Immersion heater - spot the problem

Couple of weeks ago I got a phone call from our tenant who mentioned that there was no hot water - the flat has electric only, and all hot water provided by an immersion heater in a direct tank.
They'd managed to leave the immersion switched on for about a day, and it had not only heated the 120l tank, but the large cold storage tank above it (I estimate it must be 250 or 300l at least) to a rather scary temperature. After that, they said it didn't do any better than lukewarm, and then packed up altogether.
Well, after spending couple of weeks phoning him to get access to the flat (suspect he'd been avoiding my calls, he owed us rent....) and sorted the problem out.
I'd been idly speculating upon what the problem might have been whilst waiting to do the job, perhaps the thermostat had packed up, etc etc. Heater was open circuit when tested, so drained the tank down and whacked in a replacement, no problems.
But, can anyone spot what could have been the problem with the heater....
http://www.olifant.co.uk/Olifant/PublicGallery.nsf/plinks/RSAN-62XF2E
The immersion heater is now connected via a timer...!!!
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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In an earlier contribution to this discussion,

Look like several problems to me.
1. Vent pipe dipping into water in header - thus setting up a thermo-syphon system which heated up the header tank 2. Immersion element over-heated and bent until it touched the thermostat. The stat was then driven by the element rather than the water temperature, and cut out prematurely - providing only tepid water 3. Element finally failed altogether
If I'm right about (1), you'll need to fix this, or the problem will recur.
--
Cheers,
Set Square
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thermo-syphon
I concur. Normal heating expansion shouldn't heat the cold cistern. This needs to be fixed urgently, as (a) it isn't safe and (b) it must be costing a fortune keeping the barely insulated cold cistern at 60C using electricity.
Christian.
P.S. I would always fit a dual thermostat immersion with a manual reset trip set 10C above the main operating thermostat. These are required for unvented cylinders, but heartily recommended for other types of cylinder too.
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costing
trip
unvented
this is the first time it has been known to happen, though (heating the storage tank). They say that the immersion was left switched on for a day, I have suspicions that it was far, far longer than that, though...
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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If the stat had stuck on, and it was left on without being used, the steam from the boiling water will heat the header tank over time.
--
*One nice thing about egotists: they don't talk about other people.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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In an earlier contribution to this discussion,

I guess this must be the case - since it has apparently prompted a change in the regs - but I don't quite understand it.
I would expect the steam to come out through the vent pipe and to fill the attic with steam rather than heating the header tank (unless the vent pipe dips into the water). Surely, to heat the water in the header, the steam would have to come up the cold fill pipe from the bottom of the cylinder - which seems unlikely. Am I overlooking something?
--
Cheers,
Set Square
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On Fri, 16 Jul 2004 20:57:38 +0100, Set Square wrote:

<snip>
When a cylinder boils the expanding steam in the cylinder forces hot water out of the vent pipe and thus into the header tank. Normally making loudish thumps and bangs as it does it.
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
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In an earlier contribution to this discussion,

Yes, but that doesn't really explain how the whole header gets heated up to a very high temperature.
--
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Set Square
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The vent pipe outlet could be pretty close to the top of the water, and blasting steam at that will heat it up. As the water in the cylinder boils, it will draw water in from the header tank - so it will, to a certain degree, recirculate. I've certainly seen this with a central heating system pumping over.
--
*A backward poet writes inverse.*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

thermo-syphon
recur.
Hmm, thanks, I hadn't considered this. I'll check it when I'm back there next week (there's nothing like a simple job for throwing up all sorts of nasties that need to be put right, is there!).
I'm not entirely sure that the vent pipe is dipping into the header though because when I drained the tank this would have surely either 1) siphoned water out of the header, or 2) drained the header to the depth of the vent pipe, and I didn't hear the header valve opening to replenish the level. I had thought that in this case the header heated up simply through conduction and convenctive currents set up within the feeder pipe to the HW tank.
Still, the timer will go a long way to preventing future occurrence, because it's set to heat the tank in the morning, and they are unlikely to get through an entire tank in the day (electric shower used far more than the bath).
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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because
The timer is a red herring. When I use my immersion, I leave it on 24 hours a day. Better to fit a immersion with 2 thermostats. One being a manual reset overheat type.
Christian.
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Sticky stat?, see below, ouch . . . . http://www.cibse.org/index.cfm?action=showpage&pageID92&TopSecID 
--
fred

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so the thermostat was shorted?
Regards, NT
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that
it
it
temperature.
packed
flat
the
whacked in

heater....
I dunno, but I've got it here still. If I get a spare moment, I'l stick it in a pan of water, bring to the boil, and watch how it's resistance characteristic changes.
I was just impressed by how catastrophically the whole thing had failed.
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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I came to this thread a bit late
Richard, if you really have problems, ring me on 01923 229224
--
geoff

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The thermostat switches power to the heater off when teh water reaches the desired temp. If the whole thing cooked itself to death then almost certainly it was thermostat failure that caused it. If thats whats happening, the solution is to replace the thermostat on the tank.
The stat may come as part of the new element assembly, depending on the design. If the element assembly has a temperature dial on it, and you didnt remove that bit, it will now have a new thermostat already.
Regards, NT
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it
temperature.
packed
in
This looks like a good idea for the NG. If people come up with an interesting problem that they have solved, why not post it and see if others can do it. This sort of thing is used on training programs where faults are set- in cars for example. I'd certainly like the challenge.
Peter Scott
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On Sun, 18 Jul 2004 10:34:24 +0100, "Peter Scott"

Aren't there already enough unsolved problems on here already?!
--

SJW
A.C.S. Ltd
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Leave IMM out of this.
--
*Dance like nobody's watching.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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You did ask for it.......
a hoover was being used to draw water up a pipe - yes, stupid - but it sucked rather better than expected. It sucked the water right the way up and shot it out the back. The chap who did it was unharmed, and no trip/fuse/RCD tripped. Explain.
Regards, NT
PS dont try this at home, or anywhere else, it really is dangerous
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