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Compressor won’t turn on with Wi-Fi thermostat

BigErl Member Posts: 38
Hello everyone. I’m trying to control a Trane 4YCC4048A1090A rooftop unit with a Honeywell Wi-Fi RTH6580WF thermostat.  The unit is only being used for AC.  I have the red wire to R, yellow to Y, green to G and the blue as my C wire.  The thermostat is powered and the rooftop unit responds to it but the compressor won’t turn on. I can hear it trying to turn on but it doesn’t.  I’ve read about hard start capacitors and I’ve used external transformers on boiler installations. Are either of these a solution?  


    JUGHNE Member Posts: 11,021
    Has this started and ran previously?

    If the compressor is trying to start it is probably not a tstat problem.
    Sounds like a run cap issue.

    Don't think you would suddenly need hard start components if the system worked previously.

  • BigErl
    BigErl Member Posts: 38
    The unit is a year old and works correctly with a Honeywell CT31A1003. 
  • ratio
    ratio Member Posts: 3,605
    There's a 5 minute delay before the compressor comes on that may be in effect. Make sure the jumper between R & RC is in place & the screws are tightened down (but not overtightened). Failing that, make a jumper out of a piece of thermostat wire & jump from RC to Y & see if the AC starts. Don't leave the jumper installed for more than say 30 seconds because the indoor fan won't be running. If it runs with the jumper, replace the thermostat.
    EBEBRATT-Ed Member Posts: 15,350
    Start with checking for power at the compressor contactor (line side). Make sure it matches the unit name plate and that the connections are tight.

    Also check the factory wires from the load side of the contactor to the compressor. See if the compressor contactor is pulling in and if it does check for power at the compressor contactor load side or at the compressor (better) and put an amp probe on the compressor common wire while it tries to start.

    Those units look like they are all 208/230 single phase. It looks like Trane does offer a hard start kit. Also check the capacitor

    HVACNUT Member Posts: 5,761
    The compressor comes on no problem with the old bi-metal thermostat but not with the new digital?
    But you can hear it trying start?
    I'd jump the Red and Yellow wires briefly, disconnected from the thermostat. If the compressor starts, I would think the issue is in the thermostat. 
    The programming is correct?
    Keep in mind, every time you put a load on the compressor, wait 5 minutes before you try again. It was mentioned already but I'll put a ! on it.

  • BigErl
    BigErl Member Posts: 38
    Thanks for all the replies. The jumper between R and Rc was missing.  It’s in now and the unit is working properly.