Calorifier loses heat too quickly.

[BlueChip]

Mine has a non-return valve, and works.

+1 I added one and it worked for me too

 

[Piddy]

Thanks for all the helpful comments and as an update I fitted non return check flap valves (which helped extend the pipes without major works) and routed the pipes to run vertically downwards (at the calorifier terminations) and as far below the calorifier as possible.

Now after heating the water using the immersion heater, the engine stays cold and after 24 hrs from stopping the engine, there is still nice hot water in the tank.
Mrs Piddy was very impressed!

 

[causeway]

Glad it worked out, job well done!