Boiler/Central Heating trouble

[rince]

Ive just moved into a new flat, and after having a new prv valve fitted onto the old gloworm boiler, Ive noticed that when i turn the hot water switch on the radiators come on - even though the central heating switch is turned off!!
What could be causing this, and is it a major bit of work i need to be looking at getting sorted. Im worried that ill be paying extorionate bills for heating, even though i havent even turned the heating on!!!

 

[ChrisR]

Depends what the boiler/heating system is - more dets please.
Meantime, turn the rads off at the valves if you can.

 

[rince]

Well its a gloworm boiler (really old) and in the airing cupboard i have a big black water tank, heating tank, and a wall switch where i can turn on central heating and hot water.