Two Immersion Heater Elements on One Circuit?

... 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?

Exactly! The stat length, needs to match the element length. Fit one too short, and will restrict to which the water is heated.
 
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Exactly! The stat length, needs to match the element length. Fit one too short, and will restrict to which the water is heated.
Quite - which is why I find it hard to believe that a dual one such as was suggested (with only a 'short' thermostat) would have ever been made
 
Yes, it would seem that's what should happen but again I have never heard any of my neighbours or my wife complaining ...
Indeed - and that's why, as I've just written, I find it rather hard to believe that anyone would make such a seemingly daft product - an element long enough to heat the entire contents of a cylinder but with a thermostat so high up that it couldn't !
 
Indeed - and that's why, as I've just written, I find it rather hard to believe that anyone would make such a seemingly daft product - an element long enough to heat the entire contents of a cylinder but with a thermostat so high up that it couldn't !

But,. if the cylinder is cold at the start of heating, either with the short or long element does it really matter how long, I mean short, the stat is since (IMO) the water is heated unifirmly from top to bottom, I'm pretty sure that my ancient immersion doesn't have a rod type stat in a dry pocket, its just a contact type on the top of the immersion, very like the hi limit stat on the modern immersions, marked item 6 in the attachment.
 

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But,. if the cylinder is cold at the start of heating, either with the short or long element does it really matter how long, I mean short, the stat is since (IMO) the water is heated unifirmly from top to bottom,
As I've said, although the water will be heated fairly "uniformly from top to bottom", the heated water will certainly not remain uniformly distributed "from top to bottom".

I realise it's only my personal observation/anecdote' but, as I've also said, if you start heating, 'from cold' a cylinder of water with a 'long' element, then, after, say, 30 minutes,there will be a very marked temp difference between top and bottom.
I'm pretty sure that my ancient immersion doesn't have a rod type stat in a dry pocket, its just a contact type on the top of the immersion, very like the hi limit stat on the modern immersions, marked item 6 in the attachment.
It must be at least 50 years since I first saw an immersion with a thermostat but I have to say that I don't recall having ever seen one in which the stat was not "a rod in a dry pocket" - but perhaps that's due to age-related memory failure :)
 
And heat in the tank will cause water to circulate in the tank from that point. But if the top immersion is used, and the water gets up to temperature, then you switch to the bottom immersion using the same thermostat then clearly the bottom thermostat will do nothing until water at the top cools down.

There are some tanks with baffles to stop the water circulating, often used with boilers to make tea or coffee, but most cylinders will allow water to circulate, and because the water can circulate using an immersion low down, it can take some time from when turned on, to having warm enough water at the taps.

The old idea was to have two thermostats, one in the bottom immersion being set higher to the one in the top, so the bottom immersion will inhibit the top one turning on. So only when all the water has been used will the top one work.

There is no reason why a change over relay should not be used, basic idea is if the top one activates it stops the bottom one switching on, but this would not work with a single thermostat.

But a tank thermostat 1715814322525.pngtypically has change over contacts, the one shown has a 15 amp change over contacts for a resistive load, so if hot it heats bottom or long immersion, and if cold the top one or short immersion, if set well below the thermostat built into the duel immersion heater, it will heat the top to say 50ºC then switch to bottom, which will continue to heat whole tank until say around 65ºC so a simple timer was boost would run on immersion at off peak times, with a boost button so if water is found to be cold, one can press the boost.

If one want automatic boost then it gets more complex, but a second thermostat near top of the tank in parallel with the time clock could auto switch on at say 45ºC.

But the problem is finding some one when it goes wrong to fix it.
 
As I've said, although the water will be heated fairly "uniformly from top to bottom", the heated water will certainly not remain uniformly distributed "from top to bottom".

I realise it's only my personal observation/anecdote' but, as I've also said, if you start heating, 'from cold' a cylinder of water with a 'long' element, then, after, say, 30 minutes,there will be a very marked temp difference between top and bottom.
Should be easy enough to confirm or otherwise if someone with even one (long) element is willing to drain down the cylinder and then reheat from cold.
It must be at least 50 years since I first saw an immersion with a thermostat but I have to say that I don't recall having ever seen one in which the stat was not "a rod in a dry pocket" - but perhaps that's due to age-related memory failure :)
Here is a photo of mine, taken 11 years ago, with cover removed.
 

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An HW cylinder is much, wider, more space for the heated water to mix with the cold, as it rises.
I wonders if that there "Willis" thingy shares some physics with the analogy betwixt rad and cylinder effects . Anyway, although this thread has gone on and on ( probably should have been split ages ago) I think it is very interesting and has thrown up some very interesting ideas and opinions.
Apologies to the OP if you feel hijacked or unduly long but I think that all of our opinions (perhaps some modified a bit during the course of discussions) and points of view abounded, I for one, found it amazing. Learning Curve comes to mind.
In fact I`ll admit that the bath and basin jobby I`ve seen )they used to be fairly popular a good while back in some areas, I had always wondered by the "Why one stat for two elements" puzzle, in fact all the plumbers I`ve asked about this (and the other two element setups seem to end up saying "Oh they just do work" as if magic is in play.

Anyway, that`s my opinion, others may vary
 
Heated (bottom) water allways mixes with the colder water above it in a cylinder, I've often seen this demonstrated (but to prove that hot water just rises) on u tube where a cold bottle of water is upended and placed on top of a hot bottle of water both with different food colouring in them, after a while you can clearly see the colour changing in both bottles to become the same, a hot bottle is then perched on top of the cold one and both retain their full colouring.
You will get a temperature gradient in a cylinder heated with a HW coil as the coil loses temperature from top to bottom in circulating, there is a dT of 10/12C through my heating coil, this is reflected in the HW temperature where the temperature at the top is ~ 8/10C hotter than at the cylinder stat pocket which is normally located fairly close above the coil outlet to ensure the cylinder starts reheating with the minimum of HW draw off. I just don't imagine this happening with a top mounted vertical (electric) heating element because the heat emitted per mm or inch is the same throughout its length.
 
Should be easy enough to confirm or otherwise if someone with even one (long) element is willing to drain down the cylinder and then reheat from cold.
Indeed. As I wrote ...
.....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!
 
Wonder is there a possibility that the long element heating resistive wiring only starts and ends at the portion below the length of the short element, if so, then, even though the heating still won't commence until the stat is calling, it should result in a "full" cylinder heat/reheat.
 
Heated (bottom) water allways mixes with the colder water above it in a cylinder, I've often seen this demonstrated ....
You will get a temperature gradient in a cylinder heated with a HW coil as the coil ....
There will obviously always be some 'mixing', hence heating of some of the cold water above the source if heating (but below 'the top''of the cylinder) - so this discussion is essentially about how much of that mixing occurs. However, I would expect there to be a pretty big difference between heating with a coil and heating with a near-vertical long and straight element.

With a coil, or other 'similar' sources of heat, heating is occurring across much of the 'width' of the cylinder, so heated water will be rising (and 'mixing') 'all over the place' - so I would expect a lot of that mixing (hence warming of water uis lower parts of the cylinder).

However, with a near-vertical long and straight element, I might expect that the only water heater would be that very close to the element and that would then rise in a very narrow column, therefore not really coming into contact with *(or 'mixing with') most of the water in the cylinder. That might

Do you think that is a credible theory? It might explain my person observations as to what happens with a long element in my cylinders.
 
Wonder is there a possibility that the long element heating resistive wiring only starts and ends at the portion below the length of the short element, if so, then, even though the heating still won't commence until the stat is calling, it should result in a "full" cylinder heat/reheat.
I'm not sure that I fully understand what yiou are suggesting but, in any event, the fact is that the long element (of a dual one) does heat 'a full cylinder of water to the desired temp, and I really can't see how that could happen if it were being controlled by a stat 'at the top'
 
I can understand a long element heating the full cylinder from cold but I find it difficult to understand how it can do so, if, for example, the short element has been on permanently and one decides that a full cylinder of HW is required, obviously, some of the say 30L of heated water must be run off to to allow some portion of the stat to be immersed in cold water to get it to call for heating, there will then still be maybe 15L or more of very hot water surrounding the top half of the stat, again, if the long element is heating the water at the same rate, then the stat will be satisfied when the temperature rises by say 10C, so you then have 30L of water at say 60C or more and the remaing 70L or so at the mains temp+10C, say 25C, practically cold? Assuming that the short element and stat are 11" and the long element is 27" then IF that long element is manufactured so that the heating portion (resistive wiring) is only in the lower 16" (27-11) then all the heating will commence in cold water and should result in a much greater volume of water being heated before the stat again cuts off the heating, I would be more than mildly surprised though if the long element is manufactured this way.
 
I can understand a long element heating the full cylinder from cold ....
Maybe you can, but I cannot understand how it could IF the stat controlling the long element is somewhere fairly near to the top. Amidst some disagreements and differences of opinion, I think that everyone is probably agreed that, at least during the early stages of heating (from cold), the water at the top will be much hotter that that lower down. That being the case, a stat fairly high up would turn off the element long before the water lower down in the cylinder was up to the desired temp.
but I find it difficult to understand how it can do so, if, for example, the short element has been on permanently and one decides that a full cylinder of HW is required,
As implied above, that would merely 'make things worse' - i.e. if the water at the top was already hot, a stat near the top might prevent the long element ever coming on - or, at least, would switch off that element even earlier than if the water at the top had initially been cold - but, as above, it would seem to still be a problem even if the water at the top were initially cold. don't think it makes any difference
I would be more than mildly surprised though if the long element is manufactured this way.
So would I, particularly since I have personal experience of those perceived problems not arising it practice - which is why I'm implying that the probably are not manufactured in the way that is being suggested - but who knows? !
 

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