my gas central heating and hot water (pressurised cylinder type) are controlled by the same control box thingy. at the mo the c/h is turned off but when the hot water comes on, the boiler fires up.

should it do this?

 

Yep thats how it heats the water lol

 

so whats the massive cylinder for in my cupboard upstairs??!! i thought that was what heated the water

 

Thats for holding the water or for heating it if you are using the immersion heater which is electric and independant from the central heating!
If the heating control is set to water then as soon as it needs to the boiler will fire up to heat the water!

 

ok i'll get me coat

cheers spadge