Hb on a Baxi heat pump: Three times 'PP' protection and Tw_out < 7°C
Short answer
On a Baxi heat pump, Hb means: Three times 'PP' protection and Tw_out < 7°C. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the HP40 / Baxi Assure HP50 Monobloc ASHP
The manual says: Three times 'PP' protection and Tw_out < 7°C.
Part involved: water flow / pressure.
Likely causes:
- Refer to PP. Manual caution: in winter an unrepaired E0/Hb fault can let the pump and pipework freeze.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
The manual says: 3 times PP and Tw_out < 7 °C.
Part involved: water flow / pressure.
Is it safe?
On the HP40 / Baxi Assure HP50 Monobloc ASHP
| 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: 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? | 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: 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 HP40 / Baxi Assure HP50 Monobloc ASHP
- Refer to PP. Manual caution: in winter an unrepaired E0/Hb fault can let the pump and pipework freeze.
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 Service Manual - Heat Pump air/water 'inverter' HP40/50-4-1PHMB to HP40/50-13-3PHMB (United Kingdom) ↗ · 784570-01 (v1) · page 73, 74 · 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 60 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026