1. TH4110U or TH4210U: which do you have?
The T4 Pro is Honeywell Home’s basic programmable thermostat for contractors, sold in two wiring versions. The model number is printed on the box and on the back of the thermostat. Resideo’s T4 Pro Installation Instructions (33-00187EFS-17) cover both, and mark the differences with “TH4210U only”:
| Model | Systems in the guide | AUX/W2 and E |
|---|---|---|
| TH4110U2005 | 1 heat / 1 cool conventional, heat only, cool only, 1H/1C heat pump without backup heat | Not used |
| TH4210U2002 | All of the above, plus a 2H/1C heat pump (compressor plus auxiliary heat) | Used, with an Em Heat mode |
Neither version controls a second compressor stage (Y2) or a second furnace stage: installer option 220 allows 0 or 1 cool stages, and 221 allows one heat stage. If your old thermostat had wires on Y2 or a two-stage furnace on W2, you need a model that supports them, such as the Honeywell T6 Pro. The T4 Pro is a 24 V thermostat; if the old one is wired with thick wires and wire nuts to a baseboard heater, read our line voltage thermostat guide first.
2. The UWP terminals
The UWP wall plate is shared with other Honeywell Pro thermostats, so it has more terminals than the T4 Pro uses. The guide lists each one:
| Terminal | Function on the T4 Pro |
|---|---|
| R | 24 VAC power from the heating transformer |
| Rc | 24 VAC power from the cooling transformer (joined to R by the slider tab) |
| C | 24 VAC common; on two-transformer systems, use the common from the cooling transformer |
| W | Heat (stage 1) on conventional systems; not used on heat pumps |
| Y | Compressor contactor (stage 1) |
| G | Fan |
| O/B | Heat pump changeover (reversing) valve |
| AUX/W2 | Auxiliary heat, TH4210U only |
| E | Emergency heat, TH4210U only |
| K | Connects to K on the THP9045A C-wire adaptor |
| S, S, Y2, U, U, L/A | Not used for the T4 Pro |
Two things on the plate are easy to get wrong. The R slider tab is a built-in jumper between R and Rc: push it up if you have only one R wire (from R, Rc or RH on the old thermostat), and down if you have one wire for R and another for Rc. The two U slider tabs should be left where they are on T4 Pro models. Our Rh and Rc page explains how to tell one transformer from two.
If the old thermostat had a wire on L (a heat pump fault light), the T4 Pro cannot use it: L/A is listed as not used. Tape it off; our L, S and T terminals page explains what you lose.
3. Batteries, C wire and the K terminal
The T4 Pro ships with 2 AA batteries. Resideo’s guide says it can be powered by AA alkaline batteries, by a common wire to C, or by both. With R and C connected, AC power is primary and the batteries are backup; without C, the batteries run it. The guide calls the C terminal optional if batteries are installed, “but it is recommended.”
If you want C but have only four wires (R, W, Y, G) at the thermostat, the guide’s answer is the THP9045A C-wire adaptor. It is fitted at the equipment, and on the thermostat the wire that used to be on Y or G goes to K, which carries both compressor and fan calls. The wire that is freed up becomes your C wire. Our K terminal wire saver page shows the full diagram, and the no C wire guide covers other options.
Use 18 to 22 gauge thermostat wire; the guide says shielded cable is not required.
4. Wiring steps
- Turn off power at the breaker or the furnace switch. The guide repeats this before every mounting step.
- Label each wire with the letter of the terminal it was on before you disconnect it. Go by the letter, not the colour; our wire colour guide explains why.
- Mount the UWP: level it on the wall, mark the holes and drill (7/32 in for red anchors, 3/16 in for yellow), tap in the anchors, pull the wires through the wiring hole and screw the plate on until it no longer moves. Use the cover plate and junction box adapter if you are mounting to an electrical box or covering a paint outline.
- Insert each wire into the push-in terminal with the matching letter. To release a wire, press the terminal tab.
- Set the R slider tab: up for one R wire, down for separate R and Rc wires. Leave the U tabs alone.
- Insert the AA batteries, push excess wire back into the wall, and close the UWP door; it should stay closed without bulging.
- Snap the thermostat onto the UWP and turn the power back on.
- Run installer setup (section 6) before you rely on it, especially on a heat pump.
For a typical one-transformer furnace with air conditioning, that gives R (slider up), C, W, Y and G. The two-transformer version is the same with separate R and Rc wires and the slider down, and C must come from the cooling transformer. If you are not sure which wire is live, our multimeter guide shows the 24 V checks.
5. Heat pump wiring
The guide has two heat pump diagrams. Both use R (slider up), C, Y for the compressor, G for the fan and O/B for the changeover valve. The difference is backup heat:
- 1H/1C heat pump, no backup heat (TH4110U or TH4210U): R, C, Y, G, O/B.
- 2H/1C heat pump with backup heat (TH4210U only): R, C, Y, G, O/B, plus AUX for the auxiliary heat strips and E for an emergency heat relay if your system has a separate E wire.
In both diagrams the guide says of the W terminal: “Do not use this terminal for heat pump applications!” A backup heat wire from W, W2 or X2 on the old thermostat goes to AUX on the TH4210U; a TH4110U cannot control backup heat at all. Our heat pump with emergency heat diagram and aux vs emergency heat guide explain the difference between AUX and E.
Whether your outdoor unit uses O or B is set in software, not by wiring: the same wire goes on O/B either way, and installer option 218 tells the thermostat which one you have (next section). If the old thermostat’s labels are unclear, check the wiring diagram on the outdoor unit or air handler.
6. Installer setup (ISU) after wiring
Press and hold the CENTER button together with the second button shown in the guide for about 3 seconds to open the advanced menu, then press Select to enter ISU. Change a value with the arrow buttons, press Select to confirm, and press Home to save and exit. The options that matter for wiring:
- 200 Heating System Type: 1 conventional forced air, 2 heat pump, 3 radiant (hot water or steam), 5 none (cool only).
- 205 Heating Equipment Type: the fuel or heat pump type, for example 7 air to air heat pump or 8 geothermal.
- 218 Reversing Valve O/B (heat pumps only): 0 = O (energized in cool), 1 = B (energized in heat).
- 220 Cool Stages / Compressor Stages: 0 or 1. Set 0 for heat-only systems.
- 221 Heat Stages / Backup Heat Stages: backup heat stages 0 or 1; set 1 on a TH4210U with AUX wired.
- 230 Fan Control in Heat: shown only for electric forced air or fan coil; 1 lets the equipment control the fan, 2 lets the thermostat do it.
- 387 Compressor Protection: a minimum off time of up to 5 minutes; “Wait” shows on the display while it counts down.
For a Series 20 zone valve or a power-open zone valve, the guide says to set 200 to Radiant Heat and cool stages to 0. The same menu has a TEST entry that turns heat, cool, fan and (TH4210U) Em Heat on and off one at a time to confirm each wire. Compressor protection is bypassed in the test, so the guide says not to cycle the compressor quickly.
7. If something runs wrong
- Display blank: check the AA batteries, then the breaker, the furnace power switch and the furnace door. Without C, the batteries are the only power.
- “Cool On” or “Heat On” flashing: compressor protection is engaged. The guide says to wait 5 minutes.
- Aux heat runs in cooling, or cooling runs on a heat call: on a heat pump, the guide says to check there is no wire on W.
- Heat pump heats in Cool and cools in Heat: check ISU 218; O and B work in opposite modes.
- Heat or cool does not respond: confirm the mode and set point, then allow 5 minutes for the system to respond.
For problems outside the guide, our thermostat troubleshooting checklist covers fuses and transformers, and Honeywell Home’s support site lists current contact options. If you are comparing this model with the touchscreen version, our Honeywell T10 Pro wiring guide uses the same UWP plate but requires a C wire.
8. Frequently asked questions
Does the Honeywell T4 Pro need a C wire?
No. Resideo’s guide says the T4 Pro runs on AA batteries, on a C wire, or on both, and that the C terminal is optional if batteries are installed, though it is recommended. With a C wire the batteries act as backup. If you have only four wires and want C, the guide points to the THP9045A C-wire adaptor and the K terminal; see our K terminal wire saver page.
What is the difference between the TH4110U2005 and TH4210U2002?
Wiring. Both use the same UWP wall plate, but only the TH4210U has working AUX/W2 and E terminals and an Em Heat mode. That makes it the model for a heat pump with backup heat (2 heat, 1 cool). The TH4110U handles one heat and one cool stage, either a conventional system or a heat pump with no auxiliary heat.
Where does the W wire go on a T4 Pro with a heat pump?
Nowhere. The guide says not to use the W terminal for heat pump applications. Backup heat goes on AUX and, if you have a separate emergency heat wire, E, and only the TH4210U has those. A wire on W with a heat pump is the first thing the guide checks when aux heat runs in cooling or cooling runs on a heat call.
How do I set O or B on the T4 Pro?
In installer setup (ISU), set 200 Heating System Type to 2 (Heat Pump). Option 218 Reversing Valve O/B then appears: 0 = O, energized in cool, and 1 = B, energized in heat. If the heat pump cools when you ask for heat, this setting is usually reversed. Our O/B terminal page explains which equipment uses which.
Which way should the R slider tab go?
Up if there is only one R wire (whether it was on R, Rc or RH on the old thermostat), which joins R and Rc inside the plate. Down if you have one wire for R and a separate wire for Rc, which keeps two transformers apart. Our Rh and Rc page explains dual transformer systems.