"Count de Monet" wrote in message
...
I have an unvented hot water system (all electric home with storage
heaters for central heating)

The hot water cylinder is a Kingspan Range Tribune TD180 with two
immersion heaters. The lower heater (off peak) has failed. I have
replaced a few top mounted heaters in the past (I'm a sparks) but never
on a sealed system.

It looks like I can isolate the cylinder from the rest of the cold water
system. Is it enough to close this valve then open the drain cock to
drain the cylinder. Then change the immersion heater, re-fill the
cylinder and open a couple of hot taps to vent any air.

You are correct. Shut off the supply to the cylinder and drain down, fit
the new immersion heater and refill with a hot tap open at the same time
until water comes out air free.