# Two Immersion Heater Elements on One Circuit?

You can, of course, use a kettle when doing the dishes.

You can, of course, use a kettle when doing the dishes.
As I have been for the last 10 days since the immersion heater died!

It’s not quite so good for washing hands though…

In terms of tangible benefits, it would be nice for the “on-demand”/boost water to heat up quicker (which it would with the top element of a twin element cylinder). ... This would also mean that I’d not need to leave the “on-demand”/boost element on for as long to get the water up to a suitable temperature as with a single top entry element ....
It sounds as if you may misunderstand.

A 3 kW 'top element' will not heat water any quicker than will a 3 kW 'bottom element'. Even with the bottom element, the heated water will rise to the top of the cylinder, and will hence be available 'immediately' to be drawn off and used (from the top of the cylinder) just as quickly as it would with a 'top element'.

The only difference between a top and bottom element is the matter of how much water gets heated. With the bottom element, the entire tank of water will eventually get heated. With the top element, essentially only the water at the top will ever get heated.

Indeed, when people use the dual elements (top entry, but with a short 'top element' as well as the long one) they often had a switch (which switched between the two elements - only one or the other at any time), one position labelled 'bath' (bottom element) and the other labelled 'basin'/'sink' or something like that (top element) - the difference being how much hot water was needed.

In view of the above, you may well want to re-consider whether you want/need a second element. With just one ('bottom') one, if you needed a 'boost' at a time when the element was not normally 'on', you could just switch it on for as long was needed to get as much 'immediate' hot water as you needed - and it would heat as much water, and as quickly, as would using an additional 'top element' to achieve that.

Kind Regards, John

Just as an aside , if you are still going with two elements of some kind, plumbers merchants around here quite a few years back, use to stock a "one shot switch", such an item switch the supply for the heater L on dependant upon whether the stat was calling and you pressed the button, once the stat was satisfied then it switched off and stayed off until the button was pressed again, otherwise it stayed off.

With a bog standard immersion heater that meant disconnecting the stat terminals from the heater and wiring thru this one shot switch, if I remember correctly it created an extra terminal to monitor the stat output and the relay would latch via the button and remain latched until stat drops out. Basically it interrupted the stat and used to to energise the relay coil via the push button, once the stat was happy it threw the latch open. I remember there was a capacitor in the circuit to assist in the initial button push. I have not seen them for years, it might have been one manufacturer made them or even independently manufacturer locally. I just remember they were popular amongst plumbers.
If you have the knowledge to make one safely then you might also have the knowledge to make it apply in your own situation with only one of two elements .

It had no electronics, about as simple as this :- (from Wikipedia)

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In view of the above, you may well want to re-consider whether you want/need a second element. With just one ('bottom') one, if you needed a 'boost' at a time when the element was not normally 'on', you could just switch it on for as long was needed to get as much 'immediate' hot water as you needed - and it would heat as much water, and as quickly, as would using an additional 'top element' to achieve that.
That's not quite right.
That would mean there is no point in having a top element.

The bottom element will take longer (and therefore cost more) before the water at the top of the cylinder gets "hot enough" - i.e. when the thermostat switches it off.

It sounds as if you may misunderstand.

...
I think we're saying the same thing. I am not a plumber or heating engineer, but surely if I had for argument's sake a 10" element and a 20" element of the same rating, then the 20" element is going to distribute the energy into twice as much water therefore it would take twice as long (maybe not quite twice as long as the hot water from the longer element does rise) to heat the top 10" of water.

That's the whole point in the shorter element, right? To focus the energy into a smaller area of water (thus heating faster to your target temp and saving electricity from heating more water than you need)?

edit: @EFLImpudence beat me to it.

Just as an aside , if you are still going with two elements of some kind, plumbers merchants around here quite a few years back, use to stock a "one shot switch", such an item switch the supply for the heater L on dependant upon whether the stat was calling and you pressed the button, once the stat was satisfied then it switched off and stayed off until the button was pressed again, otherwise it stayed off.
...

Very interesting, I can see the use for that. Definitely above my pay grade to build and implement though!

That's not quite right.
That's obviously literally true, since the temp of water in the cylinder below the top element will rise slightly, but only 'slightly' - so what I wrote was 'essentially true' (see 'proof of pudding' comments at end of this message).
That would mean there is no point in having a top element.
As I implied, I'm not convinced that there is much point IF one is prepared to switch power to the bottom element on/off (for a 'boost') just for the required period of time. The 'advantage' of having a top element is that, even if it is left switched on, it will not significantly hdat more than the water at the top of the cylinder.

As I understand it, countless people (like me, in the case of one of my cylinders) have E7 (or similar) and a single bottom element, and have a 'boost' button which switches on that bottom element for a period (say 1 hour) if one needs a 'boost' (heating of a relatively small amount of water) at a non-cheap time of day. If I understand correctly, that may well be what/all the OP needs.
The bottom element will take longer (and therefore cost more) before the water at the top of the cylinder gets "hot enough" - i.e. when the thermostat switches it off.
It will only take very slightly longer (because of the 'not quite true' issue above. The bottom element thermostat will only switch the element off when the entire contents of the cylinder have got up to pressure, long after a smallish usable amount of hot water is available from the top of the cylinder (having been heated by the bottom element and then 'risen up'.

"Hot water rises" (hence convection) inevitably works very well. Think of your CH system. When you switch it on from cold, pretty hot water starts being pumped into the bottom of the radiator. After a few minutes, the top of the radiator will be hot (due to that hot water having risen) whereas the bottom of the radiator will still be cold. So, if the radiator were a cylinder, with water being heated at the bottom, hot water would be available to be drawn off from the top within a few minutes of the 'heating' being switch on.

There's no need to discuss this theoretically, since I know how it works in practice. On the rare occasions when all the (heated overnight) hot water in my one-element cylinder gets used up (invariably when my daughters are hear and taking 'everlasting showers' ), if I switch on the (bottom element) useable amounts of hot water become available within about 15 minutes, probably less.

I think we're saying the same thing. I am not a plumber or heating engineer, but surely if I had for argument's sake a 10" element and a 20" element of the same rating, then the 20" element is going to distribute the energy into twice as much water therefore it would take twice as long (maybe not quite twice as long as the hot water from the longer element does rise) to heat the top 10" of water.
No. No matter where the water is heated, the amount of heat energy transferred to the water will be the same in a given period of time, and the heated water will rise to the top. See what I've just written to EFLI - in particularly my 'CH radiator analogy'and, more important, the fact that I do it here, and it works

You seem to be assuming that a large volume of water heated by a long element will initially remain 'luke warm' throughout, but that's not how physics works - heated water will rise, so that nearly all the 'heat' (energy) is then concentrated in the relatively small volume of water at the top, hence 'hot' (rather than 'luke warm' if the same amount of heat energy were distributive throughout the entire cylinder)
That's the whole point in the shorter element, right? To focus the energy into a smaller area of water
As above, it doesn't matter how much water is exposed to the element. The amount of (heat) energy transferred to the water will be the same, regardless of that, and, physics being physics, hotter water will always rise up to above colder water. If you feel a bottom-element cylinder (or CH radiator) after the heating has been 'on' (from cold) for a while, the top will be pretty hot, but the bottom (and places in-between) will remain pretty cold. That remains true (if starting from cold) for at least 30-60 mins with a cylinder, but less with a CH radiator (which contains much less water)

Believe me, it works . As I also wrote to EFLI, plenty of people with E7 and a single (bottom) element have a 'boost button' which switches the (bottom) element on for, say, 1 hour if/when one needs a bit of hot water during non-cheap electricity periods - and, as I wrote, I suspect that may well be all you need.

Kind Regards, John

There's no need to discuss this theoretically,
Right.

since I know how it works in practice. On the rare occasions when all the (heated overnight) hot water in my one-element cylinder gets used up (invariably when my daughters are hear and taking 'everlasting showers' ), if I switch on the (bottom element) useable amounts of hot water become available within about 15 minutes, probably less.
Then presumably the top element would do that in (oh I don't know) 7 or 8 minutes.

.... plumbers merchants around here quite a few years back, use to stock a "one shot switch", such an item switch the supply for the heater L on dependant upon whether the stat was calling and you pressed the button, once the stat was satisfied then it switched off and stayed off until the button was pressed again, otherwise it stayed off.
Interesting. That functionality might suit some users, but only if they only required the amount of hot water heated by the top element (until it's 'stat switched it off) and were happy for the water reverting to being cold thereafter.

If they simply left the top element 'on' for 'as long as needed', the bit of water at the top would continue to be heated (under thermostatic control) and thus some heated water would still be there when they switched the top element off.

However, as I've recently been writing, I'm not sure that there is much benefit in having a 'top element', provided a 'boost' to the bottom element only lasts for as long as needed (per a 'boost button' or manual switching on/off).

Kind Regards, John

Then presumably the top element would do that in (oh I don't know) 7 or 8 minutes.
Why do you think that? As I've said, virtually all of the heated water will rise to be at the top, very quickly, whether it is heated near the top or near the bottom. Are you perhaps thinking that 'heat rising' is a slow process (which it isn't) ?

What you're saying could only be correct if, with a bottom element, significant amounts of heated water remained below the level of the top element and, given that 'warmer water rises', that is not what Mr Physics says.

A 3 kW 'top element' will not heat water any quicker than will a 3 kW 'bottom element'. Even with the bottom element, the heated water will rise to the top of the cylinder, and will hence be available 'immediately' to be drawn off and used (from the top of the cylinder) just as quickly as it would with a 'top element'.

Not strictly true, because whilst stratification is still in evidence, with a lower element there is much more mixing. Basically - if you want a small quantity, quickly, then a high level element really is the best way to produce that.

I'm sure if that were not true, then the same effect could more cheaply be reproduced, by simply using one element, by wired in series with a choice of two stat lengths.

Why do you think that? As I've said, virtually all of the heated water will rise to be at the top, very quickly, whether it is heated near the top or near the bottom. Are you perhaps thinking that 'heat rising' is a slow process (which it isn't) ?

What you're saying could only be correct if, with a bottom element, significant amounts of heated water remained below the level of the top element and, given that 'warmer water rises', that is not what Mr Physics says.
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.

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.

Not strictly true, because whilst stratification is still in evidence, with a lower element there is much more mixing. Basically - if you want a small quantity, quickly, then a high level element really is the best way to produce that.
I've agreed that it is 'not strictly true' but am saying that, in practice, the difference is pretty small - such that, if I were the OP, I would not go to the cost and disruption of getting a new cylinder and an extra immersion (in practice, probably two new immersions) to achieve what he wants.

As I keep saying, in practice, if you start heating a cylinder with a bottom element (or switch on CH) the lower parts of the cylinder (or radiator) remain fairly 'cold to the touch' for quite a long time after the top starts feeling 'pretty hot'.
I'm sure if that were not true, then the same effect could more cheaply be reproduced, by simply using one element, by wired in series with a choice of two stat lengths.
I think it probably could but, to the best of my knowledge, such animals don't exist. In their absence, one can achieve the same by switching off (manually, or automatically after a timed period) the bottom element after ithas resuklted in enough hot water appearing at the top of the cylinder.

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