isolator valve

[confusedbrian]

I have recently had a new vailliant pro tec 28 boiler fited, by british gas, so I am hoping a reasonable job. I need to replace the isolator valve on the hot water side leading to my shower. Looking at the pipes coming from the bottom of my boiler the only one without an isolator at the boiler is the hot tap feed. If I turn off the mains feed into the boiler will this stop water coming out to let me change the valve? if not any ideas!

 

[sandybeach]

Just to be safe. Turn the cold water off at the mains stopcock.
Mind have your brolly handy for when you turn it back on!

 

[Terry7556]

The hot water out of your taps comes from the boiler,so turn off the cold inlet under the boiler and open a ground floor hot tap to drain the water in the hot pipe work. Fit the valve and turn water back on