Heat pumps suit Mill Valley’s moderate climate, and more homes here have one every year, often as the upgrade that replaced an old furnace and added cooling at the same time. But the town asks more of a heat pump than its mild reputation suggests. Higher elevations stay cooler year-round, north-facing slopes and lots under the redwoods get little sun, and damp air means frost forms on the outdoor coil on many winter mornings. So the heat pump runs in heating mode for most of the year, and the problems we repair are mostly heating problems.
Normal behavior or a fault?
Before you call, it helps to know what a healthy heat pump does:
| You notice | Normal when | A fault when |
|---|---|---|
| Steam from the outdoor unit | It lasts a few minutes during defrost | The coil stays white with ice |
| Outdoor fan stops, then a whoosh | The unit is switching into or out of defrost | The fan never restarts |
| Air at the registers feels mild | It is still warmer than the room | It is room temperature or cold for long periods |
| Long run times | It is a cold, damp night | It never shuts off on a mild day |
| Aux or emergency heat light on the thermostat | Briefly, during defrost or recovery from a setback | It stays on for hours |
What usually fails here
Defrost problems. Damp air makes frost a frequent event. If a defrost sensor or the control board fails, the coil ices over, airflow through it stops and heating collapses. On shaded lots this is the most common heat pump repair we see.
A stuck reversing valve. The valve that switches between heating and cooling can stick, leaving a system that heats but will not cool, or the other way around. Sometimes the cause is only the valve’s electrical coil, which is a simpler fix.
Backup heat doing the work. Many ducted heat pumps have electric heat strips for defrost and cold snaps. When the heat pump itself underperforms, the strips quietly take over: the house stays warm, but the electric bill jumps. A sudden rise in winter bills is a good reason to have the system checked.
Debris and drainage. Needles and leaf litter pack the outdoor coil and the base pan. Defrost water that cannot drain refreezes around the bottom of the coil.
Electrical parts. Capacitors, contactors and fan motors fail on heat pumps just as on air conditioners, and since they work all year here, they wear sooner.
Refrigerant leaks. A low charge cuts heating capacity first. We find and repair the leak rather than topping it up.
Ductless systems have their own error codes and sensors; see mini-split repair in Mill Valley.
How the repair visit works
- Schedule: book a convenient time for a heat pump diagnosis.
- System evaluation: we test heating and cooling modes, defrost operation and backup heat.
- Identify issues: we pinpoint the exact cause of the problem.
- Review options: you see the price for each repair and approve it before any work begins.
- Complete repair: we restore the system and verify it in both modes.
Repairs carry a 90-day warranty, parts keep the manufacturer’s warranty, and same-day service is available for most calls.
Repair or replace
If the compressor has failed on an older system, the system uses R-22, or it has needed several repairs in recent years, we price a replacement alongside the repair. Newer variable-speed heat pumps hold their capacity better in cool weather and run more quietly. See heat pump installation in Mill Valley and the general heat pump repair page; our Mill Valley page covers the rest of our work in town.