Can anyone suggest a guideline to set the thermostat on the return pipe from the hot water cylinder to a Dunsley Superclean 400 back boiler? It is a Honeywell and has settings from 10 to 40 degrees. The system has a pump and runs off a solid fuel 16 inch Dunsley Enterprise fire box. I have the instructions for the system but there is no mention of the thermostat. It is currently set at 25 but there is an ominous felt tip mark at around 12 o'clock on the dial which is past the 40 setting which I have no idea about. Once the correct setting is establish would it ever need to be adjusted, seasonally for example? Many thanks in anticipation.