6kW hot water cylinder

[AndyPRK]

That’s a good point John.

Op could fit a “sink bath” switch to swap between the 2 elements.

Ps I think mine is a 175l which is the last one before dual elements are fitted.
But mine is heated by gas most of the time. The element is rather slow!

 

[AndyPRK]

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/182408220739

 

[nicck]

Really appreciate the details of the information given here.
I'm going to try to source the installation instructions to see if there is a way of setting it up for efficiency.

 

[JohnW2]

Really appreciate the details of the information given here. I'm going to try to source the installation instructions to see if there is a way of setting it up for efficiency.

You're welcome.

The most crucial question, which I don't think you've addressed, is that of what sort of electricity tariff you have it - is it a 'standard' ('single rate') tariff, or is it a 'dual rate' one (like Economy 7 etc.) which gives you cheap electricity at night?

Kind Regards, John

 

[nicck]

The supply is on a standard rate tariff. Prepay meter, waiting to get changed.

The model is a UN10002 120ltr.

 

[JohnW2]

The supply is on a standard rate tariff. Prepay meter, waiting to get changed. The model is a UN10002 120ltr.

Thanks. If it's only 120 litres, and particularly given that you have a standard tariff, I would suggest that you probably wouldn't ever need to use the second immersion (i.e. both at once). Even if you totally emptied the tank of hot water, it would take a single 3 kW immersion only fractionally over 2 hours to re-heat it, so the only conceivable reason for running both would be if you were likely to get into that situation and that that speed of re-heating was not fast enough for you.

Taking baths are about the only usual situation in which there would be any risk of emptying even a 120 litre cylinder of hot water.

If it were me (and, admittedly, we, being 'shower people', very rarely take baths), I think I would probably have the second immersion wired via a manual switch, fairly sure in my assumption that, in practice, I would probably never need to switch it on!

Kind Regards, John

 

[EFLImpudence]

I still maintain that it is not 'normal' to be able to have both on at once.

As you have done that with yours, that is your choice.

 

[JohnW2]

I still maintain that it is not 'normal' to be able to have both on at once. As you have done that with yours, that is your choice.

What mechanism are you suggesting would prevent one from having both on at once - a 'changeover' switch of some sort?

Even with E7, when the main (bottom) element will normally only be on during E7 cheap hours, there's not (at least, not in my experience) anything stopping one from turning the other (top) one during that time if one really wants to.

How do you feel 2 x 3 kW immersions would 'normally' be wired, and why?

Kind Regards, John

 

[EFLImpudence]

There are various timer/switches available including the so called sink,bath switches depending on how the system works.

I think the primary reason is to prevent one forgetting to turn off the elements.

 

[AndyPRK]

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/182408220739

This is a change over switch!

Also known as sink/bath

 

[JohnD]

I still maintain that it is not 'normal' to be able to have both on at once.

Absolutely agree

If you have an immersion that you intend to use very seldom, an auto-off switch is very handy

we use this kind of thing in a sort of village hall, which runs for 1 to 4 hours,

but you can get a similar one that runs for 15/30/60/120 minutes.

Means it is never on for days or weeks on end by mistake, but very suitable for food preparation and subsequent washing up or handwashing during an event.

An immersion heater warms about 1 litre of water per minute, so about 90/120 minutes is enough for a bath. Slower in winter than in summer as the incoming supply is colder.

The Horstmann range has a good reputation
https://www.tlc-direct.co.uk/Main_I...Timers_2/index.html#Immersion_Heater_Timers_2

 

[JohnW2]

There are various timer/switches available including the so called sink,bath switches depending on how the system works.

There are, but my impression is that a high proportion of people don't have such functionality.

As I understand it, the main reason for having a second ('top') element related to dual-tariff (e.g. E7) supplies, particularly in the days when the main ('bottom') element was powered from a 'night-only CU'. The second element allowed one to 'boost' the heating during the daytime using the "24/7 CU", and since that was relatively expensive, by having the element high up one only heated a relatively small amount of water during that 'daytime boost'.

... I think the primary reason is to prevent one forgetting to turn off the elements.

I would sat that is very much a two-edged sword! If one had (literally or effectively) a change-over switch which selected which of the two (lower or upper) immersions would come on when the circuit was powered, I would think that there would be a major risk that one would often forget that one had switched to the 'top' immersion, and hence subsequently find that one did not have anything like as much hot water available than one had expected/wanted!

Kind Regards, John

 

[JohnW2]

Absolutely agree ... If you have an immersion that you intend to use very seldom, an auto-off switch is very handy ... we use this kind of thing in a sort of village hall, which runs for 1 to 4 hours, ... Means it is never on for days or weeks on end by mistake, but very suitable for food preparation and subsequent washing up or handwashing during an event.

Indeed so.

However, that, per se, is not the issue we are talking about. We're talking about having a second 3 kW immersion in the cylinder - and, in the absence of an 'intelligent' control system (which I don't think most people do have), whether one just has a manual switch or a 'time lag' switch such as you mention, there is nothing to stop one activating that second immersion at a time when the first one is already on - functionality that would require some sort of 'changeover' switch.

In theory, the total amount of energy used should be no different whether one or two (thermostatically controlled) 3 kW elements but the issue which started this whole discussion is that having 6 kW means that there are times when the peak (instantaneous) current draw will be around 26A

An immersion heater warms about 1 litre of water per minute, so about 90/120 minutes is enough for a bath. Slower in winter than in summer as the incoming supply is colder.

Indeed - and, as I've said, I would regard that as a reason for not needing more than a single 3 kW immersion in a cylinder of the sort of size we are discussing, particularly if one doesn't have E7, and even more particularly if one leaves the immersion powered continuously.

Kind Regards, John

 

[SimonH2]

One obvious solution would be a load shedding relay to hold off one element when the other is on and not tripped off by it's stat. Or with a little bit of wiring alteration, just use the post-stat switched line in one heater to power a hold-off relay for the other.
It could be wired either way round, but an obvious setup would be to have the upper element as the priority one for fast reheat of the upper cylinder, and when that reaches temperature, the lower element would take over.

 

[JohnW2]

One obvious solution would be a load shedding relay to hold off one element when the other is on and not tripped off by it's stat. Or with a little bit of wiring alteration, just use the post-stat switched line in one heater to power a hold-off relay for the other.

Well, yes - 'off-the-shelf' kit is available to do that for two electric showers. However, I'm not convinced that I really see the point in the second immersion ...

It could be wired either way round, but an obvious setup would be to have the upper element as the priority one for fast reheat of the upper cylinder, and when that reaches temperature, the lower element would take over.

... I'm not at all sure that it's true that the upper element will result in appreciably faster 'heating of the upper cylinder'. Even if one uses the lower element (for a limited period of time), the limited amount of heated water will mainly rise to the top of the cylinder. As I see it, the only real 'advantage' of having the top element is that it will only (largely) heat water at the top of the cylinder even if one leaves it on 'continuously' - whereas if one uses the bottom element to heat 'just a bit' of water, one has to remember to turn it off once enough water has been heated.

All of my cylinders do have two immersions, since they came with provision for two, so I fitted two! However, I almost never use the top ones - the main purpose they have served over the years/decades is as a very temporary (limited) 'spare' when the main (low) one has failed, whilst waiting for me to get around to replacing the failed one! Although I could (it's just a manual switch, and the wiring arrangement is adequate), I don't recall ever having had both elements in one cylinder on at the same time.

Kind Regards, John