A heat pump that runs continuously is normal on the hottest and coldest days of the year and normal all the time for a variable-speed system; it’s a problem when the house isn’t reaching setpoint, when it happens in mild weather, or when it comes with a jump in your electric bill. In those cases the usual causes, in order, are a low refrigerant charge, restricted airflow (filter, coil, ducts), a thermostat setting or wiring issue, an undersized or badly installed system, or, in winter, auxiliary heat that has taken over. Here’s how to tell which.
When constant running is fine
North Carolina design conditions — mid-90s and humid in July, teens to low 20s on a January cold snap — are exactly when a right-sized heat pump should be running near 100%. A system that cycles off on those days is oversized, which is worse for humidity and comfort. Variable-speed and two-stage systems run long low-output cycles by design. If the house holds temperature and bills look like last year’s, you don’t have a problem.
When it isn’t
Signs to act on: it runs constantly and the house is a few degrees off setpoint; it runs constantly on a mild 75° day; the air at the vents is only slightly cool (or, in winter, only slightly warm); or your bill has jumped without a change in weather or habits (high electric bill).
Check first
Filter, thermostat mode and setpoint, open vents, a clean outdoor coil with clear airflow around it, and, in winter, whether the thermostat is showing “aux” or “emergency heat” (aux heat stuck on). If the outdoor unit isn’t running at all while the indoor blower is, you’re heating with strip heat only — call.
Likely causes, ranked
Low refrigerant. The most common cause of a system that runs constantly and gradually stops keeping up. Comes from a leak; needs a technician to find and repair, then recharge.
Restricted airflow. Dirty filter or indoor coil, closed vents, crushed or disconnected ducts (common in crawlspaces), or a failing blower. Reduces the heat the system can move per hour, so it runs longer.
Thermostat. Wrong mode, a setpoint changed by a schedule you forgot about, a thermostat calling for cooling and heating in a loop, or, on some units, a “fan on” setting mistaken for the system running.
Dirty outdoor coil or blocked unit. Rejects (or absorbs, in winter) heat poorly; the system compensates with runtime.
Sizing and installation. A system that has always run constantly and never quite kept up on hot days was probably undersized or installed on undersized ducts. That’s a load calculation and duct evaluation, not a repair. How sizing works.
Winter: defrost and aux heat. In heating mode, frost on the outdoor coil triggers defrost cycles, during which the system briefly blows cool air (heat pump blowing cold air) and aux heat runs; a stuck defrost board or misconfigured aux settings can keep strip heat running for hours, which is the single biggest cause of winter bill shock in this climate.
Compressor or reversing valve problems. Less common; the system runs but moves little heat. Technician.
When to call
Call if the checks don’t change anything within a day, if the vent air is barely conditioned, if the outdoor unit isn’t running, or if the bill has jumped. Ask the technician for refrigerant charge readings (superheat/subcooling), static pressure and temperature split; those three numbers separate refrigerant, airflow and sizing problems. Every contractor on the North Carolina registry shows the license status we found.