Honeywell RTH9585WF Wiring: C Wire, K Terminal and Heat Pumps

1. Check your system first

The RTH9585WF User Guide says the thermostat works with common 24 volt systems: forced air, hydronic, heat pump, oil, gas and electric. It will not work with millivolt systems such as a gas fireplace, or with 120/240 volt systems such as electric baseboard heat. See our line voltage guide.

Then look at the old thermostat with the power off. If no wire sits on a terminal marked C, or there is no C terminal, read section 5 before you buy or install anything. The guide also warns that not every system labels the 24 VAC common as C, so check the equipment manual if the letters look unfamiliar.

2. Where each wire goes

The wallplate has ten terminals: C, K, Rc and R on the left, W-O/B, Y, G, W2-Aux/E, Y2 and L on the right. The table combines the guide’s two wiring pictures and its alternate wiring keys. Go by the letter on the old terminal, not the color, as the guide instructs. Our wire color guide explains why.

Two old labels need care. X and B mean different things on different equipment, so confirm them before you connect anything: see our pages on X and X2 wires and the B wire.

3. Wiring it, step by step

These steps follow the user guide. See also our guide to replacing a thermostat safely.

1. Switch off power at the breaker box or the system switch.
2. Remove the old faceplate, leave the wires connected, and photograph the connections.
3. Label each wire with the sticky tags as you disconnect it, using the old terminal letter. Write the letter on a blank tag if none matches.
4. Remove the old wallplate. Wrap the wires around a pencil so they do not fall back into the wall.
5. Mount the new wallplate. If you need new holes, drill 3/16 inch for drywall or 7/32 inch for plaster.
6. Connect the wires, starting with C. Straighten each wire and slide it in until it clicks. To remove one, press the terminal release with a pen tip.
7. Tick the checklist in the guide for each wire and tug each one gently. You use this list during setup.
8. Deal with the jumper loop. Remove it only if you have both R and Rc wires.
9. Push spare wire back into the wall and snap the thermostat onto the wallplate.
10. Before power-up, confirm the C wire is connected at both ends and the furnace door is firmly closed. Then switch the power back on.

If the screen stays dark, test for 24 V between R and C using our multimeter guide.

4. Heat pumps: O/B, Aux and E

A heat pump uses the same steps with different letters; the heat pump wiring guide explains each job. Three cases from the user guide’s heat pump key come up often:

• Separate O and B wires. On a single-zone heat pump, O goes in W-O/B and the guide says to attach B to C. If more than one thermostat controls the system, call Resideo’s zoning line, listed in the guide, rather than guess.
• Separate Aux and E wires. Use a wire nut to join both to a third short wire, and put that wire in W2-Aux/E. There is no separate E terminal.
• Separate Y1, W1 and W2, or V and VR wires. The guide sends you to Resideo’s site for help.

The L terminal is the system monitor. If it finds a problem, an orange alert button appears on the home screen. See our page on L, S and T terminals.

Emergency Heat appears in the MODE menu on heat pumps with auxiliary heat. It runs the auxiliary heat and locks out the compressor. See aux heat vs emergency heat for when to use it.

5. No C wire: the K terminal adapter

If a spare wire is unused at both the thermostat and the control board, connect it to C at both ends. Otherwise use a C-wire adapter. Newer boxes list a C-wire adapter and addendum in the user guide’s contents, but older printings of the guide do not, so check yours. Sold separately, Honeywell’s current adapter is the THP9045A2098.

The THP9045A2098 installation guide does not name thermostat models, so follow the addendum in your box if it differs. The adapter goes at the furnace or air handler and works like this:

1. With the power off, label the G, R, W and Y wires that run to the thermostat at the control board.
2. Move them to the adapter’s thermostat side. G goes to C and Y goes to K. R and W pass straight through. On a heat pump, the O or B wire takes the W position.
3. Connect the adapter’s equipment side to the matching board terminals, then fix the adapter in place.
4. At the thermostat, relabel the G wire as C and the Y wire as K. Connect them to C and K, with no wires in Y or G.

On a zoned system, the adapter goes between the thermostat and the zone panel, not between the panel and the equipment. The adapter guide does not cover heat-only systems. With only two wires and no G or Y, plan on new cable; our no C wire guide compares the options, and the 2-wire heat-only setup shows why that case is harder.

6. Setup screens and troubleshooting

After power-up the thermostat asks about your equipment before it joins Wi-Fi. Use your wiring checklist to answer. These are the options that depend on your wires, from the guide’s System Setup table:

• Your system type: Forced Air, Heat Pump, or Hot Water or Steam.
• Number of heating or cooling stages: 1 or 2. Y2 connected means 2 cool stages; W and W2 connected means 2 heat stages.
• Type of changeover valve (heat pump): cooling valve (default) or heating valve. Honeywell’s support article for the sister RTH6500WF says O/B is most commonly energized in cooling, and that Rheem and Ruud need it energized in heating. Our O/B page explains why.
• Number of heat pump compressor stages: 1 or 2, set by whether Y2 is connected.
• Your backup heat: Yes (default) or No, set by whether W2-Aux/E is connected.

From the guide’s troubleshooting table:

• Blank display: check the breaker and the system power switch, make sure the furnace door is closed, and make sure the C wire is connected.
• “Wait” on screen: compressor protection is running. Wait 5 minutes for the system to restart safely.
• Cool air in Heat mode, or warm air in Cool mode, on a heat pump: check Type of changeover valve.

For anything else, work through our troubleshooting checklist.

7. Frequently asked questions