..and the next thing to go wrong in the house is.......

the immersion heater

Ok, full story, so any clever heating engineers can have a think will
the full picture.

About a month or so ago, we got a monster electricity bill for the year
(12 months at £30 a month, then an additional final settlment bill for
another £312). Electricity company never sent us any bills all year,
so didnt' realise how much we were using....anyway, that's another
story.

Investigate the big electricity guzzling devices, starting with the
water tank immersion heater.

Turns out the timer is burnt out...completely, so the immersion heater
has been on for a whole year 24/7 although there is a thermostat in
there...but don't know if that was working properly or not (it was set
to about 75 degrees Celcius). Insulation on all the wires going into
and out of the timer had burnt off/melted and there were black burn
marks on the wall too where it had been.

Bought and fitted a new timer device, which works fine now.

However, since replacing the timer there doesn't appear to be as much
hot water as there was before - as you can barely have enough for 2
people to have showers before the water starts coming through the
shower cooler and cooler.

The immersion heater is currently set to come on at about 4:00am and go
off at 8:30am - so that's a good 2.5 hrs before I use the shower at
6:30am.

I've replaced the thermostat (because the temp adjusting knob snapped
off the old one), and the new one has a thermal safety cut out thing,
so if the water gets too hot it will shut off (but you can reset it).

I tried having the thermostat turned up to about 70 degrees (as it was
before I changed the timer) but the thermostat kept tripping, so I've
gradually been turning it down and down (think it's not at about 60 deg
and it hasn't tripped yet).

There's still hot water out the top of the tank going to hot water
taps, etc - and this water is still plenty hot enough to cause an
'ouch' if you have your hand under it too long!

I think the immersion heater is a 27" one, as it has an 18" thermostat.

Here's pretty much how the hot water tank is (kind of to scale to in
terms of heater and tank outlets):

http://www.randcjones.plus.com/Temp/heating.jpg

You can feel the shower feed pipe is hot, but then it obviously gets
colder as the water is used.

I'm in the shower for literally 5 mins (10 max), and my wife then uses
it after me for about 10 mins (15 max).

So not exactly running the shower forever.

I've got a new immersion heater (27" again, as the guy said that was
the largest that would fit in a tank or that size) and I'm hoping that
the problem is because the end of the immersion heater is either
damaged or the whole thing is calcified.

Just seems really weird how we never had shower hot water shortage
problems when the thing was apparently buggered - yet now with new
thermostat and timer it seems to be crap?

The only thing I can think of is that the heater was literally boiling
the water so that when cold water comes into the tank, it gets heated
by the already raosting water not only the heater. If this is the
case, I need to find a thermostat that will allow the heater to really
heat things again, so that the shower then uses less hot water to mix
with the cold (it's a thermostatic mixer valve on the pumped shower, by
the way).

Any advice or opinions would be most welcome!