Immersion heater question

[Barry Young]

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?

[Set Square]

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.

[Dave Plowman (News)]

If the top mounted one has only a short element, it will only heat some of the tank. Strange, given that water tends to circulate, but nevertheless true.

That was the idea of the two - the lower one to heat the whole tank, the top one just for when you only needed a small amount of hot water.

There are/were also twin length top mounting types to do the same thing.

It might be possible to remove the top one and replace it with a full length type - they're cheap enough.

[John]

SNIP

If you are in a hard water area the element will have most likely split and twisted out of shape. If the insulation resistance of the disconnected heater is less than a Meg its a goner regardless of whether the element shows open or a through circuit!

You have every chance of tearing a hole in the side of the cylinder if you simply apply a high torque. Use a ring or box spanner and "tap" it round with a hammer until the seal of jointing compound and corrosion detritus in the threads is released. In very extreme cases application of heat may be required as well but tapping is effective in about 95% of stuck element cases. Do it with the cylinder full until it just moves then drain off and continue to undo it.

[Barry Young]

Yep, I was half expecting that. I've just turned up the thermostat by a couple of notches and that seems to make it kick in again. I understand I'm not going to have convection currents on my side this way, but I imagine conduction through the water and the tank body itself will help to at least partially address this.

Well I've successfully run one bath from my new set-up! I'll have to try again tomorrow in the evening after the hot water's been sitting around all day.

I haven't measured the resistance, but certainly when you switch it on, there's very little response from the little turny wheel in the electricity meter. I used the same wiring on the top immersion heater and it works fine, so I'm pretty sure the actual heater's kaput.

I got one of those box spanners and applied a hairdryer for a while. Nothing happening apart from some a few groaning noises. I'll drown it in WD40 and see if it's any more cooperative tomorrow.

[Barry Young]

There's a thought. I hadn't actually bothered trying to unscrew this one as it wasn't broken.

I'm going to try my current set up for a week or two with different thermostat settings and with the timer on for different lenghts of time.

[Dave Plowman (News)]

I'd say if you've got a decent bath out of it, it's a full length type. The short ones normally fitted only give a few basins full.

[Dave Plowman (News)]

Right. I assumed the top one was in the 'conventional' position.

[Will]

The only problem here is that where two immersion bosses are fitted, they are - ime - always mounted in the side of the tank. So even if the tank were large enough to fit a longer element into in the horizontal plane, it would make no difference to the volume of water heated...

[Christian McArdle]

That isn't enough to be sure. Frequently it is the heater's internal thermostat that blows. These can usually be replaced without removing the immersion. It is worth checking to see, even if it does turn out to be the element anyway. The thermostat is usually a long thin rod that slides into a tube visible when you remove the cover cap.

Christian.

[Set Square]

. . . and wired in series with the element. That was why I suggested removing the stat and measuring the resistance of the *element* - thus eliminating any problems with the stat or connections.

[Christian McArdle]

Indeed. The OP, though, had only seen that it wasn't drawing current at the meter and hadn't done what you suggested.

Christian.