Cheapest way to run immersion heater

Section62

wittynamegoeshere said:

We had a massive tank with an immersion element.  It cost stupid amounts to heat, and lost almost all of its heat within 24 hours, despite it having foam insulation, it was ridiculous.

Strange.  My hot water cylinder (foam insulation to 1990's standards) will keep water usably hot for 48 hours.  Obviously if you are using hot water the cylinder won't stay hot without extra heat input.

wittynamegoeshere said:

IMO hot water tanks are a thing of the past, a relic from the days of cheap energy, unless you've got solar panels.  Most heat pump installations use one, but even then I'm not sure whether they make sense or not.  But it sounds like a heat pump is unlikely to be an option in a flat anyway.

I think you've got that 180 degrees out.  Whatever people do for heating hot water in the future, the cheapest solutions will involve storage of water heated either by solar/heat pump (gaining 'free' heat when it is available) or by heating when energy is cheaper (off-peak).

If you aren't sure whether a hot water storage cylinder would "make sense" with a heat pump installation then it is a bit strange you seem so confident that they are "a thing of the past" and people should get rid of them.  I'd recommend some further research.

wittynamegoeshere said:

The cheapest way to run an immersion heater is to not have one at all!  Get rid of the stupid tank altogether and get something like this...

Hot water storage tanks aren't "stupid".

Swapping one for an instantaneous water heater may be though, depending on the way in which the household uses hot water.  Without knowing how the household uses hot water, your advice is as bad as telling people they should get rid of their storage heater(s) and replace them with 'modern' panel heaters.

MattMattMattUK

QrizB said:

I'm going to disagree with MattMattMattUK here, which is a fairly rare occurrence:

MattMattMattUK said:

snowqueen555 said:

Other option of having it turn on twice a day is going to cost too much as well. It costs £2.50 to heat a 165lt tank.

It costs a lot more than that to heat a 165L tank from cold to temperature, how much it costs to heat up after your daily usage depends on how much hot water you can use.

Heating 165 litres from 10C to 60C will take a smidgen less than 10kWh. That's about £2.85 at current prices - which I wouldn't describe as "a lot" more than £2.50.

MattMattMattUK said:

snowqueen555 said:

Dread to think how much it would be in winter.

It will cost more as the input temperature of the water is lower.

The effect is fairly small. Heating those same 165 litres from 0C will take 12kWh, £3.42.

If your incoming water is below 0C you've got bigger problems to deal with.

I did the back of an envelope calculations and I am not sure what I actually did, I had it in the £6 per tank range, not sure how I managed that!

As a rough guide I have a 210L hot water tank, when I accidentally turned off the hot water, my guess from when I had knocked the switch to when I had a very cold shower one morning was that the hot water heating had been off for five or six days, with a shower every day and possibly two on one of those days. I tend not to have my showers that hot and I do not have any hair to wash so that might save me some time, but I do not rush either. 

Section62

wittynamegoeshere said:

There's little logic in heating a massive amount of water every day just in case you might need it, this is just GCSE level physics.  I don't know why people keep sticking to how we've always done things instead of progressing.  An open mind is a good place to start.

Perhaps. But you should also follow your own advice.

You don't heat "a massive amount of water every day" unless you've used "a massive amount" of hot water that day.  It is indeed "just GCSE level physics".  The energy which is used each day is only that needed to replace the heat in the water used, plus heat loss from the cylinder.  A well-insulated cylinder doesn't lose as much heat as you seem to imagine.

It isn't about "sticking to how we've always done things", it is about looking at the economics and practicalities of different methods and finding the one most suitable for the needs. I.e., keeping an "open mind" rather than dismissing something because you believe it to be outdated.

Storage of hot water allows the ability to consume energy when it is cheapest and providing a large volume of hot water in a short period of time.  For some people these are important features.

And if you believe that instantaneous water heating is something 'new' and 'modern' then I'd suggest some research into the history of water heating.  One of the only reasons why it isn't more common in the domestic setting is it is a relatively expensive way to heat water in peak times and - as you note - it isn't good where large flow rates are required.

wittynamegoeshere said:

Remembering that this thread refers to a specific situation in a flat and not a generic academic discussion, it's very likely that lots of money can be saved by heating hot water on demand instead of blindly heating a vast amount of water in case you might need it.

Quantify "lots of money".  Without knowing the household requirements you can only guess at whether one method of water heating is more efficient/cheaper than another.

wittynamegoeshere said:

One more advantage - you don't get the heat leaking from your tank and heating the home whether you like it or not, which is a very good thing during a heatwave.

If the cylinder is well insulated and installed in a cupboard (as most are) then the heat being lost into the rest of the house isn't that significant.

One more advantage of a hot water cylinder, in a cupboard, is that the cupboard can be used to store clothes, towels and bedding to keep them free of damp.

wittynamegoeshere said:

Our house had (still has actually) a cylinder with immersion element, I've used it and know that it's massively expensive and goes stone cold within about 2 days in the winter.

Quantify "massively expensive".  Mine is very cheap to run.

Given the problems you have experienced with this method it sounds like you weren't using it properly.

That isn't a reason to condemn all hot water cylinders outright - the better solution is to learn how they work and understand what you were doing wrong.

Meanwhile, your instantaneous heater may meet your needs perfectly, but when TOU tariffs become the norm you may discover the cost of heating water when you need it is much higher than heating and storing "a massive amount" during off-peak times.  Not forgetting the need to pay the cost of having a 3-phase supply installed.  Have you got a quote for that yet?

k_man

wittynamegoeshere said:

Mstty said:

As you are promoting this and I haven't looked into it fully yet.

How much is a shower costing you from this 9.6 kw model? What are you running costs for it for a couple of showers and washing up? What temperature is warm 40oC 45oC?

It uses 9.6 units per hour, so potentially it could cost you £20++ per hour, which sounds shocking  But you're not going to have a shower for an hour and, in reality, when using the shower it probably won't be running at full flow rate so will be reducing its power consumption, especially in the summer.  This depends on your shower and what flow rate you set it to.

It uses more power than an immersion element, but for a much shorter length of time.  I'm confident that this will save energy/money in almost all cases.  Economy 7 may make things more complicated, but my guess is that this would still make sense as the heat loss from a tank may cost more than the fairly mean savings for off-peak power.  But it would need lots of maths to get a good answer.

I currently have mine set to 40.  This is just right for hand washing and OK for a shower, but some may prefer about 45.  I find it simpler to just set it to my preferred shower temperature and leave the shower set to full hot.  It regulates temperature pretty reliably, although you may get a hot-cold-warm burst for a couple of seconds if you switch the shower off then back on.  This is exactly the same as most other electric showers.  Those who are expecting a jetwash-like experience will definitely be disappointed.  But that sort of thing would be an extremely expensive luxury for anyone who heats their water with electricity by any means.

Basically it's the same cost and performance as any electric shower, as that's essentially what it is.  The flow rate setting is entirely mechanical - what ever you set it to is what will come out of the tap, the unit can not control this itself.  You also set the desired temperature, and it will regulate its heat to try and achieve what you're asking for.  If you ask for the impossible then it will come out colder than you've set.  If it can easily do what you ask then it won't flow any faster but it will reduce its power.

There's also another option, which I may look into in future... You can supply it with hot water from a tank, but it must be a sealed mains pressurised tank, not an old-fashioned vented one as we have.  It then tops up the temperature as required.  So I could have a tank that was heated by solar panels whenever the sun comes out, feeding either hot, cold or lukewarm water into this, it will then do whatever's needed (or nothing) to ensure I get hot water out of the tap.  Kind of a best of all worlds option really - stored free heat, topped up on demand as needed.

Can you provide your workings out?

Do you mean up to  £2 + per hour? (So £4 ish per our on the projected October SVT?
As you say, not that many people shower for that long!

@Mstty is your hot water from the ASHP, rather than direct? So you benefit from a COP of 2 to 3 currently?

EssexHebridean

Hello OP - I'm in a 2 bed ground floor flat - not brilliantly insulated (1950's experimental build - they didn't carry on with the experiment for much longer!) and with storage heaters and an immersion for water - we do have gas but only for cooking, and realistically it's probably approaching the point where we need to consider changing to electric for that now.

Currently we use in the region of 4500 kWh a year - that is on Economy 7 though. Our immersion runs on a timer, heats overnight, and automatically goes off when the feed from the night rate switches off in the morning. For us this method works well - if we use very little water during the day, the top up heating costs the following night are tiny - the tank is a modern well insulated one. We only use the water for face washing, MrEH's shaving, and washing up - our shower is a separate electric one as is the case in the majority of all or mostly-electric properties. If we're away just overnight we leave the immersion on - if away any longer it gets turned off and reheated back from cold the first night we're back. 

Immersion heaters used to be seen as ludicrously expensive to run - but in the right setting they're really not. Sure, it would be more economical to have the ability to use gas for water heating, but that option isn't available to us. 

poppasmurf_bewdley

Just for information, I live in a 400 year old cottage with very thick walls, all electric with storage heaters and an electric immersion heater.

According to my smart meter, it is currently costs me about 25p per day to heat my new, well insulated tank on the upper of the two elements on cheap rate.  The lower element is only used as a boost and if used during the daytime can be as much as £1 to heat for an hour.  On my E7 tariff, I pay 8.9p per unit off peak and 25p peak.  

k_man

wittynamegoeshere said:

k_man said:

wittynamegoeshere said:

Can you provide your workings out?

Do you mean up to  £2 + per hour? (So £4 ish per our on the projected October SVT?
As you say, not that many people shower for that long!

@Mstty is your hot water from the ASHP, rather than direct? So you benefit from a COP of 2 to 3 currently?

Haha, absolutely right, it's £2+ per hour so not that ridiculous after all!

In response to the other points, as I say Economy 7 complicates things.  All I'm saying is to be open-minded.  The thread subject is asking the best way to run an immersion heater.  I'm suggesting that this isn't the best place to start thinking, but to start with the real aim which is to get hot water coming out of the taps.  Don't dismiss or ignore ways of doing things just because they're not the common way.

Our immersion heater is in a huge cylinder.  It takes 2 hours, flat out at 3kW to get the water hot, so uses 6 units of power.  I know this because it's on a timer that's always off, if I press Boost once for 1 hour then it's barely warm, 2 hours gets it hot so I'm assuming it takes all of the 2 hours to get hot and doesn't cut out on the thermostat before this time.  After this, even not using it, I know that it's much colder the day after and almost stone cold the day after that.  This is based on my experience, in a not very well heated house with an old cylinder.  But this is probably representative of the reality of the average property that someone may buy.  If the answer is to replace the cylinder then that is the time to think about whether there's an alternative.

This isn't a football match, it's not the league final of hot water cylinder vs every other possibility on the planet.  Just explore ideas, see what might be worth finding out more about.  I really don't get why internet discussions seem to descend into some weird kind of this vs that keyboard warrior battle.

However, I can state, categorically and without any doubt whatsoever that any kind of instant water heater will definitely cost less than using an immersion heater at the standard rate.  This is just the most basic physics possible, hopefully everyone can accept this.  This is a matter of fact, not opinion.  Whether it's the best solution for any one person may be a different matter, but you need to weigh up the options with an open mind and not just jump on one way of doing things without looking at the rest.

£20 did seem a lot 😄

What is the ballpark cost for an instant water heater like yours?

Section62

wittynamegoeshere said:

I really don't get why internet discussions seem to descend into some weird kind of this vs that keyboard warrior battle.

Generally it starts when people use words like "stupid" and tell other people to keep an "open mind". The latter implies the other people have a closed mind, and therefore is pretty insulting. The former simply doesn't make sense when applied to something inanimate like a hot water cylinder.

This is real life though, and it is important that if people are being given advice how to do things that the advice is based on facts, not people's inaccurate assertions.  Ripping out a hot water cylinder and replacing it with an instantaneous water heater could prove to be an expensive mistake - people shouldn't be advised to do something like that without a much better understanding of the circumstances and implications.

wittynamegoeshere said:

However, I can state, categorically and without any doubt whatsoever that any kind of instant water heater will definitely cost less than using an immersion heater at the standard rate.  This is just the most basic physics possible, hopefully everyone can accept this.  This is a matter of fact, not opinion.

Essentially you are asking us to accept something as "fact" which may not be true?

The 'basic physics' is that direct electrical heating of water is close to 100% efficient.  Whether you do it by an instantaneous heater, or an immersion heater, the same number of joules will be required to heat a given mass of water by a given difference in temperature.

The difference in cost between the methods will depend on the amount paid per kWh of electricity, and the 'system' efficiency - in other words how close you can get to heating the right amount of water with minimal loses.

What you state "categorically and without any doubt whatsoever" is conditional on a number of assumptions which don't hold true in all cases.  Therefore your statement is opinion, not a fact.

Maybe it would help to consider that an instantaneous heater is effectively just an immersion heater inside a teeny tiny hot water cylinder?  It doesn't do anything miraculous or different.

wittynamegoeshere

I think mine was about £180-ish at the time I bought, but I managed to nab it on ebay for £120 brand new.  But this was exceptional, and thankfully it genuinely was brand new and not broken!

I think you'll be looking at under £200 in the real world.

You will also need installation, plumbing is simple enough - if it's replacing a cylinder then it's literally a case of connecting it to the cold supply and hot pipes that are already there.  Just check the taps after, as they'll be getting more pressure than they've seen before, it probably won't be an issue unless the plumbing is really ancient.

The electrical side is more involved, you will need a ridiculously heavy duty thick cable that costs quite a bit per metre, so the cost depends on the distance from the consumer unit.  It also needs its own breaker to be added into the consumer unit.  Simple enough if you have a spare slot, more complicated but still possible if not.  Obviously this is not regular DIY territory.

wittynamegoeshere

Section62 said:

The difference in cost between the methods will depend on the amount paid per kWh of electricity, and the 'system' efficiency - in other words how close you can get to heating the right amount of water with minimal loses.

What you state "categorically and without any doubt whatsoever" is conditional on a number of assumptions which don't hold true in all cases.  Therefore your statement is opinion, not a fact.

Maybe it would help to consider that an instantaneous heater is effectively just an immersion heater inside a teeny tiny hot water cylinder?  It doesn't do anything miraculous or different.

I need one of those black curly wigs so I can say "ey ey calm down".

I do understand how electrical heat works, no need to describe the basics.  Both turn electricity into heat, one does it at the time you need it, on demand.  The other does it whether you need it or not, just in case, then leaves it sitting around going cold.  Which do you think is likely to be the most efficient and why?

Obviously an on-demand heater loses some heat.  Hot water is left in the pipe, but the same applies to a cylinder anyway, so these losses cancel out when comparing.  Some heat will be emitted from its body, but I can tell you from experience that this is negligible, presumably because the water does a very good job of carrying that heat away into the pipes, in fact it's water-cooled.

Tell me what I've missed.  Does the heater somehow put the electricity into tiny rockets and fire them into space?  Otherwise it's dificult to imagine how it could mysteriously make energy vanish somewhere so could use more power than an immersion heater.

I'm accepting as an assumption that anyone deciding what system to get will do their own research before going ahead and won't do this solely on the advice of a random stranger on the interweb.  But hopefully they'll consider all the options and not just start from where they think they should start.