Boiler on the blink, help needed :)

[sinto607276]

its on the heating side    open the old one up to release system pressure  whip the old one out and stick the new one in a bit sharp and you won't have to drain the whole system.

Brill
It's sounding easy to do but I'll have some towels/rags underneath when I do it.
So, some water should come out depending how quick I can replace it?

Read somewhere else that I could try screwing in the front screw and back again to see if that sorts it but I'm not to sure