Fitting new bath mixer so need to drain the system

[jjfresh66]

Thought i better get some reasurance from here before i go plumbing in
System is a conventional gravity fed back boiler (old mother of mary)

I just want to clear a couple a things about Isolating the hot and cold water to the bath taps before replacement.

I will be shutting off the rising main (stopcock under sink)
I've read that instead of fully draining down the system, ~I can isolate the water to the bath from the header by closing off a gatevalve either near the header tank or water cyclinder - Is this true ? or do i need to do a full drain down.
there is a red gate valve next to the water cylinder - not sure about one near the tank yet.

Thanking you all

 

[Terrywookfit]

Depending on the age and quality of the gate valve ,you may find that it does not fully turn off or will not turn back on!!

Easiest way is to drain tank! (You could then fit a full bore lever ball valve for future use) on feed to cylinder !!