immersion thermostat still playing up

[Russ]

Having just spent the weekend replacing the hot water cylinder, immersion heater and thermostat - it was all working lovely last night and this morning, but I've just come in from work to discover the thermostat has tripped and I've had to reset it.

Why does this trip exactly? The thermostat is set to it's maximum setting on the dial, so surely it's impossible to set the thermostat too high?

Under stand it trips because it gets past it's maximum temperature - although I thought the themostat measured the temperature from the tip of the device, not the top of it - and the idea was to stop the heater being stuck on should the thermostat fail on......yet it seems to be this saftey thing that causes the problem!

This is the second thermostat that's done this and it's going to get bloody annoying if I have to constantly keep checking the immersion heater has come on when I get up or get in from work.

I know it's supposedly safer to have this rest thing, but the old thermostat had been in the tank for the many years without the tank 'exploding' or whatever it's supposed to do if the heater is left on.

Starting to thing a surface mounted thermostat that sits on the outside of the tank would be better.

[Set Square]

Can we be absolutely clear just *what* has tripped. Is this the safety stat in the immersion heater itself - or is it tripping something in the Consumer Unit next to the electric meter? Just what do you have to do to reset it?

[Harry Bloomfield]

Russ presented the following explanation :

Oh yes it is!

If the set point of the regular stat is higher than the safety stat, then the safety will trip. That is its designed purpose. Set the regular one a little lower until it doesn't trip.

Health and safety concerns over the hot water being too hot mean that HW stat temperatures have been lowered anyway, so don't expect your water to be as hot as it used to be.

[Russ]

Hi Set!

It's the safety stat in the immersion heater thermostat...if that makes sense!

[Andrew Gabriel]

Not sure if this is possible, but if you had a short heater and a long stat in the top of the cylinder, I could imagine the stat would never switch off.

[Russ]

hi again Set!

Yes, it's the safety stat built into the immersion heater thermostat....if that makes sense!

[Harry Bloomfield]

Andrew Gabriel was thinking very hard :

The stat fits into a pocket built into the heater element matched to the element size, so it would only be possible to fit a shorter stat.

[Russ]

which I haven't done.

The old immersion was approx 32" with an 18" stat.

I bought a 27" immersion initially, which had an 18" stat.

The new immersion heater is 36" with an 18" stat.

I can't put any longer than that in there.

[meow2222]

then your safety stat is set to a lower temp than the adjustable stat. Turn adj stat down till it works ok. If water not hot enough when it works ok, the safety stat can not be used. A bolt on external stat can then be used to provide a secondary safety backup stat.

I can see overriding the safety stat becoming popular, since some systems simply wont work usefully at the preset safety stat temp.

NT

[Pete C]

Hi,

How hot is the water with the new heater and stat? Bear in mind that heating the water over 60C makes the scale form in the cylinder much quicker.

cheers, Pete.

[Pete C]

Does the safety stat always cut out before the adjustable one when it's on maxium? If so set the adjustable one as follows:

Turn the adjustable one up to maxium and switch on immersion. Wait until the safety stat cuts out.

Turn the adjustable one down and carefully listen for a click when the contacts of the adjustable stat open.

Turn it down a little more. Reset the safety stat.

The adjustable stat should now be set so it opens just before the safety stat. If the safety stat still cuts out just turn the adjustable one down a bit more.

Once it works reliably without the safety stat cutting out mark the position of the adjustable stat for reference.

cheers, Pete.

[Russ]

Cheers Pete

I sort of discovered/experimented with that yesterday, and it seems to be ok now.

Thanks for your advice.