Heat Pump Won't Start: Checklist Before Calling Service (2026)
Updated: 2026-10-01 · ~2 min read
Why heat pumps "refuse" — the ranking of likelihood
- Power/lockout: tripped breaker or RCD, power dip that locked the control board, or a protection lockout after repeated failed starts.
- Thermostat / schedule: the system is running fine but has no call for heat (setpoint below room temp, vacation mode, wrong period).
- Sensors: outdoor or flow sensor fault makes the controller refuse to start the compressor.
- Water side: low system pressure / no flow → flow-protection lockout.
- Outdoor unit: fan error, defrost lock, high-pressure protection in warm weather.
The 10-minute checklist (before the phone call)
| # | Check | What you'll see if it's the cause |
|---|---|---|
| 1 | Main breaker / RCD for the heat pump circuit | tripped breaker, or RCD indicator; reset once — if it trips again, stop and call (electrical fault) |
| 2 | Controller display: standby vs. error code | code → follow error codes guide; "standby/ready" → check thermostat side |
| 3 | Thermostat setpoint vs. actual room temp, period/schedule | setpoint ≤ room temp = no call; wrong schedule/vacation mode |
| 4 | Water system pressure gauge (if visible) | below nominal range → bleed the system / top up; recurring drop → pressure drop guide |
| 5 | Outdoor unit: fan spinning, clear of snow/leaves/ice, not blocked by a fence that is too close | fan silent with a running pump = fan problem; ice sheet = defrost stuck |
| 6 | Power cycle once (main switch off 1 min, on) | clears control lockouts after power dips; if it locks again immediately → real fault |
| 7 | Log: when did it last work, what happened since (storm, frost, power cut) | correlation = fastest diagnosis for the service tech |
| 8 | Circulation pump running when it should be | pump silent while system demands heat = pump/electrical issue — a very common "no start" cause |
What to send the service tech (makes the visit half as long)
- Exact model + serial (plate on the unit) and controller type.
- Error code(s) + date/time from the log, if any.
- The checklist results: "breaker OK, gauge 1.8 bar, fan OK, power cycle didn't help".
- Access notes: is the outdoor unit reachable (snow fence, balcony, basement riser)?
When to stop DIY and call immediately
- RCD keeps tripping after reset — electrical fault; don't force restarts.
- Compressor locked out repeatedly — further forcing risks the compressor.
- Refrigerant smell, oil, or heavy frosting on the outdoor unit — assume a leak until it is fixed.
Affiliate slot: none planned for this article (diagnostic/informational).
Sources
- Typical "no start" service patterns from ASHP service data (Daikin, Vaillant, Samsung); as of October 1, 2026.
- Heat pump control logic: flow protection and compressor start lockouts — manufacturer service documentation; as of October 1, 2026.