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resettable immersion heater thermostat



 
 
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  #1  
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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  #2  
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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  #3  
Old October 9th 04, 01:32 PM
Ian Stirling
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Default

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.
  #4  
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
  #5  
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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  #6  
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.
  #9  
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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