York Furnace with Honeywell WiFi Thermostat 24v Issue.

Old 10-13-14, 12:26 PM
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York Furnace with Honeywell WiFi Thermostat 24v Issue.

I have a York forced air furnace model P2MPD14N08001B.

I think it was installed around 2002. It has a condenser outside for AC.

I want to use a new Honeywell Model # RTH8580WF thermostat that requires 24V at the C teminal.

The York furnace has a small logic board with a DiagnosTech 2002 imprint and it has 5 screw connections: Y W R G and C connections. The C connection has zero volts, while the R connection shows 28V and Y connection shows 16V on a standard voltmeter. The outside condenser white wire is wired to the C connection and the condenser red wire goes to another screw connection (I forget which but can check if important).

(I can also run new wire if needed)

The thermostat never comes to life and obviously Iím not supplying 24V to the thermostat.

A few questions: does this board supply 24V ? If not, where in the furnace do I get it ? and if not available, can I plug in a standard 24V transformer external to the furnace just to supply 24V to the thermostat and then what terminal connections would I use from the York board to thermostat ?

Thanks in advance.
Old 10-13-14, 12:36 PM
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Join Date: Jul 2013
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A few questions: does this board supply 24V ? If not, where in the furnace do I get it ?
The transformer provides 24V to the R terminal on the thermostat. The C terminal at the control board is the common from the transformer. When you connect a wire between the C terminals at the control board and the thermostat it completes the circuit and allows 24V to be constantly supplied to power the thermostat. Do you have an unused thermostat wire at the thermostat and at the control board? If so, you can easily do what you are wanting to do.

If you do not have any extra wires available, you can either run a new wire from the control board to the thermostat or you can use an external transformer to achieve your goal. The best route is to install a new wire if it is relatively simple to achieve.

If the extra wire is available all you need do is to turn off power to the furnace. Then attach the wire at both ends to the C terminal and restore power.

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