E8 on a Baxi heat pump: Water flow failure
Short answer
On a Baxi heat pump, E8 means: Water flow failure. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
The manual says: Water flow failure.
Part involved: water flow / pressure.
Likely causes:
- Shut-off valves not fully open; filter needs cleaning; air in system; water pressure must be >1 bar; pump speed not highest; expansion vessel broken; circuit resistance too high for pump; during defrost check backup heater supply/fuses; pump fuse and PCB fuse.
On the HP40 / Baxi Assure HP50 Monobloc ASHP
The manual says: Water flow failure.
Part involved: water flow / pressure.
Likely causes:
- Shut-off valves not fully open; water filter needs cleaning; air in system (purge); water pressure must be >1 bar; pump speed not on highest; expansion vessel broken; circuit resistance too high for pump; during defrost check backup heater supply wiring/fuses; pump fuse or PCB fuse blown.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
The manual says: Water flow protection.
Part involved: water flow / pressure.
Is it safe?
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Power OFF/ON after correcting cause (ISM 13.4).
On the HP40 / Baxi Assure HP50 Monobloc ASHP
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Installer: correct cause then power OFF/ON (ISM 13.4).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Note the exact code shown on the controller and when it appeared.
- Read the heating system pressure gauge and report the value to your installer (the manuals quote a minimum of about 1 bar cold).
- Check the filling loop is closed.
- In freezing weather do not leave the system switched off - the manuals warn water faults left unrepaired in winter can let the pump and pipes freeze.
- If the code persists or returns, contact a heat pump engineer.
What the engineer will check
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
- Shut-off valves not fully open; filter needs cleaning; air in system; water pressure must be >1 bar; pump speed not highest; expansion vessel broken; circuit resistance too high for pump; during defrost check backup heater supply/fuses; pump fuse and PCB fuse.
Who to ask for: heating / heat pump engineer (hydraulics).
Certification: None beyond heating competence (MCS installer recommended).
On the HP40 / Baxi Assure HP50 Monobloc ASHP
- Shut-off valves not fully open; water filter needs cleaning; air in system (purge); water pressure must be >1 bar; pump speed not on highest; expansion vessel broken; circuit resistance too high for pump; during defrost check backup heater supply wiring/fuses; pump fuse or PCB fuse blown.
Who to ask for: heating / heat pump engineer (hydraulics).
Certification: None beyond heating competence (MCS installer recommended).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
- Diagnose per manufacturer service procedure (manual gives description only).
Who to ask for: heating / heat pump engineer (hydraulics).
Certification: None beyond heating competence (MCS installer recommended).
What you might notice
Sources
- Installation and owner's manual - Heat Pump air/water 'inverter' Assure Mono 5, 7, 9, 12, 16 ↗ · Back cover shows '7762758 - v05 - 19082020' (same number as the Assure Split manual - probably shared back-cover artwork) · page 72 · Reviewed 23 September 2026
- Installation and Service Manual - Heat Pump air/water 'inverter' HP40/50-4-1PHMB to HP40/50-13-3PHMB (United Kingdom) ↗ · 784570-01 (v1) · page 72 · Reviewed 23 September 2026
- Installation, user and service manual - Reversible monobloc air-to-water heat pump, outdoor unit HP60-4-1-PHMB to HP60-13-3-PHMB ↗ · 7888668 - v03 - 17072025 · page 57 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026