externally mounted hot water cylinder thermostat

[Russ]

Hi guys

Back thinking on the good old' not enough hot water' thing.

I think I'm going to try replacing the standard, immersion heater mounted thermostat with an externally mounted one. Reason being that I suspect the thermostat switches off a bit earlier than it should, as the water at the top of the tank is roasting, whilst the water further down the tank isn't.

I've been told it needs to be at least 15 or 16amp rated. Will this one be Ok?

don't really understand what SPDT means?

Can someone advise how far down the tank I should fit this - about half way at a guess? Any problems with fixing it a bit lower?

Obviously the water at the top of the cylinder will get really hot if the water at the centre is to be heater to 60 degrees - I dont' have a problem with the hot water coming out really hot, but are there any other problems I should be aware of?

I'm also not sure on how it'd be wired up, just 2 wires as the existing thermstat is?

Thanks for your time

[Roger Mills (aka Set Square)]

It means "Single Pole Double Throw" - in other words a change-over switch which connects a 'Common' terminal to one or other of two additional terminals when it switches. Have a look at

for some diagrams.

To have the desired effect, it would have to be wired in parallel with - or even instead of - the built-in stat. [If wired in series, the heater will go off when the original stat switches, even if the external stat is still calling for heat].

There are safety considerations, though! Current regs require immersion heaters to have an additional over-heat stat - which has to be manually reset if it trips. This is to prevent a potentially dangerous situation of the tank boiling if the primary stat fails 'on'. Whatever happens, you

*musn't* bypass the safety stat.

[Andy Cap]

SPDT = (S)ingle (P)ole (D)ouble (T)hrow

meaning there is a single contact which moves between two contacts which are known as NC Normally closed and NO Normally open and yes that one will do.

Your two wires needs to be connected across the NC part so that the Immersion heater initially gets hot, then at the correct temperature, the contact opens and disconnects the element.

I would say 1/3 of the way up is best and the water at the top will not get too hot. That is the way all thermostats function. Personally I prefer 55 but it's a matter of experimentation really as I doubt all stats are that closely calibrated anyway.

Andy

[Russ]

"There are safety considerations, though! Current regs require immersion heaters to have an additional over-heat stat - which has to be manually

reset if it trips. This is to prevent a potentially dangerous situation of the tank boiling if the primary stat fails 'on'. Whatever happens, you

*musn't* bypass the safety stat."

This is what I don't get. My old thermsotat that I only replaced recently didn't have this safety stat. and it'd been working merrily for years and years.

What exactly is the danger people keep mentioning, other than the water at the top getting hot?

The whole system operates on a timer, so if the thermostat did get stuck on the power would go off after a few hours anyway?

I presume these external ones don't have a separate overheat trip like the regular thermostats?

[David Hearn]

That's assuming the timer doesn't fail, or even you don't have a timer.

We never ever use our immersion heater as our boiler heats the water fine. I've never even looked at it to be sure it works fine.

However, the immersion heater runs off a flex and mains plug. We never plug it in. However, after using the socket in the airing cupboard for something else, we plugged the bedside light back in.

3-4 hours later, came back to find water dripping from the light fitting.

After looking around, it turns out we'd plugged the immersion heater back in, and steam/boiling water had been venting into the loft, splashing off the badly placed insulation jacket and onto the loft floor. From here it found a conveniently placed light fitting to drip through.

I've no idea what the thermostat setting was on the immersion heater, or whether it was working, or whether it tripped any safety mechanism (it wasn't venting when I turned it off).

Basically these safety devices are for when the normal switching devices fail, or a user does something stupid.

D

[Dave Plowman (News)]

Are you certain the actual immersion is long enough? Plumbers may just use one they had in stock as a replacement, and they come in several lengths.

switch heaters but merely pumps so don't have a 15 amp rating.

Single pole double throw. Basically a changeover switch which has three terminals and can be either off or on at 'rest'.

[Michael Chare]

immersion heater is appropriate for the size of tank that you have. Assumming the heater is mounted on the top, you might want to choose one long enough to heat the water near the bottom of the tank.

Further some immersion heaters use thermostats which are a separate internal component and where you can adjust the temperature to suit you needs.

In your proposed solution you risk causing the water at the top of the tank to boil, which may scald you.

[Russ]

guys, as per my loads of other post on this poxy hot water system that only seems to be playing up since old bits have been replaced (by me) with new bits with 'safety' features - the immersion heater is a 36" - which is the longest one I could find. Seems daft though as the thermostat is only 18" (standard, it seems).

[Dave Plowman (News)]

No - hot water rises to the top so all you need to do is measure it near there.

[Roger Mills (aka Set Square)]

There have been one or two high profile cases involving fatalities - causing the water not only in the cylinder but also in the cold header tank in the attic to boil. In one case, the plastic header tank collapsed due to the heat and dumped 50 gallons of scalding water on the people sleeping below.

It obviously needs a particular set of circumstances for the consequences to be as dire as that - but in the knowledge that it *can* happen, it's as well to take precautions.

[Pete C]

Hi,

Is the immersion heater boss perpendicular so the element is vertical in the tank, or is tilted to one side, so the element is off vertical?

Also, have you bought a thermometer yet?

cheers, Pete.

[Russ]

tilted/off vertical Pete, and no, I've not bought a thermometer?

[Christian McArdle]

But it is the temperature near the bottom that should be measured, to ensure that the entire tank has been heated through. As you say, the hot water rises to the top. If you measure there, it will satisfied almost immediately, even when the majority of the cylinder is stone cold.

Christian.

[Christian McArdle]

I hate top mounted immersions anyway. They don't work nearly as well as a

12" one stuck through the side near the bottom. That way you get the heating and measurement at the bottom, where it should be, ensuring a full tank of hot water.

Christian.

[Dave Plowman (News)]

Looking at the TLC site, 11 and 18 inch elements have dedicated stats, while the 27,30 and 36 inch share the same one.

[Christian McArdle]

Which is why long top mounted stats don't work well. They don't heat all the cylinder as the stat clicks off when it is only hot halfway down.

Christian.

[Dave Plowman (News)]

Could be - I've got one of those tall slim cylinders so probably the worse scenario but have only once used the immersion. And it certainly provided enough for a decent shower and general hot water over the couple of days when I had the boiler in bits to fix a leak.

[Pete C]

Hi,

Is the thermostat hole/'pocket' towards the outside of the cylinder or the middle?

If the latter then the thermostat might be above the tilted element, so warm water rising off the element could make it switch off before the rest of the cylinder is up to temp.

I'd try and fit the heater so the pocket is towards the _outside_ of the cylinder, eg by using an extra washer or two under the immersion heater fitting. Then it will be below the element so should only switch off when the water in the cylinder is up to temp, as warm water will rise off the element and away from it.

Ideally there should be a spacer towards the end of the thermostat pocket to hold it away from the element.

If you had one you could heat the thermostat in a pan of water to see what temp it opens, which would be useful to know.

Also to measure the water temp at the basin, if it can go above 60C there are precautions you must take.

cheers, Pete.