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Installing 5-wire thermostat on split system.

Posted by Val Thomas on October 7th, 2002 05:19 AM
In reply to New HVAC forum; click on open forum by James Darrel on August 31st, 2002 11:31 AM [Go to top of thread]

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I recently bought a home with a split Colman furnace/Carrier model #27MHI20101 AC system. It currently has a 2-wire thermostat connected at R and W, and a jumper wire between Y and G. Switching between heating/cooling modes is currently a reversing switch mounted on a control box affixed to the blower. If I read the schematic correctly, I would leave the reversing switch in the "Heat" position and remove the jumper between Y and G. I would then connect Y internally directly to the blower relay, bypassing the reversing switch and N/C reversing relay and load resistor. The external compressor relay will remain connected at Y and C. If this is correct, I should have full heat/cool/fan control at the remote 5-wire programmable thermostat by simply connecting R,W,G,Y,C wire for wire. Does this sound feasable? I have little HVAC experience, but a good background in electrical control systems. Also, every 30 min or so, the fan was running without the compressor for 3-5 min with no call from the thermostat or any other control. I noticed the area around the reversing relay load resistor was warm to the touch, and the reversing relay had pivoted on it's mount so as to be in very close proximity to the resistor. Could the heat of the resistor build up enough to drop out the reversing relay which is N/C, then when the fan runs cool it enough to engage again? I can't find any other explanation. Thanks in advance for your time and consideration.

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