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Thread: Honeywell RTH8580 Wiring Question

  1. #1
    Junior Member

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    Question Honeywell RTH8580 Wiring Question

    Hi, all.

    I am new to this site. I went through many threads but did not find my issue so I open a new one here.

    I have a Honeywell RTH8580 Thermostat. I had to use a dedicated 24 volt transformer for heating, but I had to rewire it for cooling. It is a problem when the weather is changing direction during this season.

    I had several conversations with Honeywell support and I was told that without the jumper between R and RC, the cooling would not work.
    On the other hand, I was told previously because of the issue of losing power on the thermostat after the furnace is on for a few minutes, I have to use a dedicated power supply (24 volt transformer) and I was told I have to break the jumper between R and RC.
    Now I have to have completely different wiring (changing the C connection, take away the transformer connections for cooling) for heating and for cooling which is annoying when the weather in my area changes quite quickly in this season.

    The technical question is why I cannot have the jumper on while dedicated transformer is connected? The transformer provides power to the thermostat between C and RC. It is an independent circuit between transformer and thermostat. Since C is not connected to anything else except the transformer, and RC would be the only terminal shared with the thermostat and heating/cooling system circuit, whatever happens in one system should not flow to the other. I would believe there is no reason to cut the jumper when the dedicated transformer is used to provide power for the thermostat. The Honeywell support told me if I put the jumper back on, the warranty would void. I am waiting for Honeywell to answer my question why connecting the jumper would cause damage but I don't know when or if I would get an answer.

    I cannot believe the system would be so poorly designed that I cannot use the same wiring for heating and cooling.



    Thank you for your help.

  2. #2
    Administrator

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    Without knowing how all the wiring is terminated on your system it is hard to tell what the issue is with the dedicated transformer. Most likely the issues with the RC / RH jumper is that the thermostat Y and G terminal receives 24vac from the RC and with the dedicated transformer wired to the thermostat you would have two transformers in series (A/C unit transformer and dedicated transformer). I would think that your best option would be to use the A/C wires and install a isolation relay to the heat.

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