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Thread: Thermostat RTH8500WF - Not working after installation

  1. #1
    Junior Member

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    Thermostat RTH8500WF - Not working after installation

    hello,
    I removed an old Honeywell thermostat. I checked the wiring and found out that is a conventional wiring (no heat pump) The wiring was G-Green, R-Red, W-White, Y-Yellow, C-Brown.
    See photo of the old thermostat:
    Click image for larger version. 

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    I cut-off the power from the circuit breaker and went ahead to disassemble the old thermostat to install a new one - Honeywell RTH8500WF. Obviously I went on with the same wiring configuration keeping the jumper between the R and Rc.
    For some reason, when I was inserting the C-Brown wire into the new thermostat's "C" slot, I saw a very small spark (I had the breaker's switch off already). When I installed the new thermostat and turned the breaker's switch back on, nothing happened.
    See photo of the new thermostat installed, wired and without anything on the screen:
    Click image for larger version. 

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    So I went ahead and turned it off again and re-installed the old thermostat back and to my frustration, the old thermostat didn't work either. Blank screen...

    I thought the initial "spark" may have burnt the fuses, so I went ahead and changed the fuses (they are 40A each so the chances of getting burn by such small jolt is really minimal, but then again, things tend to happen). I changed both to new fuses.

    So after all these attempts, I have failed to get my new thermostat working and for some reason my old thermostat doesn't work either.
    I don't know if there's something in between that burned because of the spark.

    I will greatly appreciate any help.... I really don't want to hire an electrician until I know more about this.
    Thanks in advance!



    Pls2k

  2. #2
    HVAC Thermostat Specialist

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    You will need to find out what the other end of the brown wire is connected to. You may have blown the low voltage control fuse or shorted the low voltage control transformer. The low voltage control fuse will be something like 3A and look like a car fuse. It will be located in the air handing unit.

  3. #3
    HVAC Thermostat Specialist

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    Conventional electric heat typically has 2 sets of breakers for the air handler. When it comes time comes consider going with a heat pump, they cost 1/2 what electric resistance heat does to run.

  4. #4
    Junior Member

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    Quote Originally Posted by HVAC View Post
    You will need to find out what the other end of the brown wire is connected to. You may have blown the low voltage control fuse or shorted the low voltage control transformer. The low voltage control fuse will be something like 3A and look like a car fuse. It will be located in the air handing unit.
    Hi, I'm not handy with the air-con system and i'm learning as I go...
    The compressor is a Carrier model #38CKC048370, fairly new... and the fuse box is located next to it. Pictures of both:
    Click image for larger version. 

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    Regarding the low voltage control fuse or the transformer, where would that be located at?
    Thanks again!

  5. #5
    HVAC Thermostat Specialist

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    The transformer and fuse should be in the control box in or on the air handling unit.

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