Two Immersion Heater Elements on One Circuit?

[JohnW2]

I see where you're coming from, but as the hot water rises through the cylinder from the bottom it will definitely dissipate heat into the surrounding body of water on its way to the top. I understand it will be hottest at the top though.

Yes, slightly - but,as I've been saying, in practice that is really not much of an issue.

With my single-element cylinder if, starting from cold, I switch on the bottom) element for long enough for the top ~20% (probably about 28 litres) to 'feel very hot', the middle, let alone bottom of the cylinder still remains pretty cold to the touch.

By minimising the distance the hot water has to travel, you minimise the heat loss into the water in the rest of the tank (which you don't need). That said you are probably anecdotally correct in that sticking a boost on for an hour will sort it either way from a practical perspective. But you will have "wasted" some energy.

Again, all theoretically/strictly true but, in practice, probably not much of an issue. As I've just written to Harry, if it were me I probably wouldn't feel that the cost/disruption of getting a new cylinder and a couple of new immersions would be 'worthwhile' - but that is obviously you'r decision.

Kind Regards, John

 

[EFLImpudence]

I've agreed that it is 'not strictly true'

Thank you.

 

[SUNRAY]

I haven't read anything like the whole thread so apologies if I'm repeating anything.
! have no experience of the switch in post#4 but I do know the Horstmann version has a changeover contact on the timeswitch (some versions of the Horstmann use 2 relays) preventing both elements working together and the later description confirms similar operation.

 

[JohnW2]

Thank you.

It's interesting. As you might imagine, in all sorts of contexts (including here and in my working life), I'm quite often criticised for the length and/or 'exhaustiveness' of my writing.

However, as I've probably said here before, the reason I tend to 'write at length' is that, if I don't, I end up having to practice 'simplifications' and not include all the caveats and discussion of endless detail and 'possibilities' - but if I do that there will always be people who will come back with "that's not strictly true", "that's not always true", "it's not as simple as that", "but you have not considered." etc. etc.

In other words, one can't really win

 

[JohnW2]

I haven't read anything like the whole thread so apologies if I'm repeating anything.
! have no experience of the switch in post#4 but I do know the Horstmann version has a changeover contact on the timeswitch (some versions of the Horstmann use 2 relays) preventing both elements working together and the later description confirms similar operation.

If DPDT (DP 'changeover') switches were readily available as electrical accessories, it would be child's play to achieve the desired functionality.

 

[Ch_152]

Yes, slightly - but,as I've been saying, in practice that is really not much of an issue.

With my single-element cylinder if, starting from cold, I switch on the bottom) element for long enough for the top ~20% (probably about 28 litres) to 'feel very hot', the middle, let alone bottom of the cylinder still remains pretty cold to the touch.

Again, all theoretically/strictly true but, in practice, probably not much of an issue. As I've just written to Harry, if it were me I probably wouldn't feel that the cost/disruption of getting a new cylinder and a couple of new immersions would be 'worthwhile' - but that is obviously you'r decision.

Kind Regards, John

After all theory is one thing and anecdotal, practical experience another. I have come round to it being the case that it would probably not be of benefit for me to change the whole cylinder. Either for comfort/ease of use reasons (because, as you say, in practice I'd notice little difference), and not for cost reasons either as outlined in my LONG post.

If I needed to change the cylinder for some other reason, then yes I think worthwhile going for 2 elements over single top entry.


Appreciate your taking the time to respond (as someone who also writes exhaustively. I am an engineering consultant and suitably caveating/qualifying information is part of the day job...).

 

[JohnW2]

After all theory is one thing and anecdotal, practical experience another. I have come round to it being the case that it would probably not be of benefit for me to change the whole cylinder. Either for comfort/ease of use reasons (because, as you say, in practice I'd notice little difference), and not for cost reasons either as outlined in my LONG post.

Fair enough., As you will realise, that would probably be my thinking.

If I needed to change the cylinder for some other reason, then yes I think worthwhile going for 2 elements over single top entry.

Given that there would undeniably be a little 'benefit', I can sympathise with that. However, you would not necessarily have to get a tank with provision for two immersions - one with provision for just a 'top' one would be OK, since one can get ones with 'dual elements' ('top' and 'bottom') which can be inserted from the top.

Appreciate your taking the time to respond (as someone who also writes exhaustively. I am an engineering consultant and suitably caveating/qualifying information is part of the day job...).

I didn't imagine that I was alone I, too, have been undertaking consultancy (albeit in fields highly divorced from anything discussed here) for many years (decades), and it's a very long time ago that I learned the need to try to pre-empt the "yes, but you have not considered." responses ... but then they complain about the length of what I write - so, as I wrote, one can't win

Kind Regards, John

 

[Harry Bloomfield]

Again, all theoretically/strictly true but, in practice, probably not much of an issue. As I've just written to Harry, if it were me I probably wouldn't feel that the cost/disruption of getting a new cylinder and a couple of new immersions would be 'worthwhile' - but that is obviously you'r decision.

We will have to agree, to disagree on that point John, because were it as simple as that - then the dual height thermostats would be an incredibly simple fix for the issue of sink/bath. Stratification, only works well, above the level of the heat source.

 

[Ch_152]

Given that there would undeniably be a little 'benefit', I can sympathise with that. However, you would not necessarily have to get a tank with provision for two immersions - one with provision for just a 'top' one would be OK, since one can get ones with 'dual elements' ('top' and 'bottom') which can be inserted from the top.

Unfortunately top entry, dual element immersion heaters seem to be relatively few and far between.

We will have to agree, to disagree on that point John, because were it as simple as that - then the dual height thermostats would be an incredibly simple fix for the issue of sink/bath. Stratification, only works well, above the level of the heat source.

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?

 

[Johntheo5]

(Disclaimer, I am in no way a qualified electrician and will get someone to look at/do this for me. Just trying to work out what the best approach is).

I'm wondering if I can run two immersion heater elements (3kW each, so 12.5A each) on the same circuit? Getting another circuit upstairs will involve drilling through a 300mm high density concrete slab - which I'd like to try and avoid...


I don't think I can have these both powered up on the same circuit at the same time (MCB is rated to 16A, cable is 32A (I think), and the timeclock after the fused spur is only 16A). And that doesn't bother me - happy to only run one at once.

So it possible to have these wired up so that they can both run on the same circuit, but so that only one can run at a time? Any risks in doing so?


Any alternative suggestions welcome.
could

Thanks


Chris

A dual element immersion may (almost) meet your requirements, as pointed out elsrwhere there is a shorter element, ~ 11" heating ~ 35L and a long(er) ~ 24/36" heating ~ 75/115L (more, depending on cylinder diameter and capacity) with a common thermostat switching the neutral, you could modify the "sink" & "bath" and connect them to a timer or whatever to allow only one element operation. The problem with these is that if the 35L is fully hot and you then change over to the longer element then no extra heating will occur as the immersion stat is only the same length as the short element, you would have to use up at least some of the 35L to get the stat to call for heating, even then, you still won't get a fully heated long element heated capacity even though the water at the top might end up say 5/10C higher than the stat setpoint and the bottom maybe 5/10C lower than the setpoint but probably not too bad once you get the longer element heating. In the second attachment, the black cable (sink) supplies the short element and the brown cable (bath) supplies the longer element.

 

[Ch_152]

A dual element immersion may (almost) meet your requirements, as pointed out elsrwhere there is a shorter element, ~ 11" heating ~ 35L and a long(er) ~ 24/36" heating ~ 75/115L (more, depending on cylinder diameter and capacity) with a common thermostat switching the neutral, you could modify the "sink" & "bath" and connect them to a timer or whatever to allow only one element operation. The problem with these is that if the 35L is fully hot and you then change over to the longer element then no extra heating will occur as the immersion stat is only the same length as the short element, you would have to use up at least some of the 35L to get the stat to call for heating, even then, you still won't get a fully heated long element heated capacity even though the water at the top might end up say 5/10C higher than the stat setpoint and the bottom maybe 5/10C lower than the setpoint but probably not too bad once you get the longer element heating. In the second attachment, the black cable (sink) supplies the short element and the brown cable (bath) supplies the longer element.

Cheers, I'd seen this image on a different thread in the past (was that your thread, or were you just replying to it?).

I never really understood how it could work well with the stat only being the length of the short element and I'm glad you've confirmed that as being a thing.

 

[Johntheo5]

I was just contributing to it, its strange but these dual elements were quite common around here and I've never seen anyone complaining about the above problem, I have a 52 year old Santon (dual element) and even though the sink only heats around 28L in a 150L cylinder, once ~ 12/15L is run off then the stat calls again for heat, it has a non adjustable (otter stat) thermostat that heats normally to 58/62C and will cut in at less than 5C lower.

 

[JohnW2]

We will have to agree, to disagree on that point John, because were it as simple as that - then the dual height thermostats would be an incredibly simple fix for the issue of sink/bath.

I must say that I would have said that they are an incredibly simple fix for the issue of sink/bath - although recent comments above make me wonder a little in terms of the positioning of the thermostat(s) in them.

Stratification, only works well, above the level of the heat source.

I'm not too sure what you are getting at there.

 

[JohnW2]

.... The problem with these is that if the 35L is fully hot and you then change over to the longer element then no extra heating will occur as the immersion stat is only the same length as the short element, you would have to use up at least some of the 35L to get the stat to call for heating ...

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

 

[Harry Bloomfield]

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?

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.