the engine "should" only heat the calorifier - if the cabin radiators are being heated by convection then your hot water wont last long. If you add anything to this circuit then you will have to bleed the circuit at any high points, and ideally the highest point should be lower than the top of the header tank - tricky to put a heater matrix on the roof !
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Also, having snapped the bleed screw on the idle governor and replaced it with a std screw, I need to adjust the idle operation, what is the procedure? or is there anywhere that sells the right screw?
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Is this the right bleed screw
http://www.ssldieselparts.co.uk/fittings-screw-pumps-c-9_123.html
springy