Cheapest way to run immersion heater

QrizB

wittynamegoeshere said:

Getting back more on-topic, I'd be amazed if nobody's ever actually measured the heat loss from a typical hot water cylinder.

Loads of people have, there's an example here on the forum:

https://forums.moneysavingexpert.com/discussion/comment/72842582/#Comment_72842582

Hot water cylinder manufacturers also calculate heat losses and cylinders are marked with an ErP rating based on this. See:
https://www.phamnews.co.uk/erp-tightens-the-rules-for-cylinder-manufacturers/

Under the 2017 regulations, the maximum allowed heat loss for the most popular 210 litre cylinder will be 87W. This roughly translates to 2kWh per day – probably a daily cost of about 10p to the householder. This is a dramatic improvement over historic systems, which will enable the country and EU to achieve its 20-20-20 target to reduce energy use by 20% and increase the share of renewable energies by 20% by 2020. In addition, it provides a real incentive for a householder to invest in a modern, well insulated hot water cylinder.

The days of 5p/kWh energy seem to be a long way behind us, but even so.

Edit to add: and here's a typical ErP label:
https://erpfichetool.jouleiot.com/api/label/Joule UK/product/pdf/CC-B3012045FEA

That's a 1200x450 166-litre tank with a heat loss of 60W (1.44kWh/day) and an ErP rating of C.

k_man

wittynamegoeshere said:

You can make the maths easier by just ignoring the hot water temperature and treating it as though the immersion heater is heating from cold to the desired temperature.  The outcome is the same however you get there in energy terms.

Anyone know the amount of kWh units needed to raise the temperature of 80L of water from say 15degC to 40?

Not trying to catch anyone out, just trying to work out what a bath actually costs.  I guess nobody's going to be having a cold bath outside of a heatwave though, so a tepid bath isn't really representative of normality.

Getting back more on-topic, I'd be amazed if nobody's ever actually measured the heat loss from a typical hot water cylinder.  Definitely lots in my case, I've checked for myself, but mine's probably at the useless end of the scale.  Presumably some here have much better tanks but I'm sure a representative average is somewhere in the middle.

You'd hope that, in the push for heat pumps and hot water cylinders versus gas combi boilers and/or existing old cylinders that somebody would have actually measured this rather than just having a massive argument among a random bunch on a forum.

Heating 80L of water from 15 to 40C is about 2.3kWh.

And heat loss from a modern tank meeting the relevent standards is between 1 and 2 kWh based on the tank being hot all the time. Once water is used, if not heated again immediately, the tank losses are lower.

(And, because someone else will mention it, when heating is required in the house, that heat loss from the tank is not completely wasted.)

 I can't find the official data atm, apart from this:

https://www.theheatinghub.co.uk/articles/hot-water-efficiency-tips

Modern cylinders must lose no more than 2kWh in per day (i.e. 24 hours) and some models lose as little as 1kWh per day.

wittynamegoeshere

Thanks, really useful to see some actual facts!

So worst case of 2kWh = 57p/day = £208 per year or £343 per year for October onwards if the 65% rise happens.

That confirms the sort of figures I was seeing with mine, despite the solid foam insulation jacket.  I don't know about anyone else, but I'd call that a lot of money.

Some may become useful home heating in the winter as pointed out above, but most are in a cupboard so much of it will end up in the loft.  It's also the opposite of desirable at some times (e.g. now).  Some may even be using air con to counter it.

Many installed cylinders will be worse than the 2kWh worst case for modern cylinders.

I think I've made the right decision, for me.  But perhaps I'm just some crackpot on the internet who believes daft conspiracy theories and upsets people on forums.

QrizB

wittynamegoeshere said:

So worst case of 2kWh = 57p/day = £208 per year or £343 per year for October onwards if the 65% rise happens.

That confirms the sort of figures I was seeing with mine, despite the solid foam insulation jacket.  I don't know about anyone else, but I'd call that a lot of money.

Well, yes and no.

Let's say that you actually use 100 litres of hot water a day. That's 6kWh/day assuming it starts at 10C and ends up at 60C. If you heat it instantaneously, on a single-rate tariff, you'lll use 6kWh. Alternatively, if you heat it in a HW tank overnight on E7 and keep it warm all day you'll need 8kWh.

• 6kWh @ 28p/kWh = £1.68
• 8kWh @ 18p/kWh = £1.44

Even though you're using more energy, you're spending less money to do so.

If you only need 50 litres a day, it bevcomes 3kWh vs 5kWh, 84p vs 90p, and single-rate is a little cheaper. But 6p/day cheaper not 56p, and it only becomes 36p/day if you use *no hot water at all* but still heat the tank.

(I chose 18p/kWh based on EDF's SVT rate card. Some night rates are a little higher than that, some are much lower.)

Edit: and yes, you're right:

Many installed cylinders will be worse than the 2kWh worst case for modern cylinders.

If you heat your water on E7 and your tank is more than 5 years old, the £200 cost of a new ErP-rated tank (assuming you can DIY the fitting) might be worth paying.

wittynamegoeshere

Totally agree and have already stated, any discounted rates, solar panels, heat pump etc etc make the situation much more complex.

But for single rate users, i.e. me and many others, it's a very expensive luxury.

It was probably something you could just not care about when it was wasting less than £100/year not so long ago.  But there's a judgement to be made against today's price, in my case I'm completely happy to save that big pile of cash every year and have warm fresh water rather than hot stale water.

Section62

wittynamegoeshere said:

But for single rate users, i.e. me and many others, it's a very expensive luxury.

You state this, despite other people using immersion heaters posting how they find it an acceptable and affordable way of heating their water.

For some people - particularly those who have no other option than electric heating - your words are borderline scaremongering, and may lead them to unnecessarily spend money they don't need to on finding other solutions. That isn't 'MSE'.

wittynamegoeshere said:

...in my case I'm completely happy to save that big pile of cash every year and have warm fresh water rather than hot stale water.

In this misrepresented 'fresh' vs 'stale' comparison you've possibly overlooked the point that 'warm' water can be the perfect conditions for legionella and other bacteria to thrive.  Your 'fresh' water may be very un-fresh.

Although it is not advisable to drink water which has been stored in domestic water systems, describing it as "stagnant" and "stale" is potentially misleading to a lay person.  Provided the storage and plumbing system complies with regulations the stored water doesn't pose a significant health risk.  People shouldn't be misled into thinking they need to make expensive alterations to their plumbing in order to get 'fresh' water for washing and bathing.  That wouldn't be 'MSE' either.

Unfortunately this forum has seen a lot of people who've been misled into replacing their perfectly adequate storage heaters with 'modern' heaters on the basis of bogus information about efficiency etc.  It would be tragic if the same thing were to be allowed to happen in relation to people's hot water needs.

Instantaneous water heaters can be the best solution in specific circumstances, but they shouldn't be oversold.

wittynamegoeshere

It's pretty simple, flowing = fresh, standing = stale.  Just like a river vs a pond really.

I've seen our cylinder's header tank, it's definitely not food grade.  Many have a layer of stuff you probably wouldn't want to know about.  It's "vented", i.e. not sealed, i.e. anything that fancies crawling in can do so.  Personally, I rather like that the water I'm splashing over my face hasn't come out of the old tank that's never been emptied.

As far as I know, the legionella risk only applies to standing water.  You're scaremongering here by suggesting there's a risk that simply doesn't exist.

Sorry if you disagree, don't like my opinion or attitude and if I don't fit your personal model of how everyone should be, but it all sounds like you're looking really hard to find things to take exception to for whatever reason.

You're also equating a perfectly sensible and "money saving" way of heating water with a whole bunch of products and companies that are definitely all scams.  I have an engineering degree, I've had shouty arguments with Fischer employees in their stand at a country show, I do know that there have been many exciting new products that are complete rubbish that are marketed in immoral and deceiving ways.  But I think you're tarring everything with the same brush in suggesting that anything unusual or novel is always bad or a scam, which is a pretty negative approach.

Besides, there's nothing new about heating flowing water.  But they've definitely advanced over the years, they were once a box on the wall over the sink with a spout sticking out.  Like everything else, things progress.  Gas combi boilers were new once, they soon established themselves once they got good enough, again from very humble beginnings.

My own personal opinion is that they've now progressed to the stage where they're (only just) worth considering against a hot water cylinder.  But I've already given a whole heap of reasons why this may not suit everyone, I'm just throwing an idea in the pot to consider with a stack of disclaimers but seem to have wound up a few people.

If the boat never got rocked then nothing would ever progress.

Alnat1

There's a water pipe connected to your heater. When your heater is not in use, the water is "stale" and "standing " (in your words) in this pipe. As it is inside your house this water will be slightly warm, so bacteria can grow in it. It will be standing there all night until you use your heater (to wash your face?) in the morning. You'll use your heater to warm it up a bit but not enough to kill the bacteria. 

Swipe

Alnat1 said:

There's a water pipe connected to your heater. When your heater is not in use, the water is "stale" and "standing " (in your words) in this pipe. As it is inside your house this water will be slightly warm, so bacteria can grow in it. It will be standing there all night until you use your heater (to wash your face?) in the morning. You'll use your heater to warm it up a bit but not enough to kill the bacteria. 

I really don't understand the obsession with legionella bacteria on this forum, it is absolutely laughable. How many people do you actually know who have had legionaries disease, let alone caught it from water standing in a pipe for 24 hours? 

wittynamegoeshere

Swipe said:

I really don't understand the obsession with legionella bacteria on this forum, it is absolutely laughable. How many people do you actually know who have had legionaries disease, let alone caught it from water standing in a pipe for 24 hours? 

Hopefully the fact that it hardly exists these days results from the fact that we take so many precautions against it.

The warm water remaining in the pipe is pretty much the same situation as the pipe from a hot water cylinder.  Either way, it remains there, in theory festering.  But so does the cold water, which may reach room temperature or above.

I'm pretty sure there's a big difference between standing and flowing water.  Electric showers heat to only a warm temperature then spray it into the air without any precautions, so presumably this is OK.

I believe the risk of contamination is a contributory factor for a vented tank, there's definitely food in there for the bacteria to eat in many cases, definitely in ours.  Also the small amount of chlorine that's added to the water supply to keep it fresh has the opportunity to evaporate off as gas into the air.

Also, with an electric shower or one of these heaters, the bit of water that remains inside can get a fair bit hotter as it gets left sitting next to the hot element, so this should sort of pasteurise everything. Some showers for infirm people have specific precautions against this hot spurt being sprayed out if it gets used again shortly after.  This is also an energy loss, but it's a fairly trivial amount.