A thermostat controlling a heat pump needs four things set correctly: the system type (heat pump, with the reversing valve direction — O or B — right), an auxiliary heat lockout so the electric strips can’t run in mild weather, small setbacks or heat-pump-aware recovery so the strips aren’t called every morning, and the fan on Auto. Get those right and a heat pump in North Carolina runs cheaply; get them wrong and it runs the strips for hours, which is the most common cause of winter bill shock in this climate.
The settings that matter
System type. In the thermostat’s installer or equipment menu, the system must be set to heat pump (not conventional/furnace) and the reversing valve to the correct O/B setting for your brand. Wrong here and the thermostat may treat aux as primary heat, or cool when it should heat.
Auxiliary heat lockout. Sets the outdoor temperature above which the thermostat won’t call the strips. Many defaults are absent or too high. In the Piedmont a lockout around 35–40°F is a sensible starting point (your contractor can tune it to the system’s balance point); the heat pump handles everything above that on its own. Some thermostats call this “aux heat threshold” or “compressor lockout / aux lockout.” Aux heat stuck on.
Setback and recovery. Raising the setpoint more than about 2°F at once tells most thermostats to bring in aux to catch up. Either keep setbacks small or use a thermostat with heat-pump-aware “smart recovery,” which starts the heat pump early and reaches setpoint on time without strips. Aggressive schedules built for gas furnaces cost money on heat pumps.
Fan. Auto, not On. Continuous fan is a real load and, in cooling season, re-evaporates moisture off the coil back into the house.
Emergency heat. Leave it off unless the outdoor unit has failed. It’s a manual override, and it’s easy to knock on and forget.
Buying a thermostat for a heat pump
Look for: explicit heat pump support with aux/emergency stages (two-stage or variable-speed support if your equipment has it); a configurable aux lockout; heat-pump-aware recovery; an outdoor temperature source (built-in weather or a sensor) so lockout works; and a C-wire or the ability to add one. Most mainstream smart thermostats do all of this now; the ones that don’t tend to be the cheapest, and the difference shows up on the January bill. Communicating thermostats supplied by the equipment manufacturer are usually the best match for variable-speed systems, because they control staging directly.
Two more settings worth knowing
Dehumidify / “cool to dry.” In our humidity, a thermostat that can run the system a little past setpoint to reach a target humidity (or slow the fan for better dehumidification) makes a real comfort difference. Requires a system and thermostat that support it.
Cycle rate / differential. Wider differentials mean longer cycles, which suit heat pumps; very tight differentials cause short cycling. Defaults are usually fine; if the system cycles every few minutes, this is one thing to check.
Have it configured at install
Ask the contractor to set system type, O/B, aux lockout and recovery at commissioning and to show you the settings; add it to your fall heating prep each year. If your winter bill is high and the thermostat has never been configured for a heat pump, that’s the first fix (high electric bill). Every contractor on the North Carolina registry shows the license status we found.