H91 on a Dimplex LA MI inverter outdoor air-to-water: Hot water overheat sensor (tank booster heater OLP) activated, or cylinder mechanical temperature dial adjusted
Short answer
On a Dimplex heat pump, H91 means: Hot water overheat sensor (tank booster heater OLP) activated, or cylinder mechanical temperature dial adjusted. 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: Hot water overheat sensor (tank booster heater OLP) activated, or cylinder mechanical temperature dial adjusted.
Part involved: Cylinder dual cut-out / tank booster heater OLP.
When it shows: Tank booster heater operating and OLP open circuit for 60 s.
Likely causes:
- Cylinder mechanical thermostat dial turned down.
- Tank genuinely >65C e.g. immersion permanently on.
- Faulty connector.
- Faulty OLP.
- Faulty indoor (water) PCB.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: unknown.
Manual reset: Clear via control panel per Dimplex article (method shown as image).
What you can check
- If the mechanical temperature dial on the front of the Dimplex cylinder is accessible without removing any cover, check it is at maximum (Dimplex end-user advice) - it is not the DHW temperature control.
- Note the code and contact the installer or Dimplex service.
What the engineer will check
- Isolate the heat pump from mains before any work.
- Check cylinder cut-out continuity and manual reset.
- Check HP13/HP14 cable continuity.
- Check CN-OLP1/CN-OLP2.
- Check for immersion permanently powered.
Who to ask for: heat pump service engineer / heating engineer.
Certification: Electrical competence (Part P / qualified electrician) for work inside control panels.
What you might notice
Sources
- GDHV Help Centre article: i have h91 error on my la mi heat pump what does this mean ↗ · GDHV article (modified 2023-02) · page n/a (web article) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026