Hd on a Baxi heat pump: Communication fault between hydraulic module parallel (cascade master/slave)
Short answer
On a Baxi heat pump, Hd means: Communication fault between hydraulic module parallel (cascade master/slave). 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 · 2 sources
What it means
On the HP40 / Baxi Assure HP50 Monobloc ASHP
The manual says: Communication fault between hydraulic module parallel (cascade master/slave).
Part involved: PCB communication wiring.
Likely causes:
- Master/slave signal wires not connected; two or more outdoor units connected to wired controller; master/slave power-on interval >2 minutes; duplicate slave addresses (S4-1 DIP procedure).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
The manual says: Communication fault between the master outdoor unit and the slave outdoor unit.
Part involved: PCB communication wiring.
Is it safe?
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.
- If the code persists or returns, contact a heat pump engineer.
What the engineer will check
On the HP40 / Baxi Assure HP50 Monobloc ASHP
- Master/slave signal wires not connected; two or more outdoor units connected to wired controller; master/slave power-on interval >2 minutes; duplicate slave addresses (S4-1 DIP procedure).
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
- Diagnose per manufacturer service procedure (manual gives description only).
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
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 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 57 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026