Immersion too hot - thermostat problem?

Hi,

I live in an old house that has no gas. Water is heated by two immersion heaters in the hot water cylinder. There is one immersion for boost during the day and the second immersion is connected to a supply that is only live during Economy 7 hours (12.30am-7.30am here). The cylinder is in an airing cupboard in the main bedroom. The header tank is in the loft above the bedroom.

When we moved into the house we noticed the water was sometimes very hot and sometimes not hot enough. A couple of years ago the thermostat failed and the water was cold. A plumber came out and replaced the thermostat with a new one.

Sometimes I wake up in the night to the sound of the immersion heating the water and I have to turn it off to get back to sleep. It doesn't seem to make a lot of noise every night, sometimes it's fine. Some days the water out of the taps seems very hot and other days it seems OK. It's pretty random.

Last night I was woken up by the heater again, and I got up to turn it off. When I was going back to bed I heard the sound of water filling into the header tank. I am in the habit of turning off the cold supply to the header tank every night (when it's nearly full the valve seems to let a drip through and it makes an annoying noise when I'm trying to sleep). So it seems like hot water was going up the escape pipe back into the header tank, which I know is a sign of overheating.

I've read this //www.diynot.com/wiki/Plumbing:faq36 and my thermostat looks more like the 'old' type without the safety, I can't see a reset button on it anywhere. Should I be concerned? Should I be looking at replacing the thermostat again? Is it possible a thermostat without the cutout was fitted even though it was only a couple of years ago? I'm not sure what to do because the problem is intermittent. If it was overheating every night I would know something is definately broken, but it's pretty random. Any ideas?

Thanks

Replace the thermostat. If they are both the older type with no cutout, replace both of them.
Thermostats are typically £10-£15 and very simple to replace.

OK, thanks. The immersion says on a sticker that it's 11". What size of thermostat would I need for it? Thanks.

You'll want a 7" thermostat make sure it has a thermal cutout. Note the one in the link is only suitable for elements installed horizontally in the side of the cylinder.

I went to buy a thermostat this afternoon. Couldn't get a branded one from Plumb Centre so I went next door to screwfix and bought this unbranded one http://m.screwfix.com/p/dual-safety-thermostat-7/21275

Will that be OK for what I need?

When wiring it up, does it matter which is the input terminal for live and which is the output to the immersion live post? It doesn't seem to be marked up in any way and neither is the old one.

Thanks

This is how the old one was wired in


This I how I have wired the new one, it won't fit in facing the right way up


Will it matter that the new one is the opposite way around?

Thanks

its fine. make sure some of the white mechanical cover of the flex is past the clamp. start stat set at approx 50-55 and increase if not hot enough

Thanks for the help, trying it out tonight.

Worth noting for the benefit of anyone else who may read this thread, it is strongly advisable to change any older type immersion thermostats for the newer type with the thermal (overheat) cutout. It may just save a life. There was a tragic incident several years ago whereby an immersion had failed, causing it to boil the contents of the hot water cylinder. This in turn caused the contents of the plastic cistern in the loft above feeding the cylinder, to heat up to the point where the plastic cistern became very soft and collapsed.

Sadly, an infant was sleeping in a Cot in the room below. As the cistern collapsed it showered the child below in boiling hot water, sadly the child later died of its injuries. For the sake of a £10 stat, dont risk it. If unsure, ask. I think I can speak for us all in saying we are happy to help.

There have been several other deaths/serious injuries from this too. The descriptions of the injuries are horrific. In all the cases I read about there were warning signs which the occupants noticed, but did nothing about as they weren't aware of the possible problems.

Seems to be working fine. Haven't heard any more overheating noises from it so far. I'm glad it's safe now. Shocking that a thermostat without a cutout was fitted only a couple of years ago, despite the new rules.

Thanks for the help.