Wasn't there a case earlier this year where exactly that had happened but the expansion tank had softened and the whole lot of hot water came through the ceiling onto a child asleep below? I don't think you should leave immersion heaters on even with a safety cut out as there is no way to test them. Fitting a ~30 minute timer is probably the best thing to do, being able to leave them on is asking for trouble.
I doubt if a short stat would cause the problem, they are contained in a metal tube so would still switch. I suppose it could reduce the average temperature by a few degrees but not by much more as long as the heater was long enough. Its the layering of the hot water above the heater that causes problems as it will never (well as far as usefulness goes) heat the water below the heater.
Some tanks have two heaters one near the top to give some hot water for washing and one near the bottom to heat the whole tank for baths. I wonder if this might be the case here?
It is for exactly this reason I have procured a GRP expansion tank at moderate extra cost - my system will have electric backup - 3x 3kW immersions capable of running the whole CH/HW at half mast so to speak so part of the design is that the whole system might be run continuously on electric if the boiler dies. In fact I intend to commission the system on electric prior to installing the boiler.
The only sensible place for the tank is over my daughter's bed (in the roof obviously).
I have a GRP tank guaranteed to withstand boiling for some time and the immersions will be controlled by a tank stat via contactors in addition to their own stats in addition to their own saftey devices.
With triple redundancy on the control and a tank that can take the abuse I feel reasonably assured.
I felt extremely uneasy when I looked at the fliddy standard plastic tanks. That set me looking first for small metal tanks - don't really exist anymore. Then I came across GRP in various ratings from "hot water", "boiling water" and "evil chemicals".
I am not sure that the over temp device in the stats will stop the tank boiling. I think they are there to cope with a dry tank and the heater going near melt down. I do admit that I haven't checked this.
When you design kit that uses redundancy to achieve uptime you soon realise that redundancy without a way to test the backup is a waste of time. The first time the primary fails is the time you find the backups were faulty. I think the GRP tank is a far better idea.
I work on the principle of not allowing it to be on long enough to boil and being able to test the timer. A one shot seems sensible as it only gets used once or twice a year, the gas boiler heats it normally.
It is possible to test the first two levels - set the immersion local stat lower than the tank stat and make sure it operates correctly.
Point taken about the emergency backup - might well be as you say.
I like both :)
When doing a PIR an immersion without the safety cut out is reported as a code 2 (requires improvement). To change the stat is a few minutes work and less than a tenner
-- Adam
While that may be true it probably doesn't solve the problem..
It say nothing of the sort in the link you supplied.
It quite clearly states that the safety cut out and the stat must work independently of each other and that does not imply in any way that they cannot be contained in the same unit.
The link even goes on to say "a safety thermostat incorporates a temperature control plus an over temperature device" which really does imply to all but you that they are one unit.
Do try to read the links before you post them.
-- Adam
I did and my opinion differs from yours. Unless you have a court case that says retro fitting a "safety stat" is OK I will still say it may be necessary to replace the heater. I don't like the idea that the stat could be bypassed and hence the safety cutout. A heater isn't expensive it just takes a bit longer to replace.
age
"Unless you have a court case that says retro fitting a "safety stat" is OK"
WTF is that supposed to mean?
-- Adam
In message , Adam Wadsworth writes
Same as usual - denise hasn't got a clue
sad little worm that he is
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