Immersion heater thermostat

[worms]

My immersion heater thermostat has blown, which should be an easy fix. However the eejit plumber who installed it sited the cylinder with the immersion heater round the back (which, to be fair, suits the pipe runs) and then mounted the header tank immediately above it. This causes two problems: I am working mainly by feel or with a mirror; and there is insufficient vertical space to withdraw the rod from the cylinder (so I don’t even know what the length of the current one is!).

In a previous house overseas, with two cylinders, each with two thermostats, I was constantly replacing thermostats. But back then, it was only the thermostat head itself that I was buying, not the whole fitting with the metal rod. Does anyone know if this is still an option on newer thermostats? It would save me draining and disconnecting my 92 litre header tank!

 

[philpjuk100]

Sorry,don`t know if the thermostats are replaceable it was the element that was open circuit on mine,I am just after sympathy,the heater is on the top left in the pic,I was considering going in through the roof but in the end disconnected all the pipes and laid the cylinder down to change it!

 

[bobby spray]

That sounds awkward. The only immersion heater I’ve replaced was the long coil type. I think that these are pretty standard in the UK. There should be a valve between the header tank and the HW tank that you can shut to enable you to drain the HW tank without having to drain the Header tank. I’m afraid it’s not and quick job. Isolate the HW tank, drain it, then disconnect all the pipes, pull the tank out remove the immersion heater. The top of the immersion heater element should have some info on it which maybe help you to have a replacement ready so that it can go straight back in. make sure you remove the heater carefully. The copper tanks are thin and easily creased.

 

[steveb]

my recollection is that the thermostat may be a probe that goes in a pocket so you can take the immersion thermostat head off and withdraw the thermostat without having to drain down or remove the whole immersion

I wouldn't fancy doing it blindfold with my arms round the back of a cylinder though!

-thought- call the plumber and ask for a fixed price quote to change the stat :twisted:

 

[fernicarry]

Last one I did, the thermostat just slid out of the inside of the element. You'll need to undo the terminal blocks and I imagine connecting up the new one by touch will be such a nightmare you'll end up taking it all apart anyway...

Try to get a quality replacement, if such things exist. The one I replaced failed after only a few weeks.

 

[worms]

Thanks all, for the replies!

Philpjuk, You win! Mine is nowhere near as problematic as that!

Bobby, it’s only the thermostat that needs replacing, the heater element works fine!

SteveB & fernicarry, my problem is really that there is not enough space above the thermostat to slide a new one in unless I drain down and remove the header tank. I could bend or break the old one to get it out, but would then be stuffed! I was hoping I could just replace the head, leaving the old probe in place. That’s what I used to do elsewhere, but I can’t see any screws to disconnect the head and can’t get it out to look at it properly! I guess I will buy the shortest possible replacement and see if there is any way to cobble the parts together. (Actually, I have a spare Horstmann cylinder thermostat for the central heating and I am tempted to just strap that to the outside of the cylinder and wire that into the immersion circuit!)

 

[philpjuk100]

I like the idea of an external adjustable thermostat but it will need to carry the element current or maybe use it to operate a relay.

 

[worms]

I like the idea of an external adjustable thermostat but it will need to carry the element current or maybe use it to operate a relay.

Yes, I was reckoning it would need to operate a relay, but the data sheet suggests the Horstmann cylinder thermostat has a 16A switch contact rating compared to the 15A on the standard immersion thermostat.

Best, though to keep it simple and just fix the b****r properly!

 

[CliffordPope]

We've got almost exactly that - the tank is in the loft under the sloping roof with a top-entry heater element. The thermostat would normally come out easily by itself, but the roof gets in the way. The element can be jiggled sideways to clear, but the thermostat is down a long straight tube. I presume that bending the thermostat and then forcing it straight to go down the tube would destroy it?

My solution was exactly as suggested - a stand-alone thermostat clamped to the cylinder, switching the element through a suitable relay.

 

[Owain]

Warning
If you use an external clamp-on thermostat, and it falls off the cylinder, the stat will never get up to temp and never turn off. The immersion could run continuously until the cylinder boils, dumping boiling water through the vent pipe into the expansion tank.

http://news.bbc.co.uk/1/hi/england/somerset/7179092.stm

Relays can also fail 'on' with contact welding.

All immersion heaters must have an integral stat with a secondary manual reset safety cutout.

A good plumber/metalworker cans older a new brass boss to the accessible side of the cylinder which will take a new immersion heater and stat if the existing one really is inaccessible.

 

[Flyfisher]

Well yes, but even after all those modifications to the tank to allow the use of a ‘standard’ thermostat, that could still fail in some way. Relays certainly can weld their contacts closed in theory but how likely is it in reality - especially with a properly rated relay? Same with the straps securing an external tank thermostat. Besides, as you rightly point out, even if the tank does boil the header tank should accommodate it. So when to stop worrying?

Having said that, I’ve known people who would never have a gas cooker in case of leaks . . . which of course do happen but are hardly commonplace. Risk perception is a funny thing - people who are scared of flying may not even give a thought about driving to the airport, yet car accidents kill far more people than flying.

 

[worms]

Owain,

Thanks, you are of course correct on the risk, and I remember reading that awful case about the baby at the time. I’ve noticed that the immersion thermostats sold now all seem to have a manual reset button - is this the type that you are suggesting or would you envisage an additional safety over-ride that is not part of the main thermostat?

In practice, our immersion is used very rarely, since our hot water is mainly provided by the central heating. The thermostat has not been working for some time and as you point out this can lead to the water getting pretty hot if you fail to turn it off in time. However, by happy coincidence, the motorised valve for our CH to hot tank connection had failed in the open position (until I fixed it last weekend) and this meant that if the immersion was left running too long, we got enough gravity circulation to dump heat round the system. Having fixed the motorised valve, it is now more important to fix the thermostat.

Further down the line I want to install a programmer/timer into the immersion circuit so that I can reduce the temperature at which the motorised valve shuts, run the normal tank HW temperature at about 45C but have a once-a-week, programmed immersion boost to bring the temp up to 60C for Legionella control. Adding a timer/programmer would further reduce the risk of the immersion remaining switched on in the event of a thermostat failure.

 

[Owain]

I’ve noticed that the immersion thermostats sold now all seem to have a manual reset button - is this the type that you are suggesting

Yes.

 

[d49xc]

Thanks, you are of course correct on the risk, and I remember reading that awful case about the baby at the time.

That case went as follow:
The thermostat failed and the backup safety reset either did not exist if it was an old unit or also failed.
The heater rather than being timed was set to be always on so so it was going full throttle 24/7
The water in the cylinder eventually boiled
This set up a thermo siphon effect with the header tank in the loft such that hot water went up the expansion tube and more cold water was dragged down from the tank....and so on.
Eventually the cold water tank in the loft got to near boiling temperatures.

Now you might have though that the fact there was near boiling water coming out the cold taps fed from the tank as indeed it was might possibly have set of some warning thoughts in the occupants heads - but apparently although this was "noticed" nowt was thought to be needing urgent attention........

The next unfortunate sequence resulted from the the plastic cold water tank in the loft instead of being mounted on a "floor" of wood to spread the load across the loft joist was sadly just dumped on the bare joists.
This created stress point where it rested on the joists and made it far weaker and its safety factor far less.
As the temperature in the cold tank increased to levels far in excess of its thermal capacity the tank sadly gave up the struggle and split dumping its entire 50 gallons of near boiling water straight down though the plasterboard ceiling into the room below where even more sadly there happen to be the said baby asleep - end of story.

 

[Flyfisher]

Certainly a tragic event, but it seems to me the weak link was an unsuitable header tank rather than its mounting arrangements. If the header tank had been proof against boiling water then it would not have split.

Surely there are regulations for such things?

 

[CliffordPope]

Warning
If you use an external clamp-on thermostat, and it falls off the cylinder, the stat will never get up to temp and never turn off. The immersion could run continuously until the cylinder boils, dumping boiling water through the vent pipe into the expansion tank.

Ours wouldn't. It's on a timer. Of course it's always possible that the thermostat, the relay and the time switch might all fail.
But then we'd notice the red light on in the kitchen, and the remote thermometer climbing.
And it's only a supplementary heater for the boiler, used for short periods.

 

[worms]

Thanks all, for the input.

So to do the job properly, I need to empty, unplumb and remove the header tank. Fairy Nuff! (It’s panto season ).

Of course, to take into account the baby case, I should add a layer of marine ply on top of the 2x4s that the header tank is sitting on...

Which will mean that the plumbing will be out by 12mm, so I should properly re-plumb the header tank and the overflow...

But then, if I am going to the trouble of replumbing the header tank, I should really lift the base of it by 11 inches to allow the thermostat to be removed the next time without needing to unplumb the header tank!

And if I’m doing all that, I should really consider just putting in a new header tank, since the current one is 20 years old...

But then, I really want to upgrade to a thermal store/accumulator tank, which would have a built-in header...

Don’t you just love old houses! :lol:

 

[tobydog]

You could just put “Buy a new combi boiler” on your list and all your problems are solved except for a having a severely depleted bank account. Unfortunately this is based on very recent experience

 

[worms]

You could just put “Buy a new combi boiler” on your list and all your problems are solved except for a having a severely depleted bank account. Unfortunately this is based on very recent experience

Unfortunately it is worse than that as the whole system is powered by a Rayburn - which could do with a replacement or a serious refurbishment. I’ll try writing to Santa for an Aga, separate combi-boiler, Thermal store and some solar thermal panels and hope he doesn’t knacker my flue liner trying to get that lot down the chimney and through the woodburner!