Connecting old immersion heater

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Can anything catastrophic happen with switching on a never used immersion heater in an unvented cylinder? Heater and cylinder both about 18 years old but water always heated by boiler (which has failed).
 
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Nothing catastrophic is likely to happen ,may trip a breaker or blow a fuse ,might even work just fine !
 
Can anything catastrophic happen with switching on a never used immersion heater in an unvented cylinder?
Yes! Our hot water, from a vented indirect cylinder, is normally on 24/7 and heated by a gas boiler. The immersion heater is hardly ever used.
I recall coming back from holiday once and switching on the immersion heater to boost the heat supplied by the boiler, to bring the water temperature up from cold quickly. There was a minor explosion as the heater split open along much of its length. I presume a pinhole in the heater sheath had allowed water ingress over time and the water had turned to steam. Perhaps if the immersion had been in frequent use it could have kept its internal insulation dry enough that the pinhole would have gone unnoticed?
 
@Alec_t I guess that your house did not have RCD protection?

Water getting into a heating element is a very common cause of RCD trips and normally would have cut the power probably well before the element got hot.

@davelondon I would have no hesitation in doing it.
 
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Apart from an IR test, also check that there is a proper overheat cut out (manual reset thermostat). IIRC it should have by that time but always worth double checking.
As to "can anything catastrophic happen, in theory yes - but in practice the safeties would take care of it.
First line of safety - the overheat cutout in the immersion. Should that fail, then the water can boil - boiling water in a sealed container = big bang :eek: But, there should be a pressure relief valve AND an overheat dump valve (commonly in one unit) - they are a requirement for unvented cylinders, but you never know if it was properly installed in the first place and if it's been properly maintained since. Either valve will avoid the big bang - one by relieving the pressure, the other by dumping water and thus allowing fresh (cold) water in.
Mythbusters did a show on this - electric water heaters are more common over in the USA I believe. They had to do some work to stop th safeties working, but after that they got one to blow itself out through the roof of their mocked up house.

As I said, if you can find the whole thing, they had to do some work to stop the safety devices working. But if they do stop working, you have a bomb in the making.
The failure mode is : temperature keeps rising, pressure keeps rising, temperature gets to well above 100˚C, the pressure eventually causes the cylinder to fail (in the video above, the join between sides and bottom fails). You now have a cylinder of superheated water which flash boils into steam which then propels the cylinder like a rocket and you don't want to be anywhere near the blast unless you enjoy a flash poaching. Lookup BLEVE - boiling liquid expanding vapour explosion.
 
you'd need the thermostat to fail, then the overheat to fail, then the pressure relief to fail.

I don't know what weak spot in a real-life plumbing system would actually go first. Maybe a seam or connector. If any tap or pipe split or burst, it would release pressure in the cylinder. Flexible hoses in particular are liable to split.

The TV programme put a lot of work into changing a household appliance into a bursting balloon.

I think it's unlikely your cylinder could boil like a kettle. In cases where such things happened (prior to current levels of safety), householders noticed boiling water coming out of the taps, and weird noises coming from the plumbing, for hours or days beforehand.

no doubt you could make your kettle or boiler explode, or your electric blanket set the bed on fire, if you put enough effort into it. Or you could saw through a leg of the Forth Bridge and make it fall down.
 
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Thanks for the replies. I'm not concerned with what happens if it works OK as I have no reason to think the thermostat won't work and in case I'll be watching it closely. I'm only concerned with whether anything could happen early doors with the fitting to the cylinder as I don't want to risk the cylinder and end up with a leak.
 
... fitting to the cylinder ...
Is the immersion heater not already installed in the cylinder ? If not, then that's another issue altogether as I understand they can be "a bit difficult" to unscrew sometimes. There's have to be something, old broken heater, blanking plug, installed in the boss - and that could be corroded in place.
 

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