I have H91 error on my LA-MI heat pump. What does this mean?

Modified on Wed, 22 Feb 2023 at 10:01 AM


Applicable to models:  LA6MI, LA9MI, LA12MI, LA16MI


Issue:  Error code H91 shown on the display. 


What this means: H91 means the hot water overheat sensor has activated, or that the mechanical temperature dial on the front of the cylinder has been adjusted when it should not have been.

Action for End User:
End user should check on the front of the cylinder to see if the mechanical temp dial has been lowered from its maximum setting. If it has, it needs to be set to maximum. It is not used to control hot water temperature! Hot water temperature is adjusted on the main heat pump LCD control screen.


If this dial is in the maximum position, try clearing the error as follows:

Action for Engineer: 

If the error persists, an engineer will need to carry out the following checks:

Heat pump must be safely isolated from mains supply before carrying out any work!  

If the hot water tank is scalding hot (above 65C), the H91 error may be legitimate. Check if an immersion element is permenantly powered on and rectify.

If the H91 error is present and the tank is not scalding hot, then the error is false and needs further investigation.

Under the ECS cylinder thermostat cover, continuity can be checked:

Disconnecting the 2 wires going back to the heat pump, when testing at these two terminals on the cylinder, there should be continuity. If there is continuity, reconnect the two wires and replace thermostat cover. If no continuity, try adjusting the control dial to maximum and manually resetting the cut-out.

Continuity should now be checked outside. Remove wires from terminals HP13 & HP14 then measure continuity. If there is no continuity, the cable has been damaged and needs repair/replacement. If there is continuity, replace the two wires into their respective terminals. 

If there H91 still persists follow the flow diagram:

CN-OLP2 is the connector for the overload stats back on the cylinder body.
CN-OLP1 is the flow boiler overload located here:

And connections on PCB here to double check:

Malfunction Decision Conditions:

During operation of tank booster heater, and tank booster heater OLP open circuit.


Malfunction Caused:

1 Faulty connector connection.

2 Faulty tank booster heater overload protector (OLP).

3 Faulty indoor unit PCB (main).


Abnormality Judgment:

Continue for 60 seconds.



Possible parts, depending on engineer diagnosis: 

• Dual cut-out assembly on Dimplex ECS heat pump cylinders part: X-R01141-1


• OLP thermostatic cut out switch (CN-OLP1)

LA6MI / LA9MI / LA12MI / LA16MI part: X-2511030


• Main Water PCB aka "Indoor PCB" (part number depends on model of heat pump)

LA6MI part: X-2511031
LA9MI part: X-2511039
LA12MI part: X-2511051
LA16MI part: X-2511058

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