Replacing my thermastat (honeywell CM737 - OpenTherm)


I currently have 14 of the Tado smart TRVs running very well here in Sweden. My house is connected to shared hot water supply (district heating) which the boiler like appliance uses for both hot water heating and central heating to the radiators.

The current thermostat is a Honeywell CM737, the heat exchanger appliance is Alfa Laval Cetetherm Mini, which uses a 3 port Honeywell M6410C2031 3 port actuator which I assume does the splitting for HW and Central Heating.

The thermostat has just a 2 wire connection, measures 48V so I assume it is using OpenTherm to talk to the heating appliance.

So, when I change over the termostat, using what I believe are the right connections, the 3 port valve inside the heating appliance goes crazy. It sounds like it is just continuously going around and around. Heating stops (when it is -30oC - not the best time to try the installation :-) ).

Any chance someone here can help?