Thanks for the replies so far, - although I'm not sure I've got an answer yet.
I thought it was a simple question, - perhaps I should add more detail.
Back boiler stove at front of boat (yes I know...) flow and return pipes feed 3 radiators down the boat,a 12v pump half way down runs continuously when the fire is in. and then into the engine room where the calorifier is. The flow and return feed into one of the calorifier coils, (almost like an 'end of line' radiator)
Just before the calorifier, there is a link pipe between flow and return with a stopcock on it, - it's normally open, thereby (i think) bypassing the calorifier, closing the stopcock would force the flow/return through the calorifier.
I dont want to switch off the pump, I just want a way of the water being directed into the calorifier if it's hotter than whats already there, and more important - not to pump cooler water around the calorifier when the fire is low,
Surely it's a common request?
Tony