How to only switch off the central heating supply

[Hummy]

We need to take off a radiator and replace it with with a smaller radiator. I've switched off the radiator valves and taken teh radiator off. However i now need to switch off the water supply to the central heating system. I have a vaillant combi boiler. Can anyone tell me how i switch off only the water supply to the central heating system. Thanks. I can't seem to figure it out other than to switch off the mains water feed into the boiler but i'm sure there must be a way of just switching the water off to the central heating.
Can i just turn off the boiler and that will only give me cold water to the taps? and nothing through to the central heating.
Hummy

 

[omegaman]

There will be two valves underneath the boiler to shut off the radiator circulation. The water will still be in the system which will need to be drained if you need to alter any pipework.

For what its worth, the heating system is only filled up once and the supply is then shut off via the filling loop. That water stays in the system and circulates. The only time you need to refill it is if you have pressure loss/leak.