725 on an alpha innotec heat pump: System error DHW. Please call installer: Domestic hot water fault: desired tank temperature substantially undershot
Short answer
On an alpha innotec heat pump, 725 means: System error DHW. Please call installer: Domestic hot water fault: desired tank temperature substantially undershot. 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: System error DHW. Please call installer: Domestic hot water fault: desired tank temperature substantially undershot.
When it shows: Domestic hot water fault: desired tank temperature substantially undershot.
Likely causes:
- DHW circulating pump.
- Tank filling.
- Shut-off valve.
- 3-way valve.
- Air in DHW/heating.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: manual acknowledgement required (rotary pushbutton 7 s) - display usually says "Please call installer".
What you can check
- Note the error number and text shown on the Luxtronik display.
- Acknowledge the message by pressing the rotary pushbutton for about 7 seconds (as the Luxtronik manual permits); if the same error reappears, contact your installer or ait customer service with the error number.
- Check the heat pump breakers/isolator at the consumer unit are on (do not open any casing).
What the engineer will check
- Check DHW circulating pump.
- Check tank filling, shut-off valve and 3-way valve.
- Vent hot water and heat pump circuits.
Who to ask for: heat pump / heating engineer.
What you might notice
Sources
- Operating manual Luxtronik heating and heat pump control Part 2 (UK) 83055400lUK - 2.1 ↗ · 83055400lUK 2.1 · page 53 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026