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Old October 9th 04, 12:43 PM
Ken Knott
 
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Default resettable immersion heater thermostat

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.

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Old October 9th 04, 01:28 PM
Set Square
 
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In an earlier contribution to this discussion,
Ken Knott wrote:

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!
--
Cheers,
Set Square
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Old October 9th 04, 01:32 PM
Ian Stirling
 
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Ken Knott wrote:
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.


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.
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Old October 9th 04, 01:42 PM
fred
 
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In article , Set Square
writes
In an earlier contribution to this discussion,
Ken Knott wrote:

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.

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.
--
fred
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Old October 9th 04, 03:17 PM
Set Square
 
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In an earlier contribution to this discussion,
fred wrote:

In article , Set Square
writes


Sounds like a secondary safety device which trips if the main stat
fails to operate for some reason.


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.]
--
Cheers,
Set Square
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Old October 9th 04, 03:51 PM
Ian Stirling
 
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Set Square wrote:
In an earlier contribution to this discussion,
fred wrote:

In article , Set Square
writes


Sounds like a secondary safety device which trips if the main stat
fails to operate for some reason.


snip
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.
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Old October 9th 04, 06:39 PM
Ed Sirett
 
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On Sat, 09 Oct 2004 11:32:41 +0000, Ian Stirling wrote:

Ken Knott wrote:
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.


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.


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.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html


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Old October 9th 04, 08:15 PM
ARWadsworth
 
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Default


"Set Square" wrote in message
...
In an earlier contribution to this discussion,
Aidan wrote:

(Ken Knott) wrote in message
. com... Since you
can
select the temperature using the dial i don't really understand what
resettable means in this context. can anyone explain? thanks.


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?!

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




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