overheat stat


Postby sido » Tue Dec 16, 2008 11:54 pm

HI, i replaced my copper cylinder because the insulation was burnt off due to an out of date thermostat fitted in the 27 inch heating element, i also replaced the old and rusty header tank,so all items new,but i found the new overheat stat, kept on activating ,so i returned complete element and stat ,and replaced them, but the problem remains overheat stat constantly activating,any thoughts would be appreciated,i have tried different, settings but no joy,water gets hot then off, its a vented cylinder hot water only thanks sido,[ a retired plummer helped me but cannot offer any help on fault]
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Postby stoneyboy » Wed Dec 17, 2008 11:09 pm

sido,
You need to clarify your question. Immersion heaters have a thermostat which turns the electricity off once the water in the cylinder has reached the preset temperature. Once enough water has been run off - cooling the water in the tank - the thermostat will sense this and turn the electricity back on.
end
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Postby sido » Thu Dec 18, 2008 1:01 pm

Thanks for your reply stoneyboy,not sure how i can say more than already said, the thermostat cuts out ,and has to be constantly reset by the reset pin,which of course means ,sometimes cold or lukewarm water depending when it switches off ,IE i go out to work at say 9,reset pin, wife checks at 12 lovely hot water i come in a 5, warm water,reset pin again,wait hour or so ,hot ,find pin needs resetting, again element and stat replaced twice, thanks colin
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Postby slapdash » Thu Dec 18, 2008 4:02 pm

Sounds like the thermostat is detecting the overheat (you say it's activating - can you hear it clicking?) but is it wired up properly to cut off the power to the element?

Check the wiring diagram you got with the thermostat. The power to the element comes off the switched terminal of the thermostat and so is turned off when it detects the correct temp.
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Postby plumbbob » Thu Dec 18, 2008 6:56 pm

It is a requirement that immersion heaters now have a safety cutout that will prevent the water from boiling should the adjustable thermostat fail. If the water in the cylinder is reaching 98 degrees and causing the safety to trip, or it trips at a lower temperature before the water has warmed, then either way, it suggests you have a faulty thermostat.

Remove it and get it exchanged. You could test it in a kettle first to see roughly at what temp the trip operates at.
Last edited by plumbbob on Fri Dec 19, 2008 5:35 pm, edited 1 time in total.
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Postby nitro23456 » Thu Dec 18, 2008 9:03 pm

what have you set it at? - should be about 60 degrees
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Postby stoneyboy » Thu Dec 18, 2008 10:18 pm

sido,
Have you got a vent pipe out of the top of the cylinder going back up to the header tank?
end
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Postby sido » Thu Dec 18, 2008 11:59 pm

Well my friend fitting the thermostat is very much idiot proof,live from the sw mains,to thermo then welded wire from element, neutral to other element pin earth fitted really impossible to get wrong,what seems to be happening is that the thermo is not switching off when it reaches the required temp, thus causing the overheat to activate,you cant hear it go off because you would have to sit for hours waiting but you can hear it click when i press the reset pin back on, iam leaning to think i may have wiring fault somewhere maybe a leakage it very old wiring but was hoping if was some identifiable plum fault.thanks colin
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Postby sido » Sat Dec 20, 2008 12:10 pm

In a word yes, its the only bit i didn't replace with new,did cross my mind it could be blocked, colin all the best for xmass,
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Postby Fredee » Sat Dec 27, 2008 12:58 pm

[quote="sido"]In a word yes, its the only bit i didn't replace with new,did cross my mind it could be blocked, colin all the best for xmass,[/quote]

Have you found a fix?

I have the same problem with a pair of hot water tanks. I can't find any problem with the wiring. There is also indirect coils in the tank. I think I am getting a hot spot at the top of the tanks above the emersion heater thermo. If both the indirect and emersion heaters are both heating the water the thermo's work, but if just the emersion heaters are being used the safety overrides pop. The tanks have three outlets. A conventional top outflow and a two lower outlets on the top/side of the tanks that are used to feed power showers.
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Postby stoneyboy » Sun Dec 28, 2008 11:09 am

sido & Fredee,
Diagnosing problems remotely is sometimes difficult, in both your cases I would suspect that you are getting an air pocket at the top of the tanks which is causing the immersion heater safety cut out to operate.
sido - suggest you hit the vent pipe with a hammer to loosen any limescale that may be blocking it or replace it.
Fredee - you are probably trying to get too much water out of the cylinder - especially if you have 2 power showers - and air is being drawn into the top of the cylinder through the vent pipe. Or you may also have a blocked vent pipe.
end
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Postby Fredee » Sun Dec 28, 2008 3:42 pm

[quote="stoneyboy"]sido & Fredee,
Diagnosing problems remotely is sometimes difficult, in both your cases I would suspect that you are getting an air pocket at the top of the tanks which is causing the immersion heater safety cut out to operate.
sido - suggest you hit the vent pipe with a hammer to loosen any limescale that may be blocking it or replace it.
Fredee - you are probably trying to get too much water out of the cylinder - especially if you have 2 power showers - and air is being drawn into the top of the cylinder through the vent pipe. Or you may also have a blocked vent pipe.
end[/quote]
Any suggestions on the vent? I have tested it and it is clear so I presume its drawing back in air. Can I submerse the vent in the cold header tank? The tank isn't pressurised so it would be clear for expansion.
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Postby stoneyboy » Mon Dec 29, 2008 10:15 am

Fredee,
Fit a 28mm pipe from the cold tank to the cylinder.
Don't use both power showers at the same time.
Extending the vent pipe into the cold tank will only give you cold water out of the hot taps.
end
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