Draining down a sealed unvented direct cylinder

[Count de Monet]

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 c*ck to drain the cylinder. Then change the immersion heater, re-fill the cylinder and open a couple of hot taps to vent any air.

A couple of pictures of the system can be seen here:

help would be appreciated

Martyn

[Heliotrope Smith]

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.

[The Natural Philosopher]

Remember it wont drain unless air can get in, so open a hot (or cold!) tap as well..

Or indeed, flush a loo..

I turned off the mains, and set to work on the hot system, opened hot tap. some came out. Then flushed a loo downstairs, and flooded the bathroom! Once the mains side could suck air in, down came all the water from the loft mounted tank!