Two Immersion Heater Elements on One Circuit?

And if you only have 1 circuit to this area, then you don’t have a dedicated E7 or separate supply so the circuit is only a 24 x 7 circuit

He doesn't have an E7 tariff, he just wants to be able to switch between two elements, whilst making sure that only one can come on at a time.
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Are you keeping this tank or planning on replacing it ?

And if you only have 1 circuit to this area, then you don’t have a dedicated E7 or separate supply so the circuit is only a 24 x 7 circuit
The plan (or at least, the idea I was exploring) is to replace the existing tank with a twin element unit.

I'm not on E7 at the moment (I was) but on Octopus Agile - which is similar in that it has low rates during the night (and for most of the day) but much higher during peak times.

The immersion is currently wired to the permanently live consumer unit, but has a manual time clock to control it to only come on at night (unless overriden).

So, for all intents and purposes, I can achieve pretty much the same as I am looking to achieve with my current cylinder (if it wasn't for the fact that the immersion element has just broken), but I was just exploring the practicalities of moving to a twin-element cylinder.

Whether it's actually worth the cost is another question. A new element for the existing tank is obviously significantly cheaper, although I can't seem to get the old one off - Bbt that's a different story...
I know my immersion heater controller can work with two immersion heaters, only one being powered at a time, in hind sight and that's easy, simple timer may have been better, my iboost+ is designed to use surplus power from solar panels, and has a host of options, however as one looks at the export rate v off peak rate, it does seem pointless.

What I have found is the hot coil heated with central heating heats the whole tank, and the immersion only heats top foot or so, so very fast the solar heated water runs out, but the CH water can last 3 days.

If I had a low immersion heater I don't think I would ever use the one in top of tank. So big question is having two immersions and all the clocks etc to be able to use them, worth while?

I have looked at converting the immersion so it can heat all the tank, how much depending on solar available, the Willis external immersion heater will it seems do this,
it will start at top of tank and work down, so even after a few minutes you get hot water, but it can heat whole tank, however the question is can I get a Welsh plumber to fit one, in Ireland not problem, what ever they say about being tick they are clearly not, but even if fitted, is it worth the expense?

I look at my energy use 1715719975427.png and it changes every day, but my tariff is so much cheaper midnight to 5 am to 5 am to midnight even with the solar, it is better to run dish washer, tumble drier, washing machine etc in that cheap rate. As I don't know what solar I will get from day to day.

So I just use the delay timers built into the machines, in the days of hot and cold fill it may have been worth heating water over night, but now I am not sure worth the effort?
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It’s worth bearing in mind that to heat water by electricity is about 3x that of using gas. Unless you have solar panels
It’s worth bearing in mind that to heat water by electricity is about 3x that of using gas. Unless you have solar panels
Maybe but I use oil, the calculations has to in summer include heating the pipe work, my cylinder is on the top floor, and boiler is two floors down, so what one has to aim for is a short heating time, in my case 20 minutes every other day. But the water temp was hit and miss, there is no thermostat on the cylinder, so time is the only control. Set for ½ hour, but return water turns off boiler after 20 minutes.

So the calculation has to include losses in the pipe work, and off peak electric around 8.95p per kWh and 7.42 per kWh for gas, that is not 3 times the cost, and with the extra losses with gas, I would say electric and gas about on par. Gas is not sold at a different price depending on time of day, same with oil at around 10.54p per kWh and LPG at 9.01p per kWh electric is cheaper.

Even with solar panels the fee paid for export means it is likely to cost more to heat the water with solar than to heat water with off peak when counting loss of money you would have been paid.

My point is any equipment to control when you use power costs, also there is a standing charge to also consider, if not using gas for other things.

I got it wrong, I will admit I got my sums wrong, many will not, heating my DHW with solar is going to cost more than heating off peak. However off peak tariffs vary, EV typically 5 hours, heating 7 or 10 hours, and the longer the time you have off peak, the more it costs. One can have a variable tariff, but working out when to turn things on/off would be a nightmare. I know what ever time of year I can charge my batteries for 4 hours at 8.95p per kWh, I don't have to watch the prices and work out day by day when to charge batteries.
.... ((MCB is rated to 16A, cable is 32A (I think),
.... Am I right in my assumption that I definitely could not run both at the same time? (Therefore I want to make sure I find some foolproof way of ensuring that this can't happen).
Re-measuring the cable, I think it might actually be 2.5mm T&E, so only 20A.
For the record, the issue may well be the avail;ability (or not) of suitably-rated MCBs, rather than the cable.

Provided it is not installed within thermal insulation, in conduit or something like that, 2.5 mm² T+E cable actually has a current-carrying-capacity of 27A - so (just) enough for 2 x 13A loads. However, the MCB protecting such a circuit would have to have a rating at least as great as the load and no greater than the current-carrying-capacity of the cable. Hence only a 26A or 27A MCB would be suitable,ans such devices do not (to the best of my knowledge!) exist.

Having said that, most (maybe all) "3 kW immersion elements" are actually 3 kW at 240V, in which case they would draw fractionally under 12A at 230 V (the voltage we use for design calculations). Since 25 A MCBs do exist for some CUs, it follows that one of those together with 2.5mm² cable (installed in an appropriate route) might theoretically be OK to supply two "3 kW" immersion elements simultaneously

Kind Regards, John
It’s worth bearing in mind that to heat water by electricity is about 3x that of using gas. Unless you have solar panels
Maybe but I use oil, the calculations has to in summer include heating the pipe work, my cylinder is on the top floor, and boiler is two floors down ...
Indeed. Some people not only assume that everyone has (natural) gas but also ignore the fact that the overall efficiency of heating water can vary appreciably between different fuels.

With electrical heating of water almost all the 'metered energy' (say, in kWh) heats the water, the only losses being in the cable (and one can theoretically get that as low as one wants by using fatter cable).

However, with gas (natural or LPG) and oil, what is metred is the volume of fuel used, and that is converted to, say, kWh from the calorific value of the fuel - and is hence the theoretical amount of energy released by the fuel when burned (in a laboratory'). However, when it is burned in a boiler to heat water, by no means all of that energy ends up in the water being heated - even with good condensing boilers, used ideally, I don't think efficiencies get much much above 90%.

A direct comparison of the cost of a metered kWh of electricity and a metered (and converted) "kWh" of gas or oil is therefore not valid, since it makes electricity appear more expensive in comparison with the other fuels than it actually is

If you add to that the issue of post-boiler losses you mention (I. too, have a very long run from boiler to DHW cylinder) and the figures become even more biased against electricity.

[ in passing, times have changed. A decade or so ago, when there was a much kore dramatic day/night difference in cost for E7 electricity, it was cheaper for me to heat water with cheap-rate E7 electricity than with LPG (my only other option) - and it may even have been cheaper than natural gas, if I had had it ]ne has to aim for is a short heating time, in my case 20 minutes every other day. But the water temp was hit and miss, there is no thermostat on the cylinder, so time is the only control. Set for ½ hour, but return water turns off boiler after 20 minutes.
There is also the option of preventing two immersion heaters energising at the same time by more simple electromechanical methods too.
Simple relay/contactor to prevent one from energising whilst the other one is energised.
If programming worries you with modern technology or you think that it might be too easy to mistakenly energise both together, then this simpler option might have some merits.
Of course you would need to consider how foolproof it needs to be in the event of a failure of a relay/contactor to work as intended.
Wired as a simple OR configuration (That is XOR = Exclusive OR in computer logic speak) so that on or the other could be energised depending when the relay/contactor coil is energised (or made capable of being energised) , note - relays with change over contacts are pretty easy to find, contactors less so, they are usually just make contacts when energised, although the coils can be controlled via a relay.

Having said all of that, and the solutions already mentioned, is it worth all the hassle and expense of doing any of this and changing to a cylinder that accepts two heaters? In your shoes I would first consider just replacing the faulty heater unless having two heaters is very important to you.

Another option might be a "Priority Shower Switch" intended for use when you have two electric showers on one circuit.
A company called "Garo" might be able to advise you on using one of their products to achieve your aim (I have never used their priority switches but have used some other products a couple of times over the years such as Double Pole switching MCBs and consumer units).
My off peak is an EV tariff, there is also Economy 7 and Economy 10 and even a variable tariff depending on demand, we hear Octopus prices being quoted many times, each results in a different off peak charge. Plus it changes area to area and supplier to supplier a quick hunt for this address got 1715755534164.pngbut for me to change I have an exit charge, and they fail to say what the solar rate is, and one in the main gets a higher payment for solar if you sell to same company you buy from. Also times vary even for same tariff, so for me in mid Wales to say what is the cheapest way to heat water, may not be valid in where @Ch_152 lives.

However it does seem he/she is talking about changing his system to suit what is cheapest option, so there are three common systems:-
1) Twin element immersion fitted into top of cylinder.
2) Two independent elements fitted into side of cylinder.
3) Element outside cylinder (The Willis system).
Much depends on the cylinder in what can be fitted, and also the header tank, the old twin element system was designed for sink or bath and often had a switch marked as such, but often the thermostat did not have the safety cutout, basic not enough room, so can't use a thermal plastic header tank as these can split if hot water gets to them which can happen with faulty thermostat. Also the central heating hot coils can get in the way of fitting a long immersion heater from the top.

So the option 2) again depends on if you can get suitable adaptation for the cylinder, which is why the Willis became so popular in Ulster where it was invented, the system heats from the top down, so the longer it is switched on the more hot water you get, but unlike the other systems even when on for a short time you have hot water, it does not gradually increase temperature, but produces hot water which slowly fills more and more of the cylinder, seems a great system.

However when I looked for a plumber to fit it, I was met with a blank expression, this is the solar version
it works it seems very well with solar, but there is also a version without solar, but in mainland UK good luck finding some one who knows how to fit it.

So nitty gritty will one get a pay back from the money spent? The conversions will not be cheap, so 31.31 - 8.95 = 23.36 p per kWh saving if using off peak, my boiler needed to run every second day, so to maintain the DHW is going to require around 10 kWh in my house with minimum insulation on the tank which means it airs clothes, every two days, including the water I use. So for whole tank cost £2.60 per week, with is switching on 3 times at off peak times heating whole tank. If it only heats top of the tank, it will cost less, but likely it will not last the day so the cost will increase 23.36 p per kWh hard to do maths as don't know how much used, however due to increased cost of power, one is likely to have around the same bill, so the question is if it's all worth the effort?

As said I fitted iboost+ all with the idea of saving money, but in fact a simple time clock would have saved more. And an iboost+ is not cheap. Let @Ch_152 do the maths for his house and say is it really worth the effort?
Yes, bath and sink style was one, well mentioned EFLI I had forgotten
Thanks for all these replies, very informative stuff.

@ericmark I'd seen a few of your posts about the Willis system - these are interesting. You're right in that it's probably worth me weighing the cost of changing the whole cylinder vs the potential savings (which are likely to be minimal) by moving to a twin independent element system. I may, as yourself and a couple of other people have noted, actually just be better off me going for a twin element in the existing tank and settling for that. The tank itself isn't broken (yet), so why fix it?

I find the Octopus Agile variable rate tariff quite usable. It's very typical that you can assume when the cheap periods are (usually anything outside of 6-10 am and 4 - 7pm - although 6-10am is about the same as your normal night rate on E7 anyway for me in London). So I make a substantial saving on any electricity used outside of those periods during the day, and usually a decent saving on the night use too. There are options for smart immersion timers which can link up to the internet and automatically activate during only the cheapest periods of the night. Message me if that interests you - it's saved me around £300 since switching in September based on my calculations.

@Murdochcat Unfortunately the property is electric only. Gas would make life easier.

@JohnW2 very useful info. on the cable and MCB capacities thanks for that.

@ebee This kind of mechanical relay is exactly what I initially had in mind initially, although seems unnecessary knowing what I know now (and that there are products which can manage this for you). And you are bang on about the cost considerations. Next post to summarise my position on that. Thanks very much for your responses.

@EFLImpudence Seems like the dual element (single entry) "bath/sink" style element could be an ideal middle ground for me as it stands then (if I don't need to replace the cylinder for any other reason).
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On cost and the wider considerations of whether this would be worth it:

My initial post was intended to figure out whether this was possible, electrically. As a number of you mention, the cost of replacing the cylinder entirely probably needs to be weighed up against any tangible benefit to me, and any long-term saving.

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 implying a cost saving. @ericmark you might like this bit...

Let’s say I needed to run the boost on a normal heating element for 2 hours, and with the top element only I could get away with only 1 hour (the element is half as long, heating half the amount of water to the same temperature).

That’s 3kWh difference per day.

Calculated, crudely, on running the element for an extra hour a day, at daytime E7 rates (~30p) comes out at around £300 a year. So sounds like a significant saving.

However, that money is not just thrown down the drain. The hot water is stored (ignoring any losses from the cylinder for the time being, which I understand should be relatively small - although the cylinder is not that new), and just means that it requires less energy put in overnight to reach the target temperature. You essentially only lose:
  • What is lost from the cylinder to the atmosphere (which also goes into my airing cupboard/spare room rather than the loft, so not truly wasted)
  • The difference between the day and night rates, multiplied by the amount of energy contained in the boosted hot water that wasn’t used.

On the same assumption of requiring running an extra hour per day using a “normal” element: If my day rate was 30p, and my night rate 12p (as per Octopus E7 tariff as of today), that would be 18p*3kW*365 days = £200 per year.

(assuming no losses from the cylinder to the atmosphere between boosting in the evening and turning back on at night).

Consider now that I won't need to boost every day, and this is not my "forever home", so the longer term savings are not quite so important.

Changing the cylinder looks like it’s going to cost me at least £750 more in parts and labour than just changing the element. So ultimately it's probably not worth it to change from the single top entry to dual immersion cylinder. The Mrs. can just turn it on half an hour earlier before she has her bath...

However, very grateful for the replies and hopefully the technical info. on the electrical side helps someone else out down the line.

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