Heating and hot water with new solar retrofit crossroads conundrum

We are very fortunate to have just had a 10 panel solar array through the Manchester LADS Grant Scheme.

Not able to afford a battery, we also doubt if it’s likely to be cost effective for us anyway - re generation and usage versus battery life.

Our energy generation forecast is between 3200 kwhs and 3800 kwhs per year.

The crossroads we now find ourselves at is 'Where to next with heating and hot water? ’

Currently we have an A star rated combi boiler for heat and hot water.

We were refused a airsource heat pump because we don’t have wall insulation.We know from the design process that a heat pump would have attached nicely to our radiators, but of course we’d need a hot water tank installing.In the future we might be able to do some IWI and be more heat pump ready.

Now having the solar we want to harness as much of our generation as possible, knowing that sending it back to the grid won’t be cost effective and we’ll save more by using the energy ourselves.

Electric storage heaters are seeming like the answer and we’re thinking of installing three for now - lounge, kitchen and daughters bedroom where she studies too. The idea being to store the heat from the solar in the day and release this from about 4pm into the evening.
We’re not home so much during the day and we also have a multi fuel stove for extremely cold days. However we really want to avoid using this as well as any gas wherever possible, being very mindful of our carbon footprint.
Heating the person via jumpers and blankets will be our go to, as well as combatting heat loss from rooms…

Our main conundrum right now is about hot water.
An Eddi Diverter has been mentioned @Tim_Gilbert :+1:t2: to divert the excess from the solar to heat the hot water in our tank - BUT we don’t have a tank!!

So these are our current questions and potentially the questions typical of many of us making the switch to electric heat and hot water.

1.Hot water tanks are a part of heat pumps too, so if we’re going to install a hot water tank, how do we ensure that down the line it will be compatible to a heat pump also?

  1. What kind of electric heating is best coupled with the solar for energy efficiency and to offer us heat flexibility as well as being a good interim before a heat pump?

  2. How much of the gas combi system should we remove and when?
    (Our daughters room doesn’t have much wall space, so we will likely have to remove her current radiator to fit an electric one)…

  3. Re hot water for washing - our shower is run off the gas combi - Do we change this to an electric shower or go with a hot water tank to be heated through the solar diverter.

  4. Will a solar diverter be cost effective? Again wondering about the life span of these things…

  5. Where are we best to learn about monitoring electricity consumption, generation and usage?

Overall, we don’t want to burn our bridges and waste money in removing any radiators or piping that might help us down the road and we certainly don’t want to install to have to remove either.
We also know that being able to do the maths around energy consumption, our usage and generation is a skill which will help us enormously.
In addition to all this, we’re really wanting to disconnect from gas and pave the way to energy sustainability and self reliance as much as possible.

Any feedback here is very much appreciated! :pray:t3::pray:t3:

Hot Water:

It is always wise to have multiple options for heating and DHW. Certainly leave as much in place as you can of your existing heating.

Taking up less space (and costing a lot more) than a hot water cylinder is a phase change storage device such as Sunamp. As you are planning/dreaming of a heat pump I can’t recommend one as they operate at very much higher temperatures than a heat pump can achieve, even in the tropics. They are ideal for flats or smaller homes with no option but to heat by direct electricity or on a very low night tariff.

When choosing a cylinder with heat pump in mind you need one with a “high gain coil” these suck more heat from the heating system’s water to put into the DHW. The return temperature is therefore less when the heating circuit water reaches the boiler/heat pump/solar thermal and therefore able to pick up a larger load of new heat. If used with a condensing boiler they will help the boiler keep in the more efficient condensing range for a higher proportion of the cycle.

Rather than a traditional cylinder you could chose a thermal store. They work more or less the same as a cylinder but act as a heat accumulator for several heat sources and can also be used towards heating a “wet” system with radiators or underfloor heating. They don’t take up more space than a large cylinder but need more changes to your plumbing. I would suggest that unless you have a re-plumb planned it isn’t worth while in your situation.

Whichever solution you choose, the storage device needs to be as close as possible to the points of use. Unfortunately “under the floorboards” isn’t a possible solution to this problem. Under the kitchen units might be possible for a Sunamp, depending on the size you need. Hopefully all the hot water plumbing in your house is close together, such as bathroom above kitchen.

Whether by immersion or Sunamp a solar diverter will be very useful. I don’t know the capabilities of other makes but a basic Eddi without the optional add ons could control the PV export by intercepting it and heating two circuits, in series. I would normally suggest hot water first and then electric heating. My own only controls an immersion at the moment but I am considering a socket in the hallway that is only live when the water is hot and there is spare PV. In winter I could plug in an electric heater and at other times non critical loads. My Eddi is under the stairs with the CU and spliced into the immersion spur. An issue with this is that in winter with low sun, clouds and short days there often isn’t adequate solar export to actually get much benefit. The Eddi does allow you to schedule boots if you need the immersion to kick in anyway. I am lucky to have solar thermal too, so my DHW gets heated to about 20-30°C from the little sun there is and I only need to top up to 50°C 6 days a week, on the 7th day I run a disinfecting cycle. In summer I don’t heat water very often. Think about it and you will see that 50°C is adequate for all normal household water uses, and safer regarding scald risk, particularly for children and the infirm. This assumes you can store enough hot water. If you can only fit in an undersized cylinder you may need to over heat it to then add cold to eek out the supply. The cost of increasing the water temperature over 50°C over the years may be significant, so try to factor that into your plans.

I haven’t written about domestic heating except a brief reference. I’ll write another post for that so they are more easily referenced individually.



Oops, I didn’t mention the showers.

If you have a bath either rip it out or fit a cover to use it as storage! There is no place for baths in an energy efficient home.

Edit: thinking about it, where the bath was is probably an ideal location for your new cylinder!

There are electric showers that take pre-warmed water and bring it to your desired temperature. Those are probably what you need unless you are going to boost the cylinder to shower temperature each time you shower. If you set the immersion thermostat it won’t come on if a boost is not needed.
Of course the electric shower will use any electricity it can, unlike the Eddi in normal use. This will impact your bills.

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Lots of good suggestions here.
This is a complex subject.
I will try to put together a few thoughts over the next week or so.

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@pottyone72, please give more details about your solar, so that I and others, can understand the situation better.

What I would like to know is the kWp of the panels, max kW of the inverter, orientation and angle of incline. Also whether any shading is possible at certain times or seasons. Did your supplier provide a likely annual output figure?

By the way, I hope you have bird netting around your panels to prevent lodgers moving in.

Thanks for posting. Finding it really helpful to see these questions published here. It really does highlight how complex retrofit is for so many people, and how increased energy costs are now making people rethink the cost benefit of different measures alongside reduction in gas use.

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Have you heard of CH radiators with an immersion heater in them? When the CH is off the radiator can still be heated. As it is fundamentally dumb it is a suitable candidate for solar diversion.

I don’t know whether replacement lock head valves exist with attached immersion.

If you decide not to go for a storage heater in that room.

Sorry for the delay answering here - technical information takes me a bit longer :thinking: :sweat_smile:

Here are the details
Tilt of panels - 30 degrees
The front of our property where the panels are situated is - 75 degrees South East , with no shading.
The KWp is 2kw - at least that how it was explained for ‘most days’ !!
Each panel is 380kwh per year and we have 10 panels. the range is between 3200 and 3800 kwh per year.

Can I ask - what is Kwp and how is it different from kwh?
Also, I do have bird netting around the panels - I would never have known about this were it not for yourself telling me to ask for it. :smiley: :+1:t3: :pray:t3:

I have also uploaded what I received when I enquired about a solar battery - I have a hunch that it’s a general specification.
I’m so aware of the need to understand the maths ourselves self to make better choices to best use what we produce, rather than exporting for a pittance and buying back at a 5 times more the price - that maths is very clear!!

BG - battery only proposal.pdf (943.9 KB)

EnSmartBuild Brochure - Batteries.pdf (370.2 KB)

Looking for C H Radiators with immersion , I found these . Is this the type of thing you’re talking about Tim ?

Immersion Heaters - UKBathrooms

Bathroom Radiators Immersion Heaters Easily applied to a range of radiators, these dual fuel immersion heaters offer an inexpensive and efficient method of activating a single radiator, bypassing the central heating for a wiser use of energy and lower running costs. Keep your bathroom and towels warm and dry without having to heat the whole house.

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Thank you. I’ll do some thinking and get back to you. I’m currently occupied with a family holiday.

The kWp is the kiloWatt peak, ie the maximum possible output for that panel. No panel in the UK is likely to reach its kWp outside a laboratory, however the figure is useful for comparing possible outputs of different panels. If you can arrange the panels so that the sun is at 90° at noon on midsummer’s day, that panel will reach its “location best” over the year. If it is sunny at mid summer you should see extraordinary output (so long as the panel doesn’t get hot!). However, what you really want is usable output in winter, so you can pretty well ignore the above! Concentrating on winter only you want to get 90° irradiance at noon on midwinters day. Real life is a compromise (unless you move your panels).

The connections shown in the summary are only for towel rails, which use a different size terminator than a radiator, but yes, that is the sort of thing I had in mind.

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Hi Carla
You have no battery, but are you next time getting a electric car? If you get a car with VTG when you come home and plug the car in the lights come on in the house.
If you can get around using the grid in peak times (5 to 10) a small battery may power the lights (and possable TV).
Good luck you are a bit further down the road than me (i think i am ahead on insulation )

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Thanks John!
The retrofit journey can indeed make unexpected twists and turns. We were looking at windows and insulation until the grant scheme.
We will insulate the walls eventually - but definitely room by room - internally.
Another project we’re completing just now is the laying of an Intello vapour control layer on the loft floor under the insulation.
Our roof and loft have thrown us many problems!! The failing fabric of the house has very much dictated our retrofit.

An electric car seems like a very long way away for us - when my husband was applying for work after Covid , we purposely looked for a work place close to home so he could walk or cycle - because of the expense of cars and to reduce our dependency.

It’s amazing though, the possibilities with the solar.
My mind is boggling!!
Just by watching the smart meter , we’re already seeing how certain appliances easily run off the solar, but others like the cooker need more.
We’re changing our behavior too - charging mobiles in the day, and learning how to stagger the usage of appliances to gain from the solar.

So much to learn !!

Thank you Tim !!
I really appreciate the feedback and support. I hope this thread is going to help others too.
Think I need to add solar into the topic name.

Hope you have a lovely holiday
Thanks again and all the best
Carla :pray:t3:

This was going to be one of my questions. In terms of the type of electricity tariff you are on it will probably make sense to combine a hot water cylinder or Sunamp (or thermal store) with an off-peak electricity tariff like Octopus Go. Thus during the winter months you could set a timer (on an Eddi control unit or similar) to store hot water from off-peak electricity. This is now often cheaper than gas, and also generally greener. This combined with PV diversion into the same HW store would give you year round low carbon hot water.
In terms of capacity for the hot water I would estimate that you will need the equivalent of 150-180L of hot water storage (at nominally 55C), this on the basis that there are 2 of you in the household and that you don’t regularly run long baths but instead take shorter showers.
All of the other advice above is very appropriate so I won’t add to that here, until I have had another think or if you @pottyone72 have any other specific questions.


I have just moved to Octopus Go Faster a little before the winter season to avoid a pre-heating season price hike. Unfortunately for @pottyone72 the cheapest night time tariffs require an EV.

@pottyone72 , are you on economy 7?

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You don’t have to have an EV, you just have to say that you have one (even if you don’t).

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:star_struck:…love this bit of information :grin:…who needs an EV :joy:

Hi Everyone :smiley:
I have been doing my homework - going through the thread and all your great suggestions.

I can concur, after reading to gain understanding, that a thermal store for the wet system and a diverter sounds like it could be a good choice for us, if the solar during winter can keep the system warm enough and is energy efficient enough to limit large bills.
This is a make or break decision for us, because of our low income.
So crunching the numbers to look at power coming in from the solar and power needed for a new system has to be our first step.

Additionally, we’re having to take some action in the bathroom where the wet room is leaking.
Currently the bathroom has the boiler, bath, toilet , sink and wet room shower - all in one space (2.35m x 3.04m).
The wet room floor needs removing to put in a shower tray where the shower is now. We’re also thinking to move the sink over to the other corner where the bath will be removed - this will keep plenty of space free by the boiler for any big system changes that might include a water tank down the road!!
We’ve decided against an electric shower too.
We’re very aware that we need to plan carefully re a bathroom design and I know we’ll benefit from looking at this.

Re the thermal store - We learned a coil type is preferred as there is a lot less that can go wrong with it versus the type which has an external plate heat exchanger.
So far we found this manufacturer - other recommendations would be very much welcomed too.
(Probably benefit from creating a new thread for thermal stores).

We thought this model by the same manufacturer seems to fit with how we could use it for the wet heating system and DHW.

Our next big question is
'How can we monitor the solar coming into our system to see peak flow times to compare this with how much power we’ll likely need to heat a thermal store for our wet system with a diverter .
How do you do your monitoring Dom. @zapaman

I’m back from holiday and I see that you, @pottyone72, are thinking along the right lines regarding your forced wet room retrofit.

If you are planning a full room retrofit (washing in a bucket in the garden while work it in progress) this would be a good time to insulate external walls too, then refit plumbing inside the new wall line. Otherwise I suggest keeping all pipe runs 150 mm inside the current wall, to simplify insulation later.

Committing to a thermal store is a big step but has several advantages. In uncertain times perhaps the biggest advantage is the multi fuel nature of the new system.

Your PV system looks to me as though it will not add significantly to your energy supply in winter, mainly due to the low angle of the panels. However an immersion heater in your thermal store with solar diverter and thermostat set higher than your boiler would still get you some benefit and on sunny winter days could give you a worthwhile boost. In fact the thermostat should be set to the legionella disinfection range of >60°C. That way you can keep your boiler running at 50-55°C or less to maintain flue gas condensation.

Edit: If you maintain a thermal store and instantaneous mains pressure hot water coil you do not need to disinfect against legionella. You can still keep the immersion thermostat high to take advantage of “free” electricity but then you wouldn’t want the water that hot from a “boost”, as it will overheat your radiators, which should be adjusted for a low temperature system.

Without a battery capable of islanding your PV will not operate in a power cut, which is a reason to keep some gas capability for now. However modern gas heating systems need electricity to operate so that plan may not help as much as we might like. Maybe a cheapish UPS could be linked into the heating circuit to stop you freezing in a power cut. You would need enough capacity for the logic board, ignition, diverter valves and pumps. All those together generally only use a few amps.