New Thermostat Wiring - No AC Control

Problems related to residential installations.

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New Thermostat Wiring - No AC Control

Post by mikeybloom20 » Mon Jan 12, 2015 9:53 pm

I recently bought a new Honeywell WiFi Thermostat (RTH9580WF) and had to change some wiring around at the furnace board in order for the Thermostat to receive power to work. I have a 6 year old Kenmore natural gas furnace with an AC unit as well. The wiring that I have coming upstairs is labeled as the - RH - (Blue); RC - (Red), Y - (Yellow), G - (Green), and W - (White). The wiring that was connected at the furnace before I switched some wiring around was Blue (RH) and Red (RC) on the "R" terminal, Incoming Red from AC and Yellow on the "Y" terminal, green at the "G" terminal, white at the "W" terminal and white incoming from the AC at the "C" terminal. As the new thermostat has a jumper between R and RC and requires that there be power to the "C" terminal at the thermostat, I removed one of the wires from the "R" terminal (Blue RH) and then put that wire at the "C" terminal on the furnace board and then did the same upstairs, moving the blue RH wire to the "C" terminal at the thermostat in order to power it up. I have heat, separate fan control, but the AC will not kick on. I could have removed the jumper wire from the thermostat and ran the RH/RC wires to both separate terminals and then moved the G from the furnace board to the C on the furnace board. I would have then had to move the G upstairs to C and put a jumper at the furnace board between Y and G. This would allow both AC and Heat control but no separate fan control. Looking for help to see if something is wired wrong? Thanks.

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- New Thermostat Wiring - No AC Control

Post by Freon » Tue Jan 13, 2015 10:04 am

You need one wire from the R terminal at the furnace to the Rc terminal at the thermostat. Leave the Rc-Rh jumper in place at the 'stat. The C (Common) terminal at the furnace needs to be connected to the C terminal at the 'stat. The Y terminal wire at the furnace needs to be connected to the Y terminal at the 'stat.

The 2 wires coming from your AC condenser energize the contactor at the condenser. One wire will connect to the Y terminal at the furnace and the other will connect to the C terminal at the furnace. The Y terminal at the furnace is a simple junction point and serves only as an easy way to connect the Y terminal from the 'stat to the AC condenser.

When you call for AC (I assume simple AC, not heat pump) go out to the condenser and you should hear a slight hum/buzzing sound of the contactor being energized.

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