Smart thermostat for a heat pump: what to check
Updated
Heat pumps need a thermostat that handles reversing valves and backup heat. Check your wires, the aux heat controls and compatibility before you buy so it does not run expensive strips.
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How a heat pump system differs from a furnace and AC
A furnace and air conditioner have separate heating and cooling equipment. A heat pump does both with one outdoor unit: it moves heat from outside to inside in winter and reverses in summer. The Department of Energy says a modern heat pump can cut electricity use for heating by up to 75% compared with electric resistance heat, and that cold-climate models are designed to work at temperatures as low as 5°F.
That design changes what the thermostat must do. It has to tell the outdoor unit which way to run (the reversing valve), manage one or two compressor stages, and decide when to bring on auxiliary heat: electric resistance strips in the air handler or a gas furnace that backs up the heat pump on very cold days. Those strips are the expensive part of a heat pump system, because they use as much electricity per unit of heat as a space heater.
Check your wiring first
Turn off power to the system at the breaker, take the old thermostat’s faceplate off and photograph the wires and terminal letters. Read the letters, not the colors. Then compare with this table.
| Terminal | What it does on a heat pump | What to check |
|---|---|---|
| R / Rc / Rh | 24 V power from the transformer | One R wire is typical; two means separate transformers |
| C | Common wire, powers the thermostat | If missing, see no C-wire options |
| Y / Y1 / Y2 | Compressor stage 1 / stage 2 | Two Y wires mean a two-stage heat pump; the thermostat must support it |
| O/B | Reversing valve | O energizes in cooling, B in heating; the thermostat must be set to match |
| AUX / W2 / W | Backup (auxiliary) heat | Needed for heat strips or a dual-fuel furnace |
| E | Emergency heat, which bypasses the compressor | Some thermostats combine it with AUX |
| G | Indoor fan | Needed for fan control |
The O/B setting is the one that trips people up. If it is wrong, the system cools when you ask for heat. Every compatible thermostat has a setup menu for it; if you are not sure which your equipment uses, check the heat pump’s manual or ask the installer. Do not guess.
Auxiliary heat: the setting that decides your bill
A heat pump’s output drops as the outdoor temperature falls, so on cold days a backup is needed. The problem is when the thermostat calls for it too early. If you set the temperature back overnight and ask for a 5°F jump in the morning, a basic thermostat sees a big gap and turns on the strips to catch up. The Department of Energy warns that setting back a heat pump thermostat can cause the unit to run inefficiently and cancel out any savings, and it points to thermostats made for heat pumps that use special algorithms to minimize backup electric heat.
Look for these controls in the specification and in the app:
- Aux heat lockout by outdoor temperature. The thermostat blocks the strips above a temperature you set, so the compressor does all the work in mild weather. Ask your installer or read the manual for the point where your heat pump needs help; do not copy a number from a forum.
- Adaptive or gradual recovery. The thermostat raises the temperature in steps with the compressor alone.
- Compressor-only mode or limits on strip heat, and clear on-screen indication when aux is running.
- Dual-fuel support. If you have a heat pump plus a gas furnace, the thermostat should switch to the furnace below an outdoor temperature you choose.
- Staging for two-stage or variable-speed units.
Emergency heat is different. It runs the strips only and skips the compressor. Use it only when the outdoor unit has failed, and call for service, because it is the most expensive way to heat.
Set it up to protect savings
- Use a modest setback, such as a couple of degrees at night, and check the app’s history for aux heat runtime during the first cold week.
- Prefer holding a steady temperature on the coldest days.
- Turn on “smart recovery” or the equivalent and make sure aux lockout is set.
- Look at your utility bill after the first month. A jump in kWh per heating degree day means aux heat is running more than it should.
ENERGY STAR notes that a smart thermostat must be compatible with your heating and cooling system. For the highest-efficiency variable-speed equipment, a controller from the same company may be worth considering.
Systems where a standard smart thermostat does not fit
- Communicating systems. Some variable-speed heat pumps use a digital bus (often four wires labeled A, B, C, D or similar) with the maker’s own thermostat. A standard smart thermostat may not work or may disable features. Ask the installer.
- Ductless mini-splits. They use their own remote and indoor controllers, not 24 V wiring. See smart control for mini-splits.
- Electric baseboard or other line-voltage heat. See smart thermostats for baseboard heat.
Test the installation
- Call for heat and confirm the outdoor fan and compressor start and warm air comes out within a few minutes.
- Call for cooling and confirm the system cools. If it heats, the O/B setting is reversed.
- Raise the setpoint a few degrees and check that the app shows heat pump stage 1 first and AUX only after a long delay.
- Confirm which mode is labeled emergency heat and that it is off.
Shop by need
Heat pump with electric heat strips
What to look for: O/B reversing valve support, AUX/E terminals, aux heat lockout by outdoor temperature, adaptive recovery, ENERGY STAR certification
Typical price: $120–$280
See options on AmazonHeat pump plus gas furnace (dual fuel)
What to look for: dual-fuel mode with a changeover outdoor temperature, two-stage compressor support, weather-based balance point settings
Typical price: $130–$280
See options on AmazonHeat pump with no C-wire
What to look for: included power extender or adapter, explicit heat pump support in the compatibility checker, O/B and AUX terminals
Typical price: $120–$250
See options on AmazonFrequently asked questions
Will any smart thermostat work with a heat pump?
No. It must support the reversing valve (O/B) and, if you have backup heat, an AUX or W2 terminal. Use the maker’s compatibility checker with your wiring photo before you buy.
What is auxiliary heat on a heat pump thermostat?
It is backup heat, usually electric resistance strips in the air handler, used when the heat pump cannot keep up. It costs more per unit of heat than the compressor, so you want it to run as little as possible.
Should I use setback with a heat pump?
Use small setbacks. The Department of Energy warns that setting back a heat pump thermostat can cancel savings if the unit then calls for backup heat to recover. Thermostats made for heat pumps manage recovery with the compressor first.
What is the difference between aux and emergency heat?
Aux heat works together with the compressor when needed. Emergency heat turns the compressor off and runs only the strips or furnace. It is meant for a failed outdoor unit.