In an earlier contribution to this discussion,
Barry Young wrote:

Hi everybody,

My flat's hot water is provided solely by a hot water cylinder with
two immersion heaters in it. Until now, only the bottom one was wired
up - the top one may have been at some point, but certainly not since
I've moved in. I'm on Economny 7, btw, and I understand the rationale
behind the two immersion heater set up.

Okay, so recently, the bottom immersion heater died on me. I think it
was probably to do with a faulty timer (which was always ON!) and
possible a suspect thermostat. I've tried putting a new thermostat in,
but no joy. I also tried removing the immersion heater to replace
after reading the many posts here on the subject. It's stuck, and I'm
scared (and there are wolves outside). Then it occurred to me, why not
just wire up the top immersion heater? So I did, and it seems to be
working fine. Now I understand that this heater is only meant as a
"top-up", but presumably if you give it long enough, the water in the
cylinder should equilibrate anyway? Because in the short-to-medium
term, I thought I might as well just use the top immersion heater,
then get a "Professional" in to replace the pair of them (or the
cylinder...) when that one goes.

What do yers reckon?

The top immersion heater will only heat the top of the tank! Natural
convection ensures that there is always a temperature gradient from the top
to the bottom of the tank. If you only heat the top, the top section can be
hot enough to turn off the heater by means of its thermostat - whilst the
bottom of the tank remains quite cool.

This is fine for handwashing or the odd shower - but not a lot of use for
baths.

Is the bottom heater definitely dead? Have you measured the resistance of
the element with a stat removed? If this is open circuit then it pretty
certainly *is* dead. Otherwise, it may just be a wiring problem.

How did you try to remove it? You will, of course, have to drain the tank -
but it's easiest to *loosen* the heater whilst the tank is still full of
water - so that the weight of water stops the tank from moving while you are
applying a *lot* of torque - probably by sliding a tube over the handle of
the spanner to increase the lever arm length.
--
Cheers,
Set Square
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