MB1 on a Worcester Bosch Greenstore ground source heat pump: Motor cut-out compressor
Short answer
On a Worcester Bosch heat pump, MB1 means: Motor cut-out compressor. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Motor cut-out compressor.
Part involved: compressor motor cut-out (Fuse 1).
When it shows: Motor cut-out compressor.
Likely causes:
- Intermittent fault/overload on supply; motor cut-out current set too low; faulty contactor/cut-out or loose connections; compressor error.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Press Ackn. on the control panel; heat pump restarts within 15 minutes if there is demand. Some causes need a reset button/MCB on the heat pump (engineer).
What you can check
- Read and note the full alarm text and code in brackets shown on the controller and pass it to your installer.
- Check the heat pump isolator and consumer-unit breaker(s) are switched on; if a breaker has tripped more than once, leave it off and call an engineer.
- Press the Ackn. button once as the manual describes (manual: no risk in resetting an alarm); contact technical support if the alarm repeats.
What the engineer will check
- Reset motor cut-out (Fuse 1) on heat pump; check current setting, contactor, wiring, compressor.
Who to ask for: heat pump installer / electrician.
Certification: Electrical work to BS 7671 by competent person.
What you might notice
What owners and engineers report
3 owner and engineer reports mention this.
- No heating on the Greenstore ground source heat pump (original, System & Combi): 2 reports
- Keeps stopping / cutting out on the Greenstore ground source heat pump (original, System & Combi): 1 report
Sources
- Worcester Greenstore 6/7/9/11 kW System Installation, Commissioning & Servicing ↗ · 8-716-112-860 Issue c · page 34 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026