Immersion heater.

The other issue is undoing without deforming the ever thinner copper of the tank. Heat up the thread section with a blow lamp before attempting to undo . Moderate torque plus tapping spanner with hammer , before upping the torque.

The local DIY store offers a very flimsy looking pressed steel spanner that won't put enough torque on the element to do any damage.

There's a more robust looking box spanner on Ebay that would apply the torque with less risk of buckling.

The last element came out surprisingly easily - so I'm remaining optimistic.

What size was it back then?.. :-)

I have a 27 inch element fitted at the top of a 42 inch cylinder. Water heating is normally by the CH coil but I occasionally test the immersion heater to make sure it works.

The manufacture's leaflet states:- "... this heater will only effectively heat water ABOVE, it is important that the correct length be fitted, i.e. if an 18" heater is used in a 36" cylinder only 45-50% of the water will be heated"

I have sensors at the top, 2/3 and 1/3 levels and a typical immersion heater test over a few hours shows that the tank is not fully heated.

Its square and not cylindrical, and the element is horizontal at the bottom - that may not accommodate as long an element as diagonal in a cylinder.

There is another threaded bush half way up the tank that I could add a second element if I wanted to.

OK - with you now. Some simulations here:-

useful only if yiou want to keep water hot for handwashing etc without heating the whole tank.

You'll get a much better response in uk.d-i-y. sci.eletronics.repair is a rather weird place to ask a question about an immersion heater.

Yebbut you need so many filters - Harry, Roddy, Denis, Toughguy etc

Aye, that's true. Can't say I blame you :)

A well insulated tank costs more to heat up from cold than keep topped up by the thermostat.

Not as bad heating half a tank from cold, but I'm guessing it would still cost slightly more.

Except the rate of heat loss is dependent on the delta T between inside and outside. Colder tanks lose less heat, if they haven't already reached minimum, than warm tanks being kept warm, so it takes less energy.