Replacing an immersion heater coil - advice.

[andrewc]

The hot water in my flat became dangerously hot, so the thermostat is knackered. 

The tank is in a tight cupboard, with a plastic water tank directly above.  There is not enough space above the immersion tank to just remove the thermostat, it's a thin tube in a vertical channel, so I'll have to remove the entire element & angle the thing out.   I'm sure I've done this before but it was over 30 years ago & I've forgotten the details! 

There is a pipe from the header tank that feeds into the bottom of the immersion tank.  This has a stopcock midway & a drain at the bottom. 

Question: How far should such a stopcock turn ?  it will only rotate a few mm either way & I'm reluctant to force it. 

As I understand it the approved process is to pop the appropriate fuse & disconnect the wiring.  Shut off the cold water inlet & then run the hot taps to partially drain the tank.  Then loosen the immersion element & drain the tank from the bottom using a hose.


Question:  Is this bit really necessary ?  I don't remember doing it last time. 


Then remove & replace the element, wire it back up close the drain valve & re-open the stopcock, then hope nothing leaks.


Should I just call a plumber ?   It's a top floor flat & I don't want any accidents!

[perpetual dan]

The stop cock should turn more than a few mm, I'm sure. The most basic valve design would be a 90 degree turn, but the red circular thing i'm picturing has a couple of turns in it. We had one that was stuck some years back. Copious WD40 and a XL size spanner were the plumber's friend (the whole, ancient, boiler was being replaced).

According to the big book of DIY Dad gave me when we got our first place, the draining the tank just needs to avoid a mess. So a top-fitted element just needs "about a gallon" to be drained. I can't picture what draining after loosening the element will achieve if you're keeping the tank.

[Kim]

If the stopcock feeding the cylinder is dodgy, then disaster can be averted by isolating the supply to the header tank and draining both.  This would seem prudent, given the top floor flat factor.  Might be a good opportunity to fit a shiny new stopcock...

[robgul]

Another potential issue is actually getting the immersion element out - it needs a very big spanner and, usually, a lot of leverage . . . very frequently it will also be corroded . . .   I have on one occasion when applying a very large, 30" long wrench" to an immersion element managed to distort the the cylinder!  I got away with it, but only just.

I'd be inclined to get a plumber as he will have the tools and experience - possibly applying heat (flame) to get it loose.

[Diver300]

If you are removing the immersion heater, loosen it off before you turn off the water, so that the tank is under pressure and as strong as possible. Once it turns 1/4 turn or so it will start to seep water and you can turn off the water.

You can just fit another thermostat. It doesn't have to be a long straight one. You can get capillary thermostats like this:- https://www.ecproducts.co.uk/product/capillary-thermostat-0-90c-ts-dbtc-090/ which would fit down the hole for the thermostat. The old one could be removed destructively by bending / cutting the bit you had slid out a few times.

You could also fit a clamp thermostat to the side of the tank. https://www.plumbnation.co.uk/site/esi-clamp-on-cylinder-thermostat/ That would need to be fitted under the insulation, so a suitable sized hole would have to be cut if it's polyurethane foam.

[Feanor]

You cannot drain the hot water cylinder by running a hot tap.

The take off is at the top of the cylinder.
Once the cold header tank is empty, (or you get the valve to close ) there will be no pressure to shove the water up out of the cylinder.

You need to use the drain off point on the cold inlet at the bottom of the cylinder.
(Open a hot tap whilst you do this, to allow air in the top of the cylinder, otherwise it can become vacuum locked.)

If you can't get that valve to move, either close the cold tank inlet valve or tie up the ballcock and run the hot taps till the header is empty, or use a rubber bung in the outlet pipe from the cold header tank.

[Wombat]

Wot Diver 300 and Feanor said, plus:
I'm puzzled about how it could be impossible to get the thermostat out, yet OK to get the much bigger and longer heater out. And surely, its not vertical, but at an angle near the top of the cylinder, as the takeoff is at the very top.

Generally, removing immersion heaters is not a task to be DIY'd unless you a) have the required spanner, and b) know exactly what you are doing.  It is more than possible to destroy the cylinder when applying enough force to undo it.

My own one is in a cupboard with 200mm of headroom above it.  There is a circular hole in the ceiling above it (its a bungalow).  Mine stopped working (it is the usual source of heat, powered by my solar PV), so I put a tester across the various terminals and decided it was the stat, so jiggled it out, and replaced it with the spare one which was lurking in the utility room when we bought the house, after testing both with a kettle of hot water, a thermometer and a multimeter.

If in the slightest doubt, and you can afford it, GAMI.

[andrewc]

Well.  I kept putting this off & finally had a go last night.    Tied the ballcock up & drained the tank down,  but couldn't shift the element with the spanner I had.   


Didn't want to leave the pipes empty so let the tank refill.   


Called a plumber this afternoon, who was already working around the corner & turned up an hour later.  Explained the problem.   


Oh, you probably don't need to replace the entire thing, just pull the thermostat out & replace that.  So he pulled the thermostat out.  And then the top of the tank started gushing water....  The long tube that held the thermostat was knackered.    Used a lot of towels, turned the mains off & it eventually stopped spouting.   


He replaced the element but then said the cable from the wall socket was in poor condition, so he's coming back tomorrow to replace that & fit an in line valve as well. 


I've got some mopping up to do, and I'm hoping my downstairs neighbours ceiling is OK.