Thinking about some solar panels. Is it a crazy idea to use the existing 300l cylinder as a thermal store.

So use the solar to heat the tank using immersion heater then turn the temp on the boiler down so that it doesn’t need to fire up but water keeps circulating.

Immersion or boiler then tops up tank temperature. As I write this I don’t think it will work but interested in thoughts!

I expect Better to get the panels to power an ASHP I expect but that would be more cost/invasive work.

I can see problems with the lack of inputs, below is an image of how mine works which may help you work through the options. These are £1500 for a 300l tank

Jp – thanks. I won’t have hot water solar panels so I’m not sure I need more inputs?

My thinking was rather than using the gas boiler to heat the radiators, the heat is taken from the hw tank.

In the winter I suppose the problem is whether the immersion can heat the tank fast enough to give hot water that day and then top it up for the evening when the heating comes on

There will be people with much better knowledge than me but what this doesn’t seem to show is that we still have an immersion heater so we have three inputs multi fuel stove back boiler, solar and immersion and we could add gas or oil in addition. It sounds like you will either have solar or immersion but not both which is where I was thinking you had a lack of inputs. This runs our central heating also so it is doing far more than you need.

Is your boiler capable of being set up vented? If so what you propose is possible although I am not sure ho.e much you would gain.

Edit just re read, you want to run Ch off the t.s? Rather than hot water via some valve Making it more like a system boiler with PV powered immersion?

Running the Ch of of it will not be worth it But the hot water might depending on your use tank size etc but I doubt it

Would solar provide enough power to run a high current immersion? You’d have to store the days energy in batteries and then have a shorter period when the inverter turns on to power the immersion wouldn’t you?

Or can you retro fit a low voltage immersion that will run longer at lower temperature?

Battery storage is the only real option then. So an additional cost to be able to fully utilise everything generated. At which point cost benefits become trickier.

Interesting point about the high current immersion. Since that’s the heating device in a dishwasher/washing machine. So if Panels don’t do that then there isn’t going to be much they do run to make them worth while (unless with a battery).

I have solar thermal that I understand they are ~80% efficient.
I believe solar pv is ~20% efficient.

My 250l tank is heated between 40-70°c for 9mths of the year.

I have no experience of pv yet, but I wouldn’t expect them to add much energy to a large amount of water.

I have no experience of pv yet, but I wouldn’t expect them to add much energy to a large amount of water

My old man has 8 odd kw – half on a sun tracker and heats the bnbs pool in the summer with an ashp and the two X 300l thermodynamic tanks in the basement from his.

He’s currently looking at 2 8kw batteries to do the heating from the main ashp to the rooms in the morning as he has so much excess solar floating about and can’t export (for money) it due to rules.

His biggest issue is he has the power when the guests are out for the day….. They call come home and want to use the kettle when the sun’s gone…

I have 4.2kw on my roof and also looking at batteries. I have mine feeding heaters off solics when there’s excess but need to make better use- reluctant to scrap my perfectly serviceable oil boiler though as that’s wasteful and the payback on capex is crazy to just be throwing a working boiler away

So it can be done. Why you’d want to connect your hot water tank up to your Ch though is beyond me. Showering in manky rusty water isn’t really much fun

@trail_rat that’s exactly my issue got a quote for a large system but not going to be able to use it when generated in reality.

The hw tank is Already connected to the central heating. S plan system, so if hot water and heating on at the same time the “central heating” water passes through the coil in the hot water tank and then the rads. Usually the heat transfer is from the circulating water to the tank but it could go the other way if the circulating water is colder.

I have solar and a controller that switches the immersion heater on/off based on the balance of electricity export/import.

Obviously the immersion heater has a cut-out so switches off when the water is hot enough.

I think it definitely heats up the water during the day.

The trouble is that the hot water cools down overnight. The ideal would be too hold enough water water overnight to cut down on hot water needs in the morning.

I have no experience of pv yet, but I wouldn’t expect them to add much energy to a large amount of water

Clearly no experience there then!
I have 3Kw in the roof here at our 100% electrically powered place by the beach.
The PV heats the 210L megaflo to 60c and then further generation is sent to a dedicated space heater.
Water temperature heat loss from a starting temperature of 60c is about 6c over a 16 hour period.
I can turn up any day of the year and have a full tank of hot water.
It’s probably the best way we can use our generation.

If you’re interested this is what’s happening with the hot water right now.
Simply being heated by the PV.
https://go.init.st/7o3th5w

(You can scroll along the temperature graph to see data at any point in the day.)

Solar with optional overnight top up from octopus night tariff might be the way to go.
You need 55c to knock legonella on the head so a wsy of getting that mid winter is needed.
Tanks of warm water can promote growth

You could fit an electric boiler to the central heating system. For example:

https://www.screwfix.com/p/heatrae-sadia-amptec-c600-electric-heat-only-flow-boiler/329FR?kpid=329FR&ds_kid=92700055281954502&ds_rl=1249404&gclid=Cj0KCQjwvLOTBhCJARIsACVldV1VzxhVEIJHLcztEwmi3bWRZjUWTSwx2tUBiIGV_hMbbO9kHTrMBKcaAiGoEALw_wcB&gclsrc=aw.ds

Cheaper/easier than an ashp but you only get 100% efficiency from electric to heat, unlike 2-300% of an ashp.

An entirely separate consideration: do you have a top immersion element only? These struggle to heat the whole tank, they generally heat only the top half. I don’t know if you can add a bottom element. Your stored hot water capacity is effectively much less than the 300l you expect.

The electric boiler would have the advantage of being able to heat the whole tank, via the coil, just like a conventional boiler.

Can’t comment on the cost effectiveness

But these devices exist to divert in used solar power to the immersion element rather than to grid

MyEnergi Eddi & iBoost solar diverters

But these devices exist to divert in used solar power to the immersion element anything you can plumb in safely. rather than to grid

Me and my old man plus I believe sharkbait have heaters hooked up to the outputs to get rid of excess rather than export. For me that means not burning as much oil far more lucrative for me than exporting at 4pence.

I have been tempted to try a storage heater solution but they are mostly ugly and need fixing to the wall compared to my oil rad.

Guy on ebay makes a copper wound immersion.
Theory being lower td need greater surface area to work. So you plumb a wet panel into this copper wound immersion and it gently warms your 300ltr through the day.

A typical gas central heating boiler is between 7 and 30 kW of heating power (depending on size)

Lets take a mid sized boiler (15kW) and say it runs for 3 hours in an evening, that is a total energy output of 45 kWh

Water has a specific heat capacity of 4.2kJ/Kg.k ie to raise 1 kg of water by 1k (1 degC) it takes 4,200 jouiles of energy. Assuming you managed to utilise all 300 litres (300kg) of water, each degree of heating is an energy storage of just 0.00035 kWh.

With a realistic deltaT. say starting at 65 degC and falling to 45 degC, you’d need 2,000 tonnes of water to replace your boiler!!

The best option, is to use a solar boosted water thermal store as a heat pump argumentator, keeping the heat pump at a lower deltaT and so leveraging a signifiantly better CoP. You’d need around 3 tonnes (3,000 litres) of water to do this for a typical property (which is 3 IBC’s buried in the ground in the garden)

Thats assuming the boiler runs constantly though. Which they dont. It will be less but by how much is an unknown. Outside air temp, thermal efficiency amd desired indoor temp wil factor in. I reckon over 3 hrs a boiler would pnly fire for half the time.
Constantly for 15mins to get heat into rads and pipes, then on and off as the air warms, but then you cany buy a 15kw combi, 24kw dhw is about the lowest
So your probably not far off with the maths.

One thing many people don’t take into consideration with ASHP is the need to upsize all your radiators or remove and replace with UFCH due to the lower heat of the water in the system. Also many older houses suffer from too much heat loss for ASHP to work effectively. The cost of installing an ASHP and altering your heating system to work with it would probably take longer to repay than the life of the system.
Converting an existing cylinder into a thermal store probably wont work very well as the coil in a TS has a much greater surface area than that in a normal indirect cylinder, allowing for a greater transfer of heat.
Personally I’d say the best use of PV solar is to use a multi channel optimiser to run an immersion heater and space heater/s. Battery storage is also very expensive and hard to justify the outlay versus return of investment, even with current power costs.