How a heat pump fails
A heat pump is an air conditioner that can run in reverse. In winter, a reversing valve flips the flow of refrigerant so the outside unit pulls heat out of cold air and moves it inside. That means a heat pump runs in both summer and winter and logs far more hours per year than a cooling-only AC, so its parts wear faster.
Common failures include a stuck reversing valve, which leaves the system cooling when you asked for heat, and a defrost control that stops clearing ice off the outside coil. Failed capacitors and burned-out backup heat strips in the indoor unit round out most winter calls.
Normal or broken?
Some heat pump behavior worries people and is normal. On a cold, damp morning the outside unit will stop, hiss and blow a cloud of steam for a few minutes. That is the defrost cycle melting frost off the coil. Air from a heat pump also feels cooler than air from a gas furnace, usually 90 to 100 degrees at the vent, compared with 120 or more from gas.
These point to a problem:
- The outside unit is encased in solid ice and stays that way for more than an hour
- Vent air in heat mode feels colder than the room
- Aux heat or Emergency heat shows on the thermostat most of the day when it is above 35 degrees outside
- The electric bill jumps by $100 or more over the same month last year
- Loud grinding, rattling or shrieking at startup
Backup heat and your power bill
Below about 30 to 35 degrees, a standard heat pump cannot keep up alone, and the indoor unit turns on electric heat strips. The strips work like a giant toaster and cost roughly two to three times as much to run for the same amount of heat. If a bad sensor or defrost board leaves the strips on all day, a January bill can double before anyone notices. A technician can test for this in one visit.
Typical costs
| Repair | Typical price |
|---|---|
| Diagnostic visit | $89 to $150 |
| Capacitor replacement | $150 to $400 |
| Defrost control board | $300 to $700 |
| Backup heat strip or sequencer | $250 to $700 |
| Reversing valve | $1,200 to $2,500 |
| Heat pump replacement | $6,000 to $12,000 |
Questions
Should I switch to emergency heat?
Only if the outside unit has stopped working or is iced solid and not clearing. Emergency heat runs the backup strips alone. It keeps the house warm and costs much more per hour, so switch back once the heat pump is repaired.
How long do heat pumps last in North Carolina?
Plan on 12 to 15 years. Units near the coast wear out sooner from salt air. Inland in Orange and Chatham counties, well-maintained units often reach 15.
Can a heat pump keep up with a Chapel Hill winter?
Yes. The average January low in Chapel Hill is around 30 degrees, which a standard heat pump handles with help from its backup strips on the coldest nights. Cold-climate models keep heating well below zero.