MB2 on a Worcester Bosch Greenstore ground source heat pump: Motor cut-out HTF pump
Short answer
On a Worcester Bosch heat pump, MB2 means: Motor cut-out HTF pump. 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 HTF pump.
Part involved: HTF (brine) pump motor cut-out.
When it shows: Motor cut-out HTF pump.
Likely causes:
- HTF pump blocked by contamination; faulty pump motor; temporary error.
Is it safe?
| 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.
- 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
- Free pump via vent screw; replace pump.
Who to ask for: ground source heat pump engineer.
Certification: F-gas certification for refrigerant circuit; competent person for electrical work.
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