The stat is a switch so you only have HOT 24 volts to it on the red R wire . You don't have the other leg of the 24 volts there.

    If you jump R & Y with the stat off the wall the indoor fan in the AHU should start and the outdoor unit should start.

    If it doesn't go to the Air handler and find the control circuit transformer. it should have 120 or 240 feeding the transformer (see Ahu name plate) and then 24 volts coming out of the transformer.

    Make sure the breaker to the trans is on and any 24 volt fuse in the ahu is good.

  • JUGHNE
    JUGHNE Member Posts: 11,473

    You need to look at the other end of the B tstat cable at the air handler.

    If that blue is connected to the common, it could be a problem.

    mattmia2
  • HVACNUT
    HVACNUT Member Posts: 6,885
    https://forum.heatinghelp.com/discussion/comment/1861488#Comment_1861488

    You'll read 24 volts across R and Y when testing at the thermostat sub base if the system is powered and the transformer and fuse is good. Y will read as Common coming through the contactor coil.

    mattmia2
  • mattmia2
    mattmia2 Member Posts: 13,614

    ~24vac

  • CTETeach
    CTETeach Member Posts: 26

    First time trying to switch on the cooling in my house since we move in. The house contains 2 thermostats controlling two outdoor HVAC Units for different rooms. (Heating for the house is using boiler)

    That being said I would think that the unit is straight A.C. If that is the case, someone hooked the blue wire up to the B terminal thinking that was a common? Remove the blue wire from the subbase and jump R and Y Indoor fan should start as well as the outdoor fan and compressor. Don't be impatient there could be a built in delay in the out door unit.

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