Water Tanks and heating them…

We have an oil combi boiler, it’s 30 years old, but works fine. We also have solar and in the summer especially, will have far more generation than we have usage. Is there such as thing as an unvented combination direct and indirect hot water cylinder? So that we can use the oil boiler when we must (winter), but rely on the solar (and off peak use) when we can?

Or do you have to go all in on a single solution? I’m massively confused by all this.

Ideally, I’d like the tank as primary hot water heat source, heated initially by solar, then off peak charge, but if not hot enough, the boiler will kick in to boost it. Is this possible?

Stick an immersion heating element in the top of the hot water tank and use a solar power diverter to dump excess solar energy into it:

eddi

So you’d add this to an indirect cylinder? The water would be then be heated by electric, then when the boiler comes on, it would fill the combi tank from the cylinder and turn off quickly IF there’s no need to heat it? Is that doable?

either fit a manual valve – or look at grant winter SOL for a remote option.

We are in the throws of doing the same – your solution is easier than what im going for.

We will be fiting a direct feed solar thermal tank with twin immersions.

Solar thermal into the bottom (25%) of the tank – switched immersion into the top and the Solar PV immersion into the middle (simply because we already have the controller)

The combi will run through a winter Sol as a branch of the heating circuit and go through the usual coil into the top 75% of the tank.

all the hot water will run off the tank except the sink in the utility room – partly down to access and partly down to still giving circulation to the water in the system and stop diverters etc in the boiler going to shit.

it would fill the combi tank from the cylinder and turn off quickly IF there’s no need to heat it? Is that doable?

who makes your boiler – many oil boilers are limited to input temps 24deg due to how the diverter works.

if yours is one of them You can do it – but youll need a combi sol pre-diverter valve to detect >42 and send it to the taps or if <42 – reduce it to 24 to pre heat the boiler feed. – thats why i chose to do it the way i have,.

So you’d add this to an indirect cylinder? The water would be then be heated by electric, then when the boiler comes on, it would fill the combi tank from the cylinder and turn off quickly IF there’s no need to heat it? Is that doable?

yes, you would need a HW tank – which you won’t have with a combi.

With our HW tank, the boiler doesn’t fire if the HW tank is up to temperature, so it only calls on the boiler when it needs heating – set by a timer and a tank thermostat in series.

How’re you getting 3 different heating loops into a single tank? Which tank are you aiming for?

I don’t understand this. Are you in the UK?

In winter, I assume solar contribution is pretty low?

In summer, we only heat water with our oil boiler. It uses next to nothing.

How many years payback is there on all that fannying around to add solar for water?

Genuine question, not trying to be a smarty pants. 🙃

We use half our 1500l in the winter for heating and water and the rest for HW in the spring/summer/autumn. 750l at 60ppl is £450 a year. The cylinder, switch and solar controller will be £3k (max). Installation maybe another £1k. ROI 8-8.5y years. But, if we can also use off peak electricity (£2 a day) for say another 80% of heating in the winter, then that moves to 7years, 100% = 6 years. And the house is ready for an ASHP as the tank is already there, so when the boiler dies, the heat pump can be easily installed. It would also save us £170 a year in boiler servicing, but I’m assuming there will be other service work than needs to be done.

gledhill , heatrae , tempest and kingspan all make them

then you have the heat bank options if you want an indirect heat exchanger model – DPS has piqued my interest but the flow rates were a bit “combi” for the differentials we have.

We seem to use quite a bit of hot water having young kids – seemed that we used a considerable amount of out with heating off. – Daffys numbers up there match mine pretty much for the consumption – over my life time that adds up – coupled with the fact that we will be fored onto heatpumps at some point which wont do the instant heat of the combi its a no brainer.

In winter, I assume solar contribution is pretty low?

Depends what you class as winter – the only month we seem to have no excess is December where its REALLY low. the rest of the time it certainly works well and pretty much eliminates all but the standing charge on our bills.

Sounds like the sort of thing a Mixergy tank would do nicely. https://www.mixergy.co.uk/products/mixergy-tank/

Does all the gubbins last that long? I thought combi boilers were dying early by the boatload and is the life of the solar stuff ~8 years?

Our basic oily boiler is ~20 years old now and has been really reliable. I’ve never measured oil consumption accurate but I’d guess it is 3/4 ‘winter’ (Nov to end Mar) and the rest just heating water. So say 750L winter and 250L the rest – just a guess.

my combis 12 years old currently and not showing any signs of issue yet.

the solar stuff -where do you get 8 years from ? has minimal moving parts – considerably less so than any boiler.

only the inverter would have anywhere close to that as a life span and even then it seems you have to be unlucky for that – my old mans inverter came on the ark and is still powering on ….. its 15 years old. it had some mosfets replaced a few years back by a man that knew what he was doing and cost a fraction of replacement of inverter.

PV panels do degrade over time and I think it’s is worse the more time they spend hot, but I think 8;years sounds very low

where do you get 8 years from

That’s Duffy’s payback calc. Ideally you’d want your kit to last a bit longer than payback…😁

Yeah, that’s the return on investment, not the life of the system. After that, you start making money. In contrast a new oil boiler (£6k) and tank (which we need) is ~£8.5k and we have to spend £1000 a year to keep it supplied and going. Over 8 years, the total cost is £16.5k.

For electric water heating backed by solar and the oil boiler that figure would be closer to £7K including oil and electric. But that assumes no requirement to replace the boiler, or install an ASHP. It also assumes that you already have the solar and that you’ll have excess gain. Should I have to install an ASHP, then the cost might be £6k, but the government grant might reduce that.

PV panels do degrade over time and I think it’s is worse the more time they spend hot, but I think 8;years sounds very low

Whilst true, the panels are by far the cheapest part of the solar install. I think replacing our 14 panels would be less than £2k and they’d pay for themselves in a 18-24 months at current prices. Batteries are the big unknown.

I’m not sure Daffy’s calcs are quite right. It looks like he’s assuming 50% of the year will be 100% supplied via solar? I think he’s also understating the cost of oil at 65p/l which should (maybe) offset the over estimate of solar effectiveness?

Does anyone here have any direct experience of solar effectiveness in the UK for heating water? If the investment is as little as £4k and you can 100% rely on it for 6 months of the year, it’s a no brainer Shirley?

Batteries are the big unknown.

Well, I think batteries are the biggest problem by far imo.

Does anyone here have any direct experience of solar effectiveness in the UK for heating water?

Our place by the beach is 100% electric and I’ve found that from early April – September the 3kWp PV array does all the hot water (200L Megaflo) for 4 or 5 of us. That’s showers though….. no baths generally.
Maybe a couple of times we need to put the boost on (i.e. import power) for an hour but that’s usually only when we’ve got friends staying.

So yeah, 6 months free hot water is feasible.

PV panels do degrade over time

It’s not noticeable on our 12yo panels.

Perhaps – but then I’m really not looking for an economic solution, I’m looking for a better, perhaps more futureproof solution than just simply (and literally) burning. I just threw some figures together based on your request for an economic incentive.

It looks like he’s assuming 50% of the year will be 100% supplied via solar?

It’s a reasonable assumption.
To heat 225L of water from 24c to 64c requires 10Kw of electricity – most PV installations will produce that every day on average for 6 months of the year.
There will be some days they don’t, but at the same time your probably not going to be heating that volume of water from 24c every day either (given that >30c is maybe too cool for a shower!)

Well, I think batteries are the biggest problem by far imo.

@TheBrick – care to explain how batteries are the problem?

Stick an immersion heating element in the top of the hot water tank and use a solar power diverter to dump excess solar energy into it:

Lowest available position surely…. why the top?

Solar thermal into the bottom (25%) of the tank – switched immersion into the top and the Solar PV immersion into the middle (simply because we already have the controller)

Why would you have a switched immersion and another immersion for the PV? You only need the PV immersion and manually* ‘boost’ it with imported power if the water is too cold for some reason.

* The Eddi looks very, very much like the Immersun v2 – which you can boost remotely by applying any voltage to an internal relay.
Being a nerd I have a Pi recording the water temperature and if it’s below a certain level at a certain time of day it switches on a remote socket that supplies 5v to the Immersun relay – so switching on the boost function automatically.

The answers in the brackets. As in I physically already own the controller and the tank has twin immersions…..

I can always turn it off but in the shoulder seasons it’ll do me a good hand.

Well that all sounds very interesting and promising, thank you.

So for a realistic £4k I can heat all my water (2 adults no swimming pool…) for 6 months. Are there any other costs/elements I should be considering or is that it? I’m not going to be faced with the ‘ahhh yeah but you’ll also need…’ and ‘oh and don’t forget xxxx, we didn’t include that as it’s obvious’ etc…

We’ve a standard oil boiler water and house heating system with a hot water tank. There’s a timed electric emersion heater in the cylinder that has been used exactly zero times since I had it installed ~20 years ago…

Ta very muchly.

So for a realistic £4k I can heat all my water (2 adults no swimming pool…) for 6 months. Are there any other costs/elements I should be considering or is that it? I’m not going to be faced with the ‘ahhh yeah but you’ll also need…’ and ‘oh and don’t forget xxxx, we didn’t include that as it’s obvious’ etc…

Such as – do you have pre existing solar ?

Such as – do you have pre existing solar ?

I’ve got wgat I said up there^ No panels, batteries, do dahs, fenagles, etc. Just an oily boiler, a cystern and a few rads. Oh, and about 1000L of myrrh (or ‘oil’ as it was formerly known)…

If you already have PV then you should just need a diverter (such as the Eddi up there, others are available but I’d look at the specs and capabilities).

You can use your existing tank but, generally, the lower the immersion the more hot water you will get.

I bought a used megaflo for our other place and it’s been perfect, unfortunately even used ones seem to be quite a lot more expensive now.

care to explain how batteries are the problem?

It’s a general statement, not specific to this application but the degree to which batteries degrade is pretty horrible. Use on power wall type application can be managed pretty well compared to use is a car for example but its still quite an issue.

I expect everything to degrade (see my solar panel comment above, I am not saying that in 8 years that the degradation will but an issue just that they do degrade).

Batteries on the other hand look like after 10 year the degradation would be significant. (I admit that I may be out of touch as this was ~ 5 years ago I delved into battery chemistry and all that jazz but I don’t think there has been any significant change over this period).

Many might consider 10 years to be ok to have significant degradation but for such a major purchase I don’t think it is. I am pretty anti the entire throw away culture though where as other are happy to buy / replace things more often so this is definitely an “opinion” as to what an exceptable life span is.

I do however consider the use of things like pumping excess electric into hot water to be a really good useful energy store as heating and hot water are the major energy use in most domestic environments.

I bought a used megaflo for our other place and it’s been perfect, unfortunately even used ones seem to be quite a lot more expensive now.

Yeah I bought a < 6 month old one including prv, expansion but no regulator for ~ £150 3 years ago. Now more like 250 without the vessel.

Soooo, I do have to buy more ‘stuff’? There’s mention of batteries and panels. I know nothing, zero, nada, zilch, naught (etc) about this subject hence the questions. I suspect it’s really not £4k and you lot were having me on…

Ignore my battery talk I am diverting the conversation

If you don’t have any PV or solar thermal then yes you’re going to need that! Solar thermal is cheaper but “only” produces hot water whereas PV can power lots of things but is more spendy up front.

You’d need to look into that cost. But once you’ve got PV you just need a diverter – this sends any spare generation to your immersion.
Just be aware that they only send spare power to the immersion, any household power requirement will be satisfied first – so if you’ve got the cooker and a couple of heaters on then you may not be heating the water even when it’s sunny!

OK. So pv (electrickery) needed dedicated to the cylinder? How many KW’s are needed and what m2 is this likely to be? Presumably roof mounted and if so, estimated ££££’s pls.

Ta.

I suggest you do some googling.

Solar thermal is cheaper but “only” produces hot water whereas PV can power lots of things but is more spendy up front.

for 200l it appears to be about 2400 materials only for the roof and pump system – youll likely need another tank for solar input so another 800 quid + fitting probably about the same 4.5K all in as a 4kw solar install cost u ( no battery)

Oh right! Solar thermal possibly isn’t such good value in that case.

Oookayyyy. So we’re at £9k and 16 years payback now. Is that right? £4.5k + £4.5k? Or have I got that wrong? I need the panels, the tank and the doobry that was up there that Daffy started with. Is that all or are there any more ‘oh, you’ll also need an xxxyyyaaabbb obv etc’. Ta.

I can Google ’till my wrists ache but I’d much rather talk directly to people that have done this rather some random sellers pretending to be satisfied consumers.

any household power requirement will be satisfied first

You can prioritise further. We have an iBoost which takes any spare solar that isnt being used elsewhere but the battery takes priority after that. So PV = use in real time > battery > iBoost > Grid. We dont have an EV yet but that can also be put in the mix using intelligent chargers.

So we’re at £9k and 16 years payback now. Is that right? £4.5k + £4.5k? Or have I got that wrong?

Yes you have. You ‘just’ need Solar PV and a diverter for the simplest set up. Battery is up to you but it does add a layer of complexity and expense.

I said to google because there are variable such as PV array size and location. Prices are slightly fluid and can vary around the country.
Lots of people have asked on the Solar thread – maybe that will help.