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immersion cylinder thermostat

Postby DUFREYNE » Sat Jan 15, 2011 8:54 pm

my plumber installed a new immersion cylinder for me and advised me to put a thermostat on the pipe. Currently the immersion is wired via an immersion switch. Neutral and earth wiring i understand. I have one live wire feeding the switch and two live wires going from the switch to the immersion, one for sink and one for bath. Iwas just wondering how to wire in the stat. It has three terminals marked C, 1 and 2.
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Postby ericmark » Sun Jan 16, 2011 8:04 am

Your post seems a little odd as normally the thermostat for an immersion heater is in the same housing as the heater and fits in a special pocket designed for it.

There is an odd system called the Willis Immersion used in Ulster very clever heats tank from top down rather than bottom up so hot water is available much quicker.

However most thermostats designed for immersion heaters have just two contacts and it is only those designed for central heating that normally have three contacts. The C and 1 would normally be used to trigger a water heater with the number 2 contact being used to work the motorised valve in a rather complex wiring in a central heating system. However most of the thermostats in central heating systems are only rated at about 4 amp and would not be suitable for an immersion heater which needs more like 13 amp.

With the Willis system there are two thermostats one inside the immersion and one in the tank the latter set at 55 degs the one inside the immersion housing is set a lot hotter and normally there is also a second over temp device which will operate should the thermostat fail. The secondary device with a header tank able to withstand boiling water like used with back boilers will normally have a reset button. However those designed where only immersion heaters are used are non resettable.

The idea is that if the immersion heater thermostat has failed then it needs renewing as we do not want anyone else killed as with the baby where the header tank failed and poured boiling water on the baby.

It seems instead of fitting header tanks able to stand boiling water builders rely on the thermostat never allowing the water to boil.

So unless you have a thermostat clearly marked 13A or above and you are using the Willis syphon system do not use a thermostat other than the one built into the immersion heater. As the current rating means it may burn out, it will not have the safety features to switch it off if it does fail and it could also be two slow acting to stop water boiling.

Out of interest do you live in Northern Ireland and have you got the Willis system? If so would be interesting to know how well it works as only seen write ups and never seen it in use. Seems only used in Northern Ireland us in the rest of UK have never adopted the system.
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