On Sun, 26 Apr 2020 10:56:10 +0100, Grumps wrote:

Hi All
Daughter has an unvented hot water cylinder in her flat which has two
3kW heaters. We believe (although I haven't checked as shouldn't really
travel) that the bottom (main) heater has failed and needs replacing.
Apart from getting the correct length and thread diameter of the new
heater, switching off the electric to the timer, and draining the tank -
is there anything else I should look out for when doing the replacement?
I have an immersion heater spanner already.

Immersion heaters for pressurised vessels must have a built in
overheat thermostat. The seal around the heater is usually made in the
factory so you don't often have the problems of plumbers in the past
slathering everything in sealant so removal isn't too difficult.
Refitting must be done carefully making sure the gasket surfaces are
absolutely clean. Make sure you know the refill procedure for the
tank and expansion vessel before you start.