1. Which Sensi you have
Sensi thermostats were sold under the Emerson name and are now made by Copeland, so older links to sensi.emerson.com may no longer open. Current help pages are on sensi.copeland.com. The model number is printed on the back of the thermostat cover. There are four models in Sensi’s help pages:
- Sensi smart thermostat: models 1F87U-42WF and ST55. Most of this article follows its installation guide (November 2024 version).
- Sensi Lite smart thermostat: battery powered like the Sensi smart, but not compatible with two-transformer systems.
- Sensi Touch and Sensi Touch 2: color touchscreen models that always need a C wire.
The wiring rules below are Sensi-specific. Other brands handle the C wire and jumpers differently, so don’t carry them over to a Nest, ecobee or Honeywell.
2. When Sensi needs a C wire
Sensi’s article Does my Sensi thermostat need a C wire? and its common wire page give these rules:
| Model | Furnace + AC (conventional) | Heat only, cool only, heat pump or zone system |
|---|---|---|
| Sensi smart | No C wire needed (batteries) | C wire required |
| Sensi Lite | No C wire needed (batteries) | C wire required |
| Sensi Touch, Touch 2 | C wire required | C wire required |
If you need a C wire and the old thermostat has none, Sensi lists three options:
- An unused wire. There may be a spare wire in the bundle that the old thermostat did not use. Connect it to C at the thermostat and to the 24 VAC common on the HVAC control board, checking the board’s wiring diagram.
- The G (fan) wire. On some systems the fan wire can be used as C. The Sensi app tells you when this is possible.
- The Sensi conversion kit (SA11). Sensi says it lets you use four wires without running new thermostat cable.
Our no C wire guide compares these with other brands’ adapters, and the C wire terminal page explains what the common wire does.
3. Terminals and the two label sets
Sensi’s terminal selection article says the type of system decides which labels you use. If the old thermostat has two sets of labels, use the set that matches your system:
- Conventional (furnace or boiler, with or without AC): R, Y, G, W, W2, Y2, C.
- Heat pump: R, Y, O/B, W, Aux, E, C.
Sensi’s rule of thumb: if the outdoor unit runs in fall or winter, you may have a heat pump. If the only heat comes from an indoor furnace or boiler, it is conventional. If you are unsure, look up the outdoor unit’s model number. The wiring diagrams in the install guide assign the terminals like this:
| Terminal | Job on a Sensi |
|---|---|
| Rc, Rh | 24 V power for cooling (Rc) and heating (Rh). Joined by the built-in jumper. |
| C | 24 V common wire. |
| Y, Y2 | First and second stage cooling, or the compressor stages on a heat pump. |
| G | Indoor blower (fan). |
| W, W2 | First and second stage heat on a conventional system. |
| O/B | Heat pump reversing valve, set to O or B in the installer menu. On a conventional system it can serve a zone panel or 3-wire hot water zone valve. |
| Aux, E | Auxiliary and emergency heat on a heat pump. |
| L | Connection to a heat pump’s L terminal. |
A wire marked B on an old thermostat is not always the reversing valve. On some systems it is the common wire, which our Rheem and Ruud B wire guide explains. Check where it lands on the control board before you choose O/B or C in the app. The heat pump wiring guide covers the rest of a heat pump install.
4. The built-in Rc/Rh jumper
The install guide says to throw away any jumper wires from the old thermostat, because Sensi has one built in. Sensi’s jumper wire article gives its location:
- Sensi smart and Sensi Touch 2: on the back of the thermostat face plate.
- Sensi Touch: on the thermostat sub-base.
Leave it in place if you have one R wire. Clip it only if two separate wires come from the wall, one to Rh and one to Rc. That means the heating and cooling equipment have separate transformers. The Sensi Lite has no option for this, as Sensi says it is not compatible with two-transformer systems. Our two-transformer wiring guide explains how to confirm the setup.
5. Wiring a Sensi, step by step
These steps follow the install guide for the Sensi smart thermostat. The app walks you through the same sequence when you choose New Installation and then Not Mounted, which Sensi recommends for DIY installs.
- Turn off power to the heating and cooling system before touching any thermostat wire.
- Remove the old thermostat cover and photograph the wiring. The app prompts you to do this, and the photo helps if you need support later.
- Throw away any jumper wires from the old thermostat.
- Choose the terminals in the app that have wires on the old thermostat, then pick your furnace type. The app tells you whether the configuration is valid.
- Label each wire with the supplied stickers as you disconnect it, then remove the old base.
- Mount the Sensi base with the supplied screws.
- Push each wire into its terminal and tighten it with a screwdriver.
- Attach the display and turn the power back on, then connect to Wi-Fi in the app.
If you are not sure a wire carries 24 V, measure it first with the multimeter guide.
6. Equipment and O/B settings
Sensi’s configuration article says to press Menu, then Next to step through the temperature scale, outdoor equipment, indoor equipment and reversing valve screens, and Exit when finished.
| Setting | Options |
|---|---|
| Outdoor equipment | AC0 no cooling, AC1 single-stage AC, AC2 two-stage AC (needs Y2), HP1 single-stage heat pump, HP2 two-stage heat pump |
| Indoor equipment | GA1 or GA2 gas, oil or boiler (GA2 needs W2), EL1 or EL2 electric, FAN no indoor heat |
| Reversing valve | O (default) or B, heat pumps only |
Leave the reversing valve on O unless the heat pump heats in Cool mode and cools in Heat mode. Then change it to B. Sensi lists wiring errors as a cause of these symptoms: the system runs past the set point, blows hot air in Cool or cold air in Heat, heats and cools at the same time, or won’t turn on.
7. Systems Sensi will not work with
- Line voltage: Sensi’s rated input is 20 to 30 VAC, so 120 or 240 V baseboard and wall heaters are out.
- Other terminal schemes: Sensi says it is not compatible if the old thermostat uses terminals 1 to 4, A to D, V+/VG/RS-/RS+, or R/1/2/C.
- Two transformers on a Sensi Lite.
When in doubt, run Sensi’s compatibility checker before you buy, or tick your wires in our wiring tool to compare other thermostats.
8. Frequently asked questions
Does a Sensi thermostat need a C wire?
It depends on the model and the system. Sensi says the Sensi smart and Sensi Lite smart thermostats work on most conventional heating and cooling systems without a C wire, because they run on batteries. They need one on heat-only, cool-only, heat pump and zone systems. The Sensi Touch and Touch 2 always need a C wire.
Do I need the jumper wire from my old thermostat?
No. The install guide says to throw away any jumper wires from the old thermostat, because Sensi has an Rc/Rh jumper built in. Only clip that built-in jumper if two separate wires came from the wall, one on Rh and one on Rc. That means two transformers. Sensi says the Sensi Lite is not compatible with two-transformer systems.
Should the Sensi reversing valve be set to O or B?
Sensi’s configuration article says to leave the default, O. Change it to B only if the heat pump heats in Cool mode and cools in Heat mode. Your outdoor unit’s manual says which one it uses. See the O/B terminal page for what the setting does.
Why does the Sensi app say my wiring is an invalid configuration?
The app checks the terminals you tick against the systems Sensi supports. Common causes are a wire on a terminal that was only a jumper, picking the conventional labels on a heat pump (or the other way round), or a missing C wire on a system that needs one. Recheck the photo of your old wiring, then use Sensi’s compatibility checker.
Will Sensi work with baseboard heaters?
No. Sensi says line voltage systems cannot be used, and its rated input is 20 to 30 VAC. Electric baseboard heaters usually run at 120 or 240 V. See our line-voltage thermostat guide for what to use instead.