P21 on a Grant Aerona 290: Abnormal operation of DC pump
Short answer
On a Grant heat pump, P21 means: Abnormal operation of DC 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: Abnormal operation of DC pump.
Part involved: DC circulating pump / hydraulic PCB.
Likely causes:
- The water pump is faulty or stuck.
- The system lacks water or is blocked.
- Faulty connection between water pump and hydraulic PCB.
- The hydraulic PCB is faulty.
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: Not stated in DOC 0204.
Manual reset: DOC 0204 s11.1: once the fault is resolved, reset by turning the unit OFF and back ON; s11.5: power cycle can be done from the Aerona Smart Controller touchscreen instead of the external isolator (installer procedure).
What you can check
- Note the exact code shown on the controller display and tell your installer or Grant-trained engineer.
- Check the heating system pressure gauge (if one is fitted) and report a low reading to your installer; make sure the filling loop valves are closed.
What the engineer will check
- Check operation of water pump; replace if faulty.
- Check system water pressure, blockage (filter) or closed valves; refill, clean filter, open valves.
- Check connections between water pump and hydraulic PCB.
- Check the hydraulic PCB; replace if faulty.
Who to ask for: Grant-trained heat pump engineer.
What you might notice
Sources
- Grant Aerona 290 Air to Water Air Source Heat Pump Range - Installation & Operating Instructions (UK DOC 0204) ↗ · Rev 1.9 · page 55 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026