Can pre-warming water from a heat battery reduce the need for a higher kw boiler?

[Pete lea]

We have a 4 bed house that's reasonably well insulated.  Our family uses quite a lot of electricity already. I work remotely from home and my wife is home all day and both my teenage daughters need to be illuminated by at least 2 devices at all times whenever they're here.

 

Having said that, we want to get rid of our old gas combi boiler and electrify everything.  We're getting Solar PV (6kw+) and 2 Tesla Powerwalls for around 28kwh of battery storage.

 

The idea is to charge up the batteries as much as required in the 4 off-peak hours and try not to draw any excess from the grid during the day.

 

I want to apply this same logic to my heating (Worst case scenario we were using ~70kwh of gas a day on average during january - does this sound about right?), so i'm looking at getting a Sunamp Thermino (or 2 in series? for more than I would realistically need) and charging them up at night.  This will supply my hot water directly, but also be connected to a Heatrae Sadia Amptec direct electric boiler; from my understanding this will pre-heat the water so the boiler doesn't need to put as much energy into it (if any) to get it up to temperature for the central heating.  I'd like to try and keep my households maximum output below the capacity of what the batteries can provide (10-14kw peak) so that I don't draw from the grid.  Perhaps a 6kw electric boiler? Does this sound totally illogical? 

 

The sunamps and amptec were chosen mainly due to the fact i'm limited on space so I can't really have a large cylinder and a boiler in the same cupboard.

 

My worry is that it will be undersized but I figure the heat batteries are doing most of the work.  Any insight would be greatly appreciated.

 

[Ferdinand]

Hmmm.

 

Interesting. That's a lorra lorra gubbins.

 

As a ballpark, what is your current EPC value?

 

I think you want a lifecycle cashflow model, working out eg the cost of replacing your Powerwalls after 10 (?) years and so on, to model the economics at various electricity prices.

 

That is in addition to the thermal model of your house you need to build to understand your demand. We have a striaghforward Jeremy Harris ss available here that seems to be quite good as an approximate model.

 

The central heating sounds unorthodox (others will comment), but I'd suggest that more solar pv might be useful.

[Pete lea]

  On 28/08/2022 at 21:28, Ferdinand said:

Hmmm.

 

Interesting. That's a lorra lorra gubbins.

 

I think you want a lifecycle cashflow model, working out eg the cost of replacing your Powerwalls after 10 (?) years and so on, to model the economics at various electricity prices.

 

The central heating sounds unorthodox (others will comment), but I'd suggest that more solar pv might be useful.

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Unfortunately that's the maximum size Solar PV array they can fit on my roof  They're looking at maybe sourcing higher watt panels to eek out a bit more though but it won't be much more.

 

Our current electricity on a non time-of-use tariff is looking to be astronomical (650+ then maybe 800+ a month...) so I think the batteries will pay themselves back within the 10 years if I can get the majority of my electricity at 7.5p/kwh as opposed to 30..40..50p/kwh+ (or whatever it will stop at).

[SteamyTea]

A quick look at the Thermino data shows it can take a maximum input temperature of 80°C and has the equivalent of a 284 litre water cylinder.

So if one assumes an incoming water temperature of 8°C in the winter, then:

 

4.2 [kJ.kg^-1.K^-1] x 284 [kg] x 72 [K] = 85,881.6 kJ

 

Convert to kWh

 

85,881.6 [kJ] x 0.00027778 = 23.86 kWh

 

Now if you are burning though 70 kWh.day^-1 of gas, you would need 3 of the TRP-FGW-AVZ-1

 

To electrically charge them in 4 hours, you need a 17.5 kW supply.

 

Even at 7.5p.kWh^-1, I don't think it would make much sense financially, let alone at 20p.kWh^-1, which is the more realistic current E7 rate.

 

I think you would be better off with a large, traditional, thermal store of some sort, if you have room for it.

 

There are also thermal and electrical loses to consider, so it may well be easier to up the size of the electric boiler and run that to the batteries are depleted (or close to it).  The loses on an empty (cold) battery are lower than a full one.

 

Just seems a very expensive way to coble together a heating system.

 

As @Ferdinand says, find out your heat loses and work on getting them down, and remember that power [kW], energy [kWh] and temperature [K] are not the same thing.

[Conor]

You could just get a heatpump that will do all your heating and hot water. Run off the batteries during the day if needed. Then no complex heat battery setup. And 3x more efficient than the direct electric heating. With a monblock you just need a cylinder inside. And of course a solar diverter will cover your joteater needs for a chunk of the year.

[Ferdinand]

* Further perhaps disconnected thoughts:

 

Adding it up and guestimating, you are looking at something like a £50k investment at a time of peak demand and high prices, plus other stuff to glue it together.

 

2 x PW2 installed = 15-20k.

2 x Thermino = 5k + VAT + install.

Electric boiler = 2k installed (?)

6 kWp Solar = 10k installed

 

I think this feels over-complex.

 

That's the sort of sum that people do quite substantial renovations for, so it is great that you are taking the time to think it through. There's a French horse jumping proverb - reculer pour mieux sauter - which applies, "pause in order to jump better".

 

* Suggestions:

 

- I would suggest looking at recasting the problem - rather than treating it as a fixed demand that must be met. Can demand be reduced, so making the problem to be addressed smaller? See below. One of the basics with going all-electric is that the house *,must* be fundamentally efficient.

 

- You also need to check timing considerations - can you get it in for this winter? In the context I can see an argument for scheduling the solar in for the spring, as it will deliver little in the winter. Is this programme better in phases?

 

- How does that electric boiler compare with your combi gas boiler? Presumably if nothing in demand is changing you need it to be the same power, discounted for however much the water is preheated. The biggest load challenge is always water heating.

 

- I'm skeptical about projected prices. But my own strategy is based on reducing winter gas demand, and I already have a large solar array on the roof.

 

- I'd keep an eye on what Fizzy Lizzy is going to do when she is the new PM in a few days. I have little faith at present as she is putting out naive statements about tax reductions mainly when the govt strategy for the last 10 years has to take people who now need the most help *out* of income tax, but she may be mugged by reality.

 

- I think you are not thinking separately enough about water for human use, and central heating. If you plan to use Thermino preheated water to feed heat to your central heating and permit a smaller electric boiler, you will need a tank with a thermal element of some sort to transfer the heat from your Thermino preheated water to your circulating water. That sounds complex, and it may just be better to eliminate the Heat Battery and use a Powerwall driving the electric boiler directly.

 

Others are better than me on that. 

 

- Will your rads need replacing?

 

* Reduce, Reuse, Recycle 

 

I think it's also going to be worth you doing a runaround the traditional reducing demand options again perhaps; increased prices have moved a lot of balance points.

 

eg Are you *fully* LED bulbs, insulated and sealed loft hatch?, how old is your fridge & freezer (modern ones are *much* more efficient), are your hot water pipes insulated?, does your extractor fan have a backdraft shutter? or Heat Recovery, are your window reveals insulated?, do you have underfloor draughts? - all of the classic small reductions that are now worth 3-5x as much as they used to be.

And bigger things such as whether your floors are insulated etc.

Ferdinand

[ProDave]

Forget the idea of the resistance heating electric boiler.  If you are going to heat the house by electricity then a heat pump is what you need.  In very approximate figures that will give you 3kW of heat for every 1kW of electricity it consumes, bringing the cost of electric heating close to the cost of gas heating (at current rates, who know about the future)

 

The solar PV and batteries make sense and you will have plenty of electrical devices to use all that the solar PV can produce.  

[Pete lea]

  On 29/08/2022 at 07:51, Conor said:

You could just get a heatpump that will do all your heating and hot water. Run off the batteries during the day if needed. Then no complex heat battery setup. And 3x more efficient than the direct electric heating. With a monblock you just need a cylinder inside. And of course a solar diverter will cover your joteater needs for a chunk of the year.

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I'm not sure ASHP is an option, I have microbore all through the house and I don't think I could take them opening up all the walls to replace them and would probably have to replace all my radiators as well. I've read that it might be doable though with microbore but they have to up the flow rates? Does that sound right?  Perhaps it would be worth getting a company in to do some calculations.

[Pete lea]

To answer some of the other questions/additional info:

• The house was built in 2007 and has okay insulation, but I want to maybe looking at some additional cavity insulation if possible as well as some additional roof insulation once all the work is done up there.
• I'm also looking at an MVHR (mainly so my wife doesn't open every window in the house every day for fresh air while I freeze to death).  I think we can also put a bio filter in this system as my youngest daughter and I suffer badly from Hay Fever each year.
• I'm looking to schedule in the Solar PV and Batteries as soon as possible as all the companies are getting so many enquiries and my fixed tariff ends in May
• I'm getting 3 phase power upgrade in 8-10 weeks so I can draw as much as I need in the 4-5 hours at night.  If all goes well with the night-time tariff pricing we might sell our car and go full electric as well.
• I've been quoted 7200 for the ~6kw solar PV install and 15300 for the 2xPowerwalls which I think is pretty reasonable.  These are the only things I've made a decision on so far as I think they're a no-brainer for us.
• We have full LED bulbs throughout the house and new fridges/freezers (we've been renovating the entire house). 
• I need to expand and insulate loft hatch before all the work gets done up there.
• I would definitely go for an ASHP if I thought I wouldn't have to rip my entire house open to do it.

[SteamyTea]

@Pete lea

 

Have a go at calculating your existing pipe flow rates.

https://sciencing.com/flow-rate-vs-pipe-size-7270380.html

Then see if an ASHP is viable.

[Pete lea]

  On 29/08/2022 at 10:57, SteamyTea said:

@Pete lea

 

Have a go at calculating your existing pipe flow rates.

https://sciencing.com/flow-rate-vs-pipe-size-7270380.html

Then see if an ASHP is viable.

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That looks pretty complicated and I'm not sure I trust myself to do those calculations properly when my plumbing skills extend to being able to replace a radiator.

 

The company that's doing my Solar PV also specialise in ASHP so I might shoot them an email so they can come and do a survey.

[Ferdinand]

  On 29/08/2022 at 10:45, Pete lea said:

To answer some of the other questions/additional info:

• The house was built in 2007 and has okay insulation, but I want to maybe looking at some additional cavity insulation if possible as well as some additional roof insulation once all the work is done up there.

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What's your EPC value? You can check it on the public register.

https://www.gov.uk/find-energy-certificate

I'm not *totally* convinced by 2007 - aiui the big jump in Building Reg insulation etc requirements was in 2010. My house was converted from a bungalow in 2008-9, and he built to better than regs but I still only call it OK (eg 70mm Celotex in the walls).

 

Put in a diary note to take thermal photos this autumn to look for gaps in your insulation.

 

My EPC as converted was 74C, which is still above average but not *that* above average. Average EPC for all UK housing stock is now just under 70.

 

  On 29/08/2022 at 10:45, Pete lea said:

• I'm also looking at an MVHR (mainly so my wife doesn't open every window in the house every day for fresh air while I freeze to death).  I think we can also put a bio filter in this system as my youngest daughter and I suffer badly from Hay Fever each year.

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Don't forget existing extractor fans. If no shutter they are a hole in the wall. There are models which recover 80% of the heat and put it into incoming air.

 

  On 29/08/2022 at 10:45, Pete lea said:

• I'm looking to schedule in the Solar PV and Batteries as soon as possible as all the companies are getting so many enquiries and my fixed tariff ends in May
• I'm getting 3 phase power upgrade in 8-10 weeks so I can draw as much as I need in the 4-5 hours at night.  If all goes well with the night-time tariff pricing we might sell our car and go full electric as well.
• I've been quoted 7200 for the ~6kw solar PV install and 15300 for the 2xPowerwalls which I think is pretty reasonable.  These are the only things I've made a decision on so far as I think they're a no-brainer for us.

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Those prices look OK.

 

  On 29/08/2022 at 10:45, Pete lea said:

• We have full LED bulbs throughout the house and new fridges/freezers (we've been renovating the entire house). 
• I need to expand and insulate loft hatch before all the work gets done up there.
• I would definitely go for an ASHP if I thought I wouldn't have to rip my entire house open to do it.

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Good responses. I hope you don't mind lots of questions.

 

Two other checks:

1 - What is your heating flow temperature? If it is above 60C your condensing gas boiler will not be condensing, and will be using maybe 10% extra gas to cover the lower efficiency. Needs to be 

2 - One more checkpoint is your radiator sizes. I am probably nearly ASHP ready as I have ufh downstairs, and large rads upstairs to match the lower than normal flow temperature for the ufh.

3 - I'd still say that you need to do your own heat calculations as a check on any suppliers.

 

I think your basic issue is still the complexity. I'm really not convinced by the electric boiler and preheater as described.

 

F

[PeterW]

A lot of the newer ASHP can get to 60°C at a CoP of 2, so that is twice as efficient as the Sunamp and the electric boiler - still not as good as the gas boiler though. Instead of the sunamps why not fit a 200 litre slimline tank and a solar diverter and then use that to feed the gas boiler for DHW..? No point in using direct grid power for it unless you can get it below the gas kWH price though. 

[Ferdinand]

  On 29/08/2022 at 12:52, PeterW said:

A lot of the newer ASHP can get to 60°C at a CoP of 2, so that is twice as efficient as the Sunamp and the electric boiler - still not as good as the gas boiler though. Instead of the sunamps why not fit a 200 litre slimline tank and a solar diverter and then use that to feed the gas boiler for DHW..? No point in using direct grid power for it unless you can get it below the gas kWH price though. 

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Question for your Peter, as I am looking at that option.

 

Do I need a specialist company for that, or just a normal plumber?

[SteamyTea]

  On 29/08/2022 at 12:52, PeterW said:

A lot of the newer ASHP can get to 60°C at a CoP of 2,

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At what external temperature?

[Radian]

  On 29/08/2022 at 12:50, Ferdinand said:

What is your heating flow temperature? If it is above 60C your condensing gas boiler will not be condensing, and will be using maybe 10% extra gas to cover the lower efficiency. Needs to be 

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I keep meaning to start a topic on this. If I set my boiler below 65^oC then unless I also drop my HW cylinder stat the same number of degrees below 55^oC the boiler spends a long time going on and off as it's modulated down to its minimum but still seeing too small a delta between flow and return. E.g. to run the boiler sensibly at 60^oC I can only have the HW at 50^oC which is a bit lower than I'd like.

[Gone West]

  On 29/08/2022 at 12:50, Ferdinand said:

he built to better than regs but I still only call it OK (eg 870mm Celotex in the walls).

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Struth, you're a difficult person to please, 870mm of Celotex and it's not enough for you. 😁

[Pete lea]

  On 29/08/2022 at 12:50, Ferdinand said:

What's your EPC value? You can check it on the public register.

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it says 76 - c, with a potential 86 - B (not sure what that means).

 

  On 29/08/2022 at 12:50, Ferdinand said:

Don't forget existing extractor fans. If no shutter they are a hole in the wall. There are models which recover 80% of the heat and put it into incoming air.

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The extractor fans in both upstairs bathrooms have seen better days... i'm not sure one even works properly. Both bathrooms need to be replaced - they're the last rooms that need renovating.

 

  On 29/08/2022 at 12:50, Ferdinand said:

1 - What is your heating flow temperature? If it is above 60C your condensing gas boiler will not be condensing, and will be using maybe 10% extra gas to cover the lower efficiency. Needs to be 

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I honestly have no idea - it's a potterton powermax HE and it just has a dial on it with no numbers (it's about 80% of whatever the range is).

 

  On 29/08/2022 at 12:50, Ferdinand said:

2 - One more checkpoint is your radiator sizes. I am probably nearly ASHP ready as I have ufh downstairs, and large rads upstairs to match the lower than normal flow temperature for the ufh.

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We don't have UFH and we basically replaced our downstairs radiators a couple of years back, although they were mainly chosen for aesthetics than performance, a mistake on my part.  They seem to work about as well as they old ones did but I wish I'd got larger ones thinking about it now. 

 

  On 29/08/2022 at 12:50, Ferdinand said:

I think your basic issue is still the complexity. I'm really not convinced by the electric boiler and preheater as described.

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I think I just looked at one of the diagrams for that sunamp to see how it was configured and said "yeah that's the one I need". Something like the picture below but without the heat pump 

I can't find many installers for the Sunamp though although the videos I've seen have all been largely positive.  I'm space constrained so the idea of stacking a couple of these on top of each other for 400+ L or hot water with minimal heat losses (advertised at least) seemed like a good idea) with just enough room to put a slim boiler in there.  Unfortunately there's not enough room to put a big cylinder and a boiler in the same cupboard.

 

What i'm confused about is people telling me to switch our the sunamps for a water cylinder, I thought they were meant to be direct replacements? For example, I can get a mixergy 300L smart tank but I can get the 300 Sunamp (meant to be 284L) and combine it with another one for way more (maybe 400-450L). The advantage being that I could also use this extra capacity to help with heating? but I think the general consensus is that this wouldn't be enough anyways?  

 

I'm definitely going to get some pros in to do the installation - I'm just trying to think through some options that would work for our house. 

Edited August 29, 2022 by Pete lea

[PeterW]

  On 29/08/2022 at 16:26, Pete lea said:

What i'm confused about is people telling me to switch our the sunamps for a water cylinder, I thought they were meant to be direct replacements?

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 they are - but they are not all they are cracked up to be.  You have no issue with the losses from a cylinder into the house which is what Sunamp is designed to prevent. A pair of sunamps won’t do your heating and hot water and they are way more expensive than a 300 litre unvented hot water tank. 

[Pete lea]

  On 29/08/2022 at 16:29, PeterW said:

A pair of sunamps won’t do your heating and hot water and they are way more expensive than a 300 litre unvented hot water tank. 

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Is this because the demand for heating is just way more than hot water?

 

I know they're a lot more expensive - more than I was wanting to pay for sure!

[JohnMo]

What's the reason for dumping gas?

[Pete lea]

  On 29/08/2022 at 17:55, JohnMo said:

What's the reason for dumping gas?

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I could lie and say eco reasons etc, but really I just don't like it, I didn't want my kids cooking with it (switch to induction and electric ovens already) and I don't like the small chance that it could leak carbon monoxide (even though we have a detector and have regular services etc).  So not necessary a logical argument for most people.

[Pete lea]

  On 29/08/2022 at 12:52, PeterW said:

A lot of the newer ASHP can get to 60°C at a CoP of 2, so that is twice as efficient as the Sunamp and the electric boiler - still not as good as the gas boiler though. Instead of the sunamps why not fit a 200 litre slimline tank and a solar diverter and then use that to feed the gas boiler for DHW..? No point in using direct grid power for it unless you can get it below the gas kWH price though. 

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I assume if the newer ASHP can reach 60°C then I could keep my microbore piping and radiators? (perhaps upsize radiators as well) and it could run off the stored energy from my batteries which will be cheaper per KW than gas? Even if it's only at a Scop of 2.

[Trw144]

  On 28/08/2022 at 21:55, Pete lea said:

electricity at 7.5p/kwh as

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can I ask who this is with? I thought night time rates had also jumped up significantly

[Pete lea]

  On 29/08/2022 at 20:12, Trw144 said:

can I ask who this is with? I thought night time rates had also jumped up significantly

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I believe Octopus Go is still at this rate? Although it may have jumped up since, I'm fixed till may 2023 but hopefully going to switch when my Batteries and Solar PV get installed.