The whole system worked really well in fact the heat exchanger worked so well it set up a thermo siphon with the engine water jacket. The Hurricane was so powerful it eventually produced enough heat to get the 450kg engine block up to 40c in an un-insulated engine room. Clearly this is not ideal unless the engine needed preheating in arctic conditions which is unlikely at 52N degrees.
The answer was to fit some additional control. (Yes more control systems!) will it ever end? I decided a motorised ball valve would fit the bill. I was unable to find a 3/4" valve that had a sprung return so opted for a relay controlled power to open power to close.
Valve with relay above
This is the circuit diagram.
The relay is energised when the ignition is powered up and killed when it switches off. Once the valve motor has reached it's travel no power is drawn.
The other relay (top behind) in the system is also powered from the ignition to interrupt the live feed from the heat sensitive switch to the CH pump. Once the engine reaches 50c the pump to the CH switches on and through the heat exchanger heats the radiators inside the boat. This extra relay is there to interupt the live feed to the pump so it switches off when the engine stops. This stops the pump running on until it drops below 50c at the sensor which results in the engine not re-absorbing heat from the rads and calorifier. Before this additional relay was in, the pump would run on pointlessly for up to a totally counterproductive hour until the heat switch cut out.
Hopefully that's it for the CH system.