The thermostat says aux heat all day, and the electric bill jumped

Aux heat, emergency heat and normal

Auxiliary heat comes on automatically when the heat pump falls behind: on the coldest mornings, during a defrost cycle, or when the thermostat is turned up several degrees at once. Short runs are normal.

Emergency heat is a setting someone chose. It shuts the outdoor unit off and runs strips only. If the thermostat is on emergency heat and nobody remembers why, that alone may explain the bill.

Around Bastrop, January averages a low of 38.6°F at the Smithville station, and freezing nights number about 34 a year. A heat pump in working order handles most winter hours without strips. Aux heat on a 50°F afternoon is a fault.

What the outdoor unit might be doing

• Not running in heat mode. A capacitor, contactor or fan motor fault, or a tripped breaker. The same parts fail in winter as in summer.
• Running but blowing cool indoors. A reversing valve that has stuck in cooling position, or its solenoid.
• Encased in ice. Frost that never clears points at the defrost control, a sensor, or low refrigerant.
• Short on charge. A leak that shows up as weak cooling in summer becomes weak heating in winter.

What you can check

The thermostat mode (heat, not emergency heat), the filter, and whether the outdoor unit runs and its fan turns when heat is called. Then call, rather than running on strips for weeks.

What a technician checks

Whether the outdoor unit starts in heat mode, the reversing valve, pressures, the defrost board and its sensors, and the thermostat's wiring and settings. A thermostat set up for a different kind of system can call aux heat too early, and that is a settings fix.

What to say when you call

• That it is a heat pump, its install year and refrigerant
• What the thermostat shows and since when
• Whether the outdoor unit runs, and whether it is iced
• How much the last electric bill rose
• Your ZIP code

Common questions

Is it safe to keep running on aux heat? The house stays warm; the cost is the problem. Get it looked at before a full month of strip heating.

Is it time to replace the heat pump? Not because of this symptom alone. It depends on what failed and how old the unit is; see heat pump repair and replacement.