how can i fix my puma 80 boiler ?

in summer my puma 80 boiler is only on for hot water and central heating knob is in the middle position on off point.Room thermostate is also on 10 digree.When i open any hot water tap after finishing usege of hot water boiler start central heating itself.
so any body know that what is the cause of this fault and please explain me how to fix it by calling a gas central heating engineer.with many many thanks