1. Which T10 this covers
Honeywell Home’s T10 Pro is sold through contractors, so the wiring instructions live in Resideo’s professional literature rather than a retail box. Resideo’s current T10 and T10+ Pro install guide (33-00605EFS) covers four models: THX321WFS2001W and THX321WF2003W (T10, with and without the room sensor) and THX321WFS3001W and THX321WF3003W (T10+). The original T10 Pro guide (33-00423EFS) covers the first two. The wiring is the same on all four. The T10+ adds an optional Equipment Interface Module (EIM), THM04R3000; when an EIM is used, the guide says to wire only R and C to the thermostat and to follow the EIM’s own diagrams.
Both guides list what the T10 Pro does not do: electric baseboard heat at 120 to 240 V and millivolt systems. If your old thermostat is wired to L1 and L2, read our line voltage thermostat guide instead. If you were looking at the retail T9, its base is similar but not identical; see our Honeywell Home T9 page.
2. The UWP terminals
The T10 Pro clips onto Honeywell’s UWP mounting system. Wires go into the inner hole of each terminal while you press the tab, one wire per terminal, and you tug each one to check it holds. The “Terminal designations” table in the install guide lists them by system type:
- R and Rc: 24 V from the heating transformer and the cooling transformer. One transformer uses R alone with the slider tab joining Rc.
- C: the 24 V common, taken from the cooling transformer if there are two. See our C wire page.
- Y and Y2: compressor stage 1 and 2 (cooling on a conventional system, cooling and heating on a heat pump).
- G: fan relay.
- W and W2: heat stage 1 and 2 on conventional systems only.
- O/B: the heat pump changeover (reversing) valve.
- Aux and E: backup heat and emergency heat on a heat pump. On the base these share positions with W2 and W, which is why the heat pump column is different.
- L/A: heat pump fault input (the outdoor unit’s L wire).
- K: connects to K on the THP9045A C-wire adapter, replacing Y and G.
- U/U: a relay for a humidifier, dehumidifier or ventilator.
- S/S: input for a wired indoor or outdoor sensor.
If your old thermostat has letters this list does not, our terminal letter index says what each one usually means.
3. The R and U slider tabs
The UWP has two small sliders that replace the metal jumpers of older bases. Resideo’s “Setting Slider Tabs” section is specific:
- R slider up (1 wire): only one power wire, which was on R, Rc or RH on the old thermostat. Put it on R.
- R slider down (2 wires): one wire on the old R and another on the old Rc, meaning separate heating and cooling transformers. Put them on R and Rc.
- U slider down (2 wires): the U terminals are a dry pair of contacts for an accessory with its own transformer.
- U slider up (1 wire): for an accessory powered by the cooling transformer; the lower U is then jumped internally to Rc.
Leave the U slider down unless you are wiring a humidifier, ventilator or damper. The product data’s dual fuel and heat pump figures all show the R/Rc switch up with one transformer.
4. Resideo’s diagrams as one table
The install guide draws each system separately. This table reads across four of them. Each cell is what the terminal connects to; your old thermostat’s wire with the same function goes there.
| Terminal | Furnace + AC, 1 transformer | Furnace + AC, 2 transformers | Heat pump, 1H/1C | Heat pump with backup heat (2H/1C) |
|---|---|---|---|---|
| R | Power | Heating transformer | Power | Power |
| Rc | Joined by slider (up) | Cooling transformer (slider down) | Joined by slider (up) | Joined by slider (up) |
| C | 24 V common | Common from cooling transformer | 24 V common | 24 V common |
| Y | Compressor contactor | Compressor contactor | Compressor contactor | Compressor contactor |
| G | Fan relay | Fan relay | Fan relay | Fan relay |
| W | Heat relay | Heat relay | Empty | Empty |
| O/B | Empty | Empty | Changeover valve | Changeover valve |
| Aux | Empty | Empty | Empty | Auxiliary heat |
| E | Empty | Empty | Empty | Emergency heat relay (if a separate wire) |
| L/A | Empty | Empty | Empty | Heat pump fault input |
Two-stage equipment adds W2 (heat stage 2) or Y2 (compressor stage 2) in the same way. The guide also draws heat-only with fan, cool-only with fan, a hot water relay panel and two zone valve cases; if your boiler uses Y to power a valve closed, the guide says to configure the thermostat as radiant heat without cooling.
5. Wiring steps
- Turn off power to the heating and cooling system at the breaker or the furnace switch.
- Photograph the old wiring and label each wire by its terminal letter, not its colour. Our wire colour guide explains why colours cannot be trusted.
- Remove the old base and mount the UWP, with the junction box adapter if the box is exposed. Resideo specifies 18 to 22 gauge thermostat wire; shielded cable is not needed.
- Set the R slider up for one power wire or down for two, and leave the U slider down unless you wire an accessory.
- Insert each wire into the inner hole of its terminal with the tab pressed, then tug to confirm. Use the table above; on a heat pump, nothing goes on W.
- Push excess wire back into the wall and close the UWP door so it does not bulge.
- Snap on the thermostat, restore power and follow the on-screen setup, which asks about the system type and, on a T10+, whether an EIM is used.
Not sure a wire is live, or whether a spare wire really reaches the furnace? Our multimeter guide shows the 24 V check between R and C.
6. No C wire: the K terminal adapter
Resideo’s wiring note is firm: “This thermostat requires a C-Wire. If a C-Wire is not available and the system uses Y and G, use C-Wire adapter accessory THP9045A1098 or THP9045A1023.” The terminal table explains how it works: the adapter is for heat/cool systems with only four wires at the thermostat. You use the K terminal in place of Y and G, so one wire carries both the fan and compressor calls, and the wire that was G becomes your common. The adapter at the furnace splits K back into Y and G.
That means the adapter does not help a heat-only system with no cooling, because there is no Y to combine. In that case a spare wire or new cable is the fix; see our guide to running new thermostat wire. The step-by-step furnace side of the wire saver is on our K terminal and wire saver wiring page.
7. Heat pumps: O/B, Aux, E and the W trap
The heat pump diagrams in the install guide put the changeover valve on O/B, the compressor on Y, and backup heat on Aux with a separate E for an emergency heat relay. Below them is the rule most installs get wrong: “Do NOT use W for heat pump applications. Auxiliary heat must wire to AUX or E.” The product data repeats it in capitals and adds the reason: a wire on W can make heat run continuously. The guide’s footnote covers the common case of one backup wire: “If you do not have separate wires for the Aux and E terminals, connect the wire to the Aux terminal.”
So if your old thermostat had its strip heat on W2 or AUX, it goes on Aux. If it had a wire on E, that goes on E. If it had wires on both W and W2 and you have a heat pump, the W wire was usually the first backup stage on an older layout; confirm what it does at the air handler before deciding. Our aux heat vs emergency heat guide explains the difference in operation, and the E terminal page covers the E wire alone.
The guide does not say whether your valve is an O or a B type. The product data states the test: the valve is labelled O if it is energized in cooling and B if it is energized in heating. Our O/B terminal page shows how to tell from the old thermostat and the outdoor unit. Rheem and Ruud heat pumps are the usual B type; most others are O.
Two-stage heat pumps add Y2, and the fault wire from the outdoor unit goes on L/A, which the diagrams label “heat pump fault input”. For what the L wire does, see our L, S and T terminals page.
8. Setup numbers that match the wiring
The T10 Pro walks through its installer setup (ISU) on screen. The ISU table in Resideo’s T10 and T10+ product data (33-00462) gives the numbers you may be asked for by a contractor or support:
- ISU 2000, System Type: Conventional Forced Air Heat, Heat Pump, Radiant Heat, Fan Coil (T10+), Other, or None (cool only). Pick Heat Pump whenever a wire is on O/B.
- ISU 2010, Equipment type: for heat pumps, Air to Air or Geothermal.
- ISU 2060, Reversing Valve: O (O/B on Cool) or B (O/B on Heat).
- ISU 2175, Backup Heat: drive Aux and E together, or keep them independent. With only an Aux wire, this setting does not appear.
- ISU 2180, Backup Heat Type: electric, or gas/oil for a dual fuel system.
The product data also notes that Emergency Heat mode is only offered when the system type is Heat Pump and the backup heat stages are set to 1 (ISU 2071).
9. If the system runs wrong
The install guide’s troubleshooting table ties three symptoms to the same wiring mistake. If aux heat runs during cooling, cooling runs on a call for heat, or heat runs with cooling on a heat pump, it says to verify there is no wire attached to W. Other rows:
- Blank display or no response: check the breaker, the power switch at the furnace, and that the furnace door is closed securely (the door switch cuts 24 V).
- “Cool On” or “Heat On” flashing: compressor protection is running; wait five minutes.
- Sensor will not connect: hold Connect on the sensor for 15 seconds until the LED turns amber, then add it under Menu, Devices and Sensors.
If the heat pump itself misbehaves after a clean install, our aux heat running too much guide covers the settings side, and the general thermostat troubleshooting checklist covers fuses and transformers. Honeywell Home’s support site has the current app and firmware notes.
10. Frequently asked questions
Does the Honeywell T10 Pro need a C wire?
Yes. Both Resideo install guides say the thermostat requires a C wire. If none is available and the system uses Y and G, the guide points to the THP9045A1098 or THP9045A1023 C-wire adapter: at the thermostat, one wire goes on K in place of Y and G, and the freed wire becomes C. Our no C wire guide covers the other options.
Where does the aux heat wire go on a T10 Pro with a heat pump?
On Aux, or on E if you have a separate emergency heat wire. Resideo’s note reads “Do NOT use W for heat pump applications. Auxiliary heat must wire to AUX or E.” If you have only one wire for backup heat, the guide says to connect it to Aux. See aux heat vs emergency heat.
Which way does the R slider tab go?
Up (1 wire) if your old thermostat had a single power wire on R, Rc or RH. Down (2 wires) if one wire was on R and another on Rc, which means two transformers. Our Rh and Rc page explains how to tell.
What is the L/A terminal on the T10 Pro?
In the heat pump diagrams it is the heat pump fault input: the outdoor unit’s L wire lands there so the thermostat can show a fault. In the product data it also serves as a dry-contact changeover input on the T10+ for 2-pipe fan coils. On a heat pump, use it only if your old thermostat had a wire on L.
Can the T10 Pro run a dual fuel system?
Yes. The short install guide has no dual fuel diagram, but Resideo’s T10 product data includes one, with the heat pump and furnace on separate boards. During setup, choose Heat Pump at ISU 2000 and set the gas or oil furnace as the backup heat type at ISU 2180. Our heat pump wiring guide explains the dual fuel terminals.