A8 on a Worcester Bosch Greenstar Plus Hybrid: No communication with EMS heat appliance / No communication with system controller (status B)
Short answer
On a Worcester Bosch heat pump, A8 means: No communication with EMS heat appliance / No communication with system controller (status B). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: No communication with EMS heat appliance / No communication with system controller (status B).
Part involved: hybrid control module / FW200.
When it shows: No communication with EMS heat appliance / No communication with system controller (status B).
Likely causes:
- Communication cannot be established between hybrid control module and boiler; or with system controller - check contacts, cables and connections; repair/replace bus cable; replace appliance PCB or system controller.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes.
Manual reset: Hold Back button on hybrid control module 5 s to restart after fault is rectified (installer).
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
- Check whether the boiler display also shows a fault (manual: if the boiler shows a fault the heat pump is not faulty) and report both to the engineer.
What the engineer will check
- Communication cannot be established between hybrid control module and boiler; or with system controller - check contacts, cables and connections; repair/replace bus cable; replace appliance PCB or system controller.
Who to ask for: Gas Safe registered engineer + heat pump installer.
Certification: Gas Safe registration for any work on the boiler; F-gas for refrigerant circuit.
What you might notice
Sources
- Installation and Maintenance Instructions Greenstar Plus Hybrid ↗ · 6 720 803 687 (2012/11) · page 58 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026