“Aux heat” on a heat pump thermostat means the electric resistance heat strips in your air handler are running to help the heat pump keep up; “em heat” (emergency heat) means the heat pump is switched off and the strips are doing all the work. Both are normal in short bursts on North Carolina’s coldest nights and during defrost cycles; aux heat running for hours, or coming on in mild weather, is the single most common cause of winter electric-bill shock here, and the usual culprits are thermostat settings (large setbacks, no lockout, wrong system type), a heat pump that isn’t heating properly, a defrost problem, or emergency heat left switched on by mistake.
Aux vs emergency heat
Auxiliary heat is automatic: the thermostat calls for it when the heat pump alone can’t raise the temperature fast enough. Emergency heat is manual: you (or someone) selected it, and it disables the outdoor unit entirely. If your thermostat says EM HEAT or Emergency, switch it back to Heat unless the outdoor unit is broken; you’re paying two to three times the running cost. Thermostats and heat pumps explains the settings.
Check first
Thermostat mode: Heat, not Em Heat. Setpoint changes: raising the setpoint more than about 2°F at once tells most thermostats to bring in aux heat to catch up; if you use big setbacks at night, that’s aux heat every morning. Set a smaller setback or use a thermostat with heat-pump-aware recovery. Filter and vents: restricted airflow drops the heat pump’s output and triggers aux. Outdoor unit: is it running when the thermostat calls for heat? Is it a block of ice? A unit that isn’t running or is heavily iced means the strips are carrying the house.
Likely causes, ranked
Thermostat configuration. Not set up for a heat pump (treats it like a furnace), no outdoor-temperature lockout for aux, an aggressive setback schedule, or a “comfort” recovery mode that leans on strips. The most common and cheapest cause. Fix: settings, or a heat-pump-aware thermostat.
Heat pump not heating well. Low refrigerant, dirty coils, a failing compressor or reversing valve. The heat pump runs but delivers little, so aux fills the gap continuously. Technician.
Defrost problems. Frost on the outdoor coil is normal in heating; a defrost cycle clears it and briefly runs aux. A failed defrost board or sensor leaves the coil iced and aux running. Technician. Heat pump blowing cold air covers the defrost cycle.
Undersized heat pump / oversized strips. A heat pump sized for cooling can be short on heating capacity on cold nights, and strip heat sized generously fills in expensively. Not a repair; a note for the next replacement, where a load calculation should size for both seasons.
Emergency heat left on. After a service call or a power outage, someone flipped it. Check.
Stuck sequencer or relay. The strips are energised regardless of the call. Uncommon; a technician’s find.
The bill
Resistance heat is 100% efficient; a heat pump in NC winter conditions delivers two to three times the heat per kWh. So every hour aux runs instead of the heat pump costs two to three times as much. If your bill doubled in January without a change in weather or habits, aux heat is the first suspect (high electric bill); if your utility offers hourly usage data, look for the flat high-draw blocks.
When to call
Call if aux runs continuously above roughly 40°F, if the outdoor unit isn’t running or is a block of ice, if the vent air is cool with the system in Heat, or if settings changes don’t fix it in a day. Ask the technician to check the aux lockout and thermostat configuration, refrigerant charge, and defrost operation, and to tell you the strip heat kW installed. Every contractor on the North Carolina registry shows the license status we found.