H06.28 on a Baxi HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP: Comm IDU - ODU Communication error between the indoor unit and outdoor unit
Short answer
On a Baxi heat pump, H06.28 means: Comm IDU - ODU Communication error between the indoor unit and outdoor unit. 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: Comm IDU - ODU Communication error between the indoor unit and outdoor unit.
Part involved: PCB communication wiring.
Likely causes:
- The main PCB B and the main control PCB of the hydraulic module are not connected. Connect the wire. Check the value of signal HM024. If the value of HM024 is less than 75%, the communication errors are too severe. It is essential to use a shielded communication cable. If there is a strong magnetic field or strong interference, (e.g. lifts, powerful transformers), add a barrier to protect the unit or move the unit to another location. 1. Switch off the outdoor unit and the indoor unit. 2. Wait 3 minutes for the outdoor unit capacitors to discharge. 3. Switch on the indoor unit then the outdoor unit.
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 - system restarts automatically when the cause disappears (may run in degraded/back-up mode meanwhile).
Manual reset: not required once cause removed.
What you can check
- Note the exact code shown on the controller and when it appeared.
- The manual's user steps allow switching the system off and on once to see if the error clears; if the code returns, contact the installer.
- If the code persists or returns, contact a heat pump engineer.
What the engineer will check
- The main PCB B and the main control PCB of the hydraulic module are not connected. Connect the wire. Check the value of signal HM024. If the value of HM024 is less than 75%, the communication errors are too severe. It is essential to use a shielded communication cable. If there is a strong magnetic field or strong interference, (e.g. lifts, powerful transformers), add a barrier to protect the unit or move the unit to another location. 1. Switch off the outdoor unit and the indoor unit. 2. Wait 3 minutes for the outdoor unit capacitors to discharge. 3. Switch on the indoor unit then the outdoor unit.
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
What you might notice
Sources
- Installation, user and service manual - Indoor unit ASHP Control ↗ · 7887625 - v04 - 22072025 · page 129 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026