i have just fitted a resettable immersion heater thermostat. It has a dial to select the temperature and a reset button. Since you can select the temperature using the dial i don't really understand what resettable means in this context. can anyone explain? thanks.
Sounds like a secondary safety device which trips if the main stat fails to operate for some reason.
Another explanation - albeit unlikely - is that it heats the water only once, and then turns off until reset. I'm not aware that anyone makes any like this, but it could be useful if you have a combined gas-powered heating and hot water system, but need the occasional boost from an immersion heater when needing a lot of hot water in a hurry. No? OK, I'll get my coat!
In case it gets too hot for some reason, you can press the reset button when the overtemp thermostat cuts it out. Otherwise for one with a thermal fuse, you'd need to replace it.
In article , Set Square writes
I'll go with this one, it's all to do with health & safety (the good kind) and the single fault principle. The idea is that a single fault (say the main thermostat contacts welding closed with age) should not cause harm to the user, say by the water boiling in the cylinder & consequent risks. In this case, the second safety device will cut out, removing the power and protecting the user. It's such a fundamental principle in the certification of all potentially hazardous electrical & electronic equipment that I'm surprised it's taken so long for these to appear in immersion heaters - kettles & coffee makers all have them as standard.
I'm sure you're right. It seems a *bit* odd for it to be re-settable though because if the main stat has welded contacts you shouldn't be able to turn it on again! [An electric kettle is different because some fool might turn it on with no water in. When it's re-set and re-filled, it's fine.]
If for example there are a couple of heaters in one cylider, then it's quite possible to trip the device with out it being faulty itself.
Alternatively, it is of course possible if the water fails, to run the heater dry.
It has a secondary, manual-reset thermostat, a safety device that operates at a higher temperature than the first adjustable thermostat. The contacts on thermostats can often fail by spark-welding themselves together, so they fail to an unsafe condition. If the re-settable thermostat operates, it can be reset by pressing a small button. This is supposed to alert the operator to the fact that the system has suffered a failure and it needs fixing.
A woman was killed in Cornwall , about a year back, when boiling water from a defective immersion heater thermostat accumulated in the water storage tank in the loft. The plastic tank collapsed, sending a cascade of boiling water through her bedroom ceiling. The relevant BS was changed after this, but the older pattern, non-resettable, type can still be sold.
Surely , it would be safer for the secondary device to be *non*-resettable?!
When you say the older pattern can still be sold, does this have a non-resettable secondary device, or doesn't it have one at all?
I think, the idea is that the reset button requires non-user intervention (the cover must be unscrewed to gain access). So when the main thermostat welds permanently on as they can and do the heater will stop working until someone (presumeably competent) fixes it and resets it.
There was a case of a fatality the other year due to a stuck immersion and a stack of other problems.
*non*-resettable?!
You can get non resettable versions. As Ed said, the idea is that you need to remove covers to reset the stat (to stop the plebs messing about). I understand it is now compulsory to only fit stats with a secondary (overheat) thermal cut out.
Adam
Many thanks to you all. I don't think I'll spend too long worrying about it!
In article , Set Square writes
FWIW I've just changed one in my daughters flat, had a small BI-metallic switch type thingy on in series with the thermostat, over temp cut out I suppose.....
Many thanks to you all. I don't think I will be worrying too much about this!
Hmmm, yes. I don't know the logic behind this. It might be that the device could be tripped by a another fault, say the thermostat being incorrectly wired or having been removed from the immersion heater pocket. If the secondary thermostat has tripped it wouldn't necessarily follow that the primary thermostat is defective, although that would be the most likely cause.
They don't have one at all, only the primary control thermostat. I think the older type can still be bought and fitted. I believe the legislation was rushed through and there are still stocks of the old type in the distribution system.
I think the old stocks were removed from the distribution chain a few months ago. I think someone asked in this newsgroup about this, as they were looking for a replacement and could not obtain one. Places like B&Q had notices to the affect that they were not allowed to sell them.
It's only recently that B&Q have immersion thermostats on there shelves. If, as Ed mentioned, someone was killed by hot water (probably one of the worst ways to die) it makes sense that the 'stats were removed and new regulations were rushed in.
My parents have had a couple of immersion 'stats fail, twice resulting in the supply flex burning and cutting off the supply. But once it had managed to heat the cold tank upstairs till it was quite warm. At the time I had suggested a second external tank 'stat...
Very Scary!!!
-- Peter
In article , fred writes
It is now law that that immersions without such a device can not be supplied. On 1 Apr 2004 BS3456 section 2-21 was withdrawn BS EN 60335-2-73:1997 was introduced. The thermal trip must not be automatically resettable (like those in kettles etc). Wholesalers and retailers will only have the new standard ones in stock. There was a shortage of immersion stats in the sales outlets when the new standard came in but all old standard stocks were removed from the market.
The introduction of non resetting trips was because of serious scalding injuries and in some cases fatalities ( an elderly lady in Aberdeenshire died of a scalding incident just days before the introduction of the new standard)
The really old stats were much more reliable than cheap recent ones which I was forever replacing.
Sounds like its intended to be used in series with a standard immersion stat for use in an unvented cylinder. I recently replaced an immersion in an unvented cylinder and it had two thermostats inside, one of which was manual reset.
I agree: Sounds like a resettable 'over temperature safety switch'! The OP will probably never have to look at it again? Similar 'overheat' device are scattered through electrical appliances. As far as I have seen all electrically operated hot water tanks/cylinders here have this 'overheat' device; typically it's small black plastic block which mounts just above the upper thermostat wired in series with one leg of our 230 volts supply, (I guess in the UK that would be the 'hot/live' 230 volt input lead). If the tank overheats for any reason which might include low water and therefore maybe steam, in top of the tank, a thermostat goes short circuit etc. a bi-metallic disc inside the device 'clicks' open a single contact disabling the input voltage. The bi-metallic (when everything has cooled down!) can be reset by a red button. The upper thermostats and this gadget are revealed by removing the cover on the side of the tank where the wires are connected. In over 30 years I don't think I've seen one bi-metal that has operated and they never seem to go faulty, but since they seem to come as part of a new upper thermostat replacement, I must have several in the junk box! Unlike thermostats and heater elements. A typical tank/cylinder, often referred to as "A hot water heater" or "Hot water tank", contains about 35 Imp. Gals and has upper and lower 3000 watt
230 volt heating elements each with its own upper/lower thermostat which are adjustable. Instructions regarding setting the water temperature vary. Depts. of Health demand and will test for a minimum of 160F. But Safety Organisations will recommend a max. of 120F "To avoid children scalding themselves"! Conservationist recommend low temperatures, such that you then need to use more 'hot' water mixed with cold for a reasonable shower! As shipped the upper and lower heaters are wired 'Flip-Flop', in other words the upper heater heats top of tank first. When top reaches the set temperature the upper thermostat flips over and the lower heater under control of the lower thermostat then heats the lower section water to its set temperature. etc. It is possible by moving one wire to have both upper and lower heaters work independently, each under control of its own thermostat, for faster recovery, using a maximum of 6000 watts, which may require heavier wiring!In article , Set Square writes
It just came to me there, a resetable secondary device is testable whereas a non-resetable device is not testable, except by batch testing, that probably makes the resetable device safer. Zzzzzzzzzzz, scared myself for a moment there . . .
"Aidan" wrote in message news: snipped-for-privacy@posting.google.com...
news:...
*non*-resettable?!Seen, installed and used electric hot water tanks here (Eastern Canada) for almost 50 years. As far as I can recall, at least since 1960, of the eight to ten of our own or other family members, never seen one that did not have that 'overheat' bi-metal disc cut-out mounted, under cover, just above the upper thermostat! They never seem to go faulty and I've always assumed they are there as a heat cut-out of 'last resort'. "Better the tank don't work than steam come out of the bath taps!" Deteriorated tanks and corroded heater elements have been something of a problem here; probably due to the frequent presence of iron in some of our water supplies and/or ground-water or drilled wells. In fact, while manufacturers will warranty a tank for five years elsewhere they will, now, only do so for three years in this part of Canada! We are also starting to hear of some 'pin-hole' leaks in copper pipes of older houses possibly due to the same. Over the years numerous tanks have been discarded, often someone, such as myself take the thermostats as spares, the upper thermostat always seemed to include the overheat device, no matter what manufacture or whether it was Canadian or US made. So I've always assumed the overheat device was 'standard'. Previous postings interesting though; cos. British Standards in many respects, especially electrical, seem to be stringent? Also UK electrical devices are generally, it seems, substantial and well made? Although I still don't like those little tiny brass screws and the two individual base spring plunger contacts for bayonet type bulbs! Are they still used? Or am I out of date? Edison threads seems to be so much simpler. And yet cars over here use some bayonet (and wire-ended) lamps! Anyway safety of the best made equipment conforming with the best standards can be invalidated by improper/stupid installation and practices. This is great group for learning; enjoy the informative postings. Terry.
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