1. T6 Pro terminals used by a heat pump
This applies to the three T6 Pro models in Honeywell’s T6 Pro installation instructions (form 33-00181EFS): TH6320U2008, TH6220U2000 and TH6210U2001. The wall plate (UWP) has more terminals than a heat pump needs. These are the ones in Honeywell’s heat pump diagrams:
| Terminal | Honeywell’s description | Heat pump use |
|---|---|---|
| R | 24 VAC power | The power wire; R and Rc are joined by the R slider tab |
| C | 24 VAC common | Optional with batteries, recommended |
| Y | Compressor contactor (stage 1) | Runs the heat pump in both heat and cool |
| Y2 | Compressor contactor (stage 2) | Two-stage heat pumps, TH6320U only |
| G | Fan | Indoor blower |
| O/B | Changeover valve | Reversing valve; O or B is chosen in ISU 218 |
| Aux (shared with W2) | Auxiliary heat | Backup heat strips, or the furnace on dual fuel |
| E | Emergency heat | Separate emergency heat wire, if there is one |
| L/A | Heat pump fault input | Optional; needs a C wire |
| W | Heat (stage 1) | Not used with a heat pump |
Not every model has every option: the manual notes that the TH6210U has one compressor stage, and the TH6220U has one compressor stage when set up as a heat pump. The 2H/2C and 3H/2C heat pump diagrams are TH6320U only.
2. Moving each old wire to the T6 Pro
Go by the letter on the old thermostat’s terminal, not the wire color; colors are not standardized (see thermostat wire colors). Take a photo before you disconnect anything.
- R, Rh or Rc (one wire) → R, with the slider tab up (1 wire). Two separate R wires go on R and Rc with the tab down. More in Rh and Rc explained.
- Y or Y1 → Y. Y2 → Y2 (TH6320U).
- G → G.
- O, or B on a Rheem or Ruud heat pump → O/B. The O/B wire page explains which brands use which.
- B on an old thermostat that also has an O wire → usually C, not O/B. That B is the transformer common; on O/B it would short the transformer. Confirm with the B wire guide.
- W2, Aux or X2 → Aux. E → E.
- W or W1 on an old heat pump thermostat → don’t put it on the T6 Pro’s W. On some thermostats (ecobee in heat pump mode, for example) W1 is the backup heat output, so check where the wire lands at the air handler; if it is the heat strip or furnace input, it goes on Aux.
- C → C.
If any old label isn’t listed, the wiring tool will tell you what it most likely is, and the heat pump thermostat wiring guide covers O/B, Aux and E in more depth.
3. Wiring steps
- Turn off power to the heat pump and air handler at the breaker. Honeywell’s manual warns of shock or equipment damage otherwise.
- Label and photograph the old wiring, then remove the old thermostat.
- Mount the UWP on the wall and pull the wires through it.
- Set the R slider tab: up for one R wire, down for separate R and Rc wires.
- Insert each wire in its terminal as mapped above. Leave W empty.
- Insert the two AA batteries if you have no C wire. With a C wire they act as backup.
- Close the UWP door, snap the thermostat on and turn the breaker back on.
- Run installer setup as described in the next section before you test heating or cooling.
Honeywell specifies 18 to 22 gauge thermostat wire; shielded cable isn’t needed. If you are running new cable, see thermostat wire gauge and running new wire.
4. Installer setup (ISU) for a heat pump
To open installer setup, press and hold the center button and the arrow button shown in the manual for about 3 seconds to open the advanced menu, then press Select to enter ISU. Select moves through the settings, the arrow buttons change values, and Home saves and exits. The settings that matter for a heat pump:
| ISU | Setting | Heat pump value |
|---|---|---|
| 200 | Heating System Type | 2 = Heat Pump |
| 205 | Heating Equipment Type | 7 = Air to Air Heat Pump, 8 = Geothermal Heat Pump |
| 218 | Reversing Valve O/B | 0 = O (O/B in Cool), 1 = B (O/B in Heat) |
| 220 | Compressor Stages | 1 or 2 (2 only on TH6320U) |
| 221 | Backup Heat Stages | 0 with no backup heat, 1 with Aux |
| 253 | Aux/E terminal control | 0 = Aux and E together (fixed except on TH6320U), 1 = independent |
| 255 | Backup Heat Source | 1 = Electric, 2 = Gas/Oil (dual fuel); TH6320U and TH6220U |
Honeywell’s footnotes match the diagrams: a 2H/1C heat pump is 1 compressor and 1 stage of backup heat, 2H/2C is 2 compressors and 0 backup stages, and 3H/2C is 2 compressors and 1 backup stage. For a gas or oil furnace as backup (dual fuel), the manual’s diagram adds an outdoor sensor on the S terminals, set in ISU 130. Which O/B setting your heat pump needs is in the outdoor unit’s manual; when in doubt, try one and test.
5. Problems after installing a T6 Pro on a heat pump
- Cold air in heat mode, or warm air in cool mode. ISU 218 is the wrong way round for your heat pump. Switch it and test again.
- Aux heat runs while cooling, or cooling runs on a call for heat. Honeywell’s troubleshooting table points to a wire on W. Move it to Aux or remove it.
- “Cool On” or “Heat On” flashing. The compressor protection delay is running. Honeywell says to wait 5 minutes.
- Blank display. Check the breaker, the system power switch, that the air handler door is closed securely, and the AA batteries.
- Aux heat runs a lot in cold weather. That can be normal, but check the stages and backup heat settings; our guide to aux heat running too much goes through the causes.
For other faults, use the thermostat troubleshooting checklist or test the wires with a multimeter.
6. Frequently asked questions
Where does the O/B wire go on a Honeywell T6 Pro?
On the O/B terminal. The wire is the same whether your heat pump uses O or B; what changes is installer setting 218, Reversing Valve O/B: 0 = O (energized in cool), 1 = B (energized in heat). Most brands use O; Rheem and Ruud use B. See the settings table.
Can I use the W terminal on a T6 Pro with a heat pump?
No. Every heat pump diagram in Honeywell’s T6 Pro manual marks W “Do not use this terminal for heat pump applications”. Backup heat goes on Aux (the same terminal as W2) and emergency heat on E. The manual’s troubleshooting table says aux heat running in cooling, or cooling running on a call for heat, means a wire is on W.
Does the T6 Pro need a C wire for a heat pump?
Not for basic operation. Honeywell says the C terminal is optional if AA batteries are installed, but recommended. The exception is the L/A heat pump fault input, which the manual says needs a C wire. If you have no C wire, read the no C wire guide.
My heat pump blows cold air in heat mode after installing a T6 Pro. What is wrong?
Usually ISU 218 is set the wrong way for your heat pump. Change it from O to B, or B to O, and test again after the 5-minute compressor protection delay. A wrong O/B setting doesn’t damage anything; it just runs the opposite mode. If the setting is right, check that no wire is on W.
Do I need a jumper between Aux and E on the T6 Pro?
Honeywell’s manual doesn’t call for one. ISU 253 controls the two terminals: 0 drives Aux and E together, and the manual says it is fixed at 0 on every model except the TH6320U. With one backup heat wire, put it on Aux. On a TH6320U with separate Aux and E wires, setting 253 to 1 runs them independently.