Burner fires up and CH comes on, even though not selected at switch ?

Having been impressed with the diaphragm solution I was given here in 2010, I have another problem now with my Halstead Ace High. The burner fires up and CH comes on, even though not selected at the selector switch and the thermostat is set to zero. I have to have the selector switch turned off until I want some hot water, then switch it on to hot water (the central heating comes on at this point) while I have my shower or whatever, then switch it off again.