# Two Immersion Heater Elements on One Circuit?

Unfortunately top entry, dual element immersion heaters seem to be relatively few and far between.
Oh, are they? Maybe I'm behind the times, since I'm sure there was a time when they were pretty common and readily available.
What do you mean when you refer to "dual height thermostats would be an incredibly simple fix for the issue of sink/bath"? Presumably that if JohnW's simplified theory held then you would only need one element with a dual height thermostat rather than two elements?
Indeed, per my 'simplified theory' that approach would presumably work - but I don't think one would find a 'dual height thermostat' or an element that had one, would one?

I've never really thought of that but, if it's true, it would surely mean that the long/bottom element would never be able to heat the whole cylinder of water, since a stat fairly near to the top would turn the element off long before all of the water in the cylinder (lower down) was hot, wouldn't it?

An element stat, senses the temperature, mostly at the very tip, the bottom end of the device - which is slight higher than the bottom end of the element.

That was exactly my point. If the heated water behaved perfectly, and spread uniformly from the top down, irrespective of where the element was located - then all you would need, is stats at various heights, to determine how much of the cylinder contents was heated.
As I've just written, I think that 'stats at various heights' probably would work reasonably well, but I don't know one could implement that with currently available products.

The heated water obviously does not 'behave perfectly' (any more than hardly anything 'behaves perfectly' ) but I've been suggesting that the deviation from 'perfect' is not very great.

What do you have to say about my radiator analogy? There is no doubt that the hot water 'appears' at the bottom, but is it not your experience (as it is mine) the top very quickly gets hot whilst the bottom initially remains cold?

An element stat, senses the temperature, mostly at the very tip, the bottom end of the device - which is slight higher than the bottom end of the element.
That's what I'd always assumed, but CH_152 Johntheo5 seemed to be implying that, with dual elements, the 'bottom' element was controlled by a thermostat near the 'top' - which, if true, would seem totally daft.

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I've never really thought of that but, if it's true, it would surely mean that the long/bottom element would never be able to heat the whole cylinder of water, since a stat fairly near to the top would turn the element off long before all of the water in the cylinder (lower down) was hot, wouldn't it?

What am I missing?

Kind Regards, John
I think what happens is (assuming just say long element in use) that the water stays almost completely stratified because the element is emitting the same watts per unit length from top to bottom, I have a very short (in height) solar coil that only occupies ~ 30L in the bottom of the cylinder and I have very accurate (to within 0.1C) PT1000 sensors in 3 different locations on the cylinder, as the solar coil starts heating the 3 sensors (assuming water at the same starting temperature) will rise almost exactly at the same rate, to within 0.1C , even when the solar coilis emitting ~ 2kw on a very sunny day, if the water is allready stratified, then the temp at the hottest sensor will remain exactly as is until the others "catch up", I have seen the top sensor at 50C, the middle sensor (oil fired coil) at say 40C and the solar coil (store) sensor at 25C, as the solar coil starts heating, the "40C" sensor will remain at exactly 40C until the solar sensor reaches 40C, they both the rise together, the top sensor will remain at exactly 50C until both the other two reach almost exactly 50C the three will then rise uniformly, together. I would imagine the same should happen with a low level side mounted immersion, but as the stat is at the same level then will cut in/out when a few litres of HW is drawn off.

What do you have to say about my radiator analogy? There is no doubt that the hot water 'appears' at the bottom, but is it not your experience (as it is mine) the top very quickly gets hot whilst the bottom initially remains cold?
The radiator analogy is interesting, if you were to leave one valve shut on any rad until the boiler reaches full operating temperature and then open it, its uncanny, the hot water will rise up in the first say 4/6ins of the rad, it will then move along the top and reduce in temperature as it "falls" down through the rad.

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As I've just written, I think that 'stats at various heights' probably would work reasonably well, but I don't know one could implement that with currently available products.

Perhaps inadequate current rating, but clamp on cylinder stats are easily available.

The heated water obviously does not 'behave perfectly' (any more than hardly anything 'behaves perfectly' ) but I've been suggesting that the deviation from 'perfect' is not very great.

Guesswork, but I would suggest an element at the bottom, will take twice as long, to raise the water to temperature to a comparable, as an element located at the top - simply due to dilution as the heat rises.
What do you have to say about my radiator analogy? There is no doubt that the hot water 'appears' at the bottom, but is it not your experience (as it is mine) the top very quickly gets hot whilst the bottom initially remains cold?

Radiators have quite narrow passages, water enters at the base, then rises up to the top, via the nearest passage to the entry. From the top, the hot water then spreads/pushes its way down.

An HW cylinder is much, wider, more space for the heated water to mix with the cold, as it rises.

I think what happens is (assuming just say long element in use) that the water stays almost completely stratified because the element is emitting the same watts per unit length from top to bottom,
The water will presumably be heated fairly uniformly along the length of the element, but that obviously does not, in itself, mean that much of the heated water will 'stay where it was heated'

have you never felt the top, middle and bottom of a DHW cylinder say, 30 minutes after (starting from cold) you have turned on a long immersion?

The water will presumably be heated fairly uniformly along the length of the element, but that obviously does not, in itself, mean that much of the heated water will 'stay where it was heated'

have you never felt the top, middle and bottom of a DHW cylinder say, 30 minutes after (starting from cold) you have turned on a long immersion?
No, but I have seen it many times on very large HW cylinders heated with steam coils, (Saturated) Steam gives up its heat isothermally, that means there is no change in the steam/coil temperature from the steam entering the coil until it leaves the coil as water, it just gives up its latent heat, there was little or no differences in the heated water temperature rise throughout the cylinder. I do have a long element, unfortunately my old Santon is a 3 wire with the change over switch mounted very inconveniently on top of the immersion itself!, I have no easy access to this now and have only used the short element since I installed the cylinder with the solar coil ~ 13 years ago. It would be interesting though to monitor the long element heating of a cold cylinder.

I think what happens is (assuming just say long element in use) that the water stays almost completely stratified because the element is emitting the same watts per unit length from top to bottom, I have a very short (in height) solar coil that only occupies ~ 30L in the bottom of the cylinder and I have very accurate (to within 0.1C) PT1000 sensors in 3 different locations on the cylinder, as the solar coil starts heating the 3 sensors (assuming water at the same starting temperature) will rise almost exactly at the same rate, to within 0.1C , even when the solar coilis emitting ~ 2kw on a very sunny day, if the water is allready stratified, then the temp at the hottest sensor will remain exactly as is until the others "catch up", I have seen the top sensor at 50C, the middle sensor (oil fired coil) at say 40C and the solar coil (store) sensor at 25C, as the solar coil starts heating, the "40C" sensor will remain at exactly 40C until the solar sensor reaches 40C, they both the rise together, the top sensor will remain at exactly 50C until both the other two reach almost exactly 50C the three will then rise uniformly, together. I would imagine the same should happen with a low level side mounted immersion, but as the stat is at the same level then will cut in/out when a few litres of HW is drawn off.
But in this sense, if the stat is in the short element only (which it is: https://www.heatrodshop.com/product/27-dual-copper-immersion-single-stat) it seems true that you would not be able to get the long element to run if the top is already at temperature?

But in this sense, if the stat is in the short element only (which it is: https://www.heatrodshop.com/product/27-dual-copper-immersion-single-stat) it seems true that you would not be able to get the long element to run if the top is already at temperature?
Thats correct, you would have to use or run off some HW to allow cold water to come in contact with the thermostat, it depends then on the switching differential as the stat is switching on the average rod temperature, based on mine its probably around 1/2 the small heated vol that requires to be used or run off but of course it also depends on the "cold"water temperature.

No, but I have seen it many times on very large HW cylinders heated with steam coils, (Saturated) Steam gives up its heat isothermally, that means there is no change in the steam/coil temperature from the steam entering the coil until it leaves the coil as water, it just gives up its latent heat,...
Fair enough,m and the means of heating obviously makes no difference to what we're discussing. ...
there was little or no differences in the heated water temperature rise throughout the cylinder.
Do you mean 'initially'? Is that coil (or part of it) close to the bottom of the cylinder? If so, in the absence of heated water being ';drawn off' the entire contents of the cylinder (above the lowest part of the heating coil) would obviously eventually rise to the same temp - it's what happens in the early stages of 'heating from cold' that we are discussing.

Thats correct, you would have to use or run off some HW to allow cold water to come in contact with the thermostat,
... but the stat would then surely turn the ('long') element off again as soon as a small amount of water at the top had got up to temp, wouldn't it? In other words, the long element would never be able to het anything like all the water in the cylinder?

Perhaps inadequate current rating, but clamp on cylinder stats are easily available.
True - I'd forgotten them (even though I have some)
Guesswork, but I would suggest an element at the bottom, will take twice as long, to raise the water to temperature to a comparable, as an element located at the top - simply due to dilution as the heat rises.
That obviously implies that the lower element would have to heat double the volume of water than would a higher element in order for the temp at the top to be the same. I wouldn't have thought that, but you may be right.
Radiators have quite narrow passages, water enters at the base, then rises up to the top, via the nearest passage to the entry. From the top, the hot water then spreads/pushes its way down. An HW cylinder is much, wider, more space for the heated water to mix with the cold, as it rises.
There certainly is a fair degree of apparent logic in what you are suggesting. My cylinder with two immersions is very rarely used (heated) 1at all but the next two times I fire it up (obviously 'from cold') I'll try to remember to do that initially with each of the elements, and make some measurements and timings - but don't hold your breath, because it could be quite a long time before the opportunity to undertake those experiments arises!

... but the stat would then surely turn the ('long') element off again as soon as a small amount of water at the top had got up to temp, wouldn't it? In other words, the long element would never be able to het anything like all the water in the cylinder?
Yes, it would seem that's what should happen but again I have never heard any of my neighbours or my wife complaining as many years ago when we had a open fire/back boiler for CH/HW, we used the immersion throughout the summer, and whenever a bath was required the "bath" immersion was turned on but I don't know if the water was stone cold or not in the cylinder, we certainly had enough to "fill" the bath every time.

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