Hybrid ASHP / Heat battery option: thoughts appreciated.

Fink_Nottle

As my oil boiler is misbehaving, I’m thinking this might be a good time for me to start planning a move to an ASHP. I suspect my running costs could end up being a bit higher than with oil and though I can accept this, I’d like to narrow the gap as much as I can.

I use about 2000l per year at a cost of – let’s say - c £1400. Let’s also say this is about 2000kWh, so about 7p per kWh. Let’s say my electricity (Octopus Intelligent Go) averages out at maybe 21p per kWh (off peak @ 7p, rest at c29p but I’m rounding to keep the numbers easy).

To break even, I’d need a SCOP of 3, yes? I have underfloor heating downstairs, and some big rads upstairs (and can easily change the rest myself). BUT I live in an old stone (solid wall) house, 360m up a windy hill in the North Pennines, and I wonder if a SCOP of 3 is ambitious in the cold months (Sept-May!).

Additionally, given that space for a h/w cylinder is limited, I’m considering the advisability of using a heat battery (say 150L) for DHW, powered solely by overnight electricity. At 7p per kWh, this more or less matches oil, and saves space, but I understand that this has a higher up front purchase cost than a cylinder (though installation feels a bit easier than a cylinder).

I’ve also read* that using an ASHP for low temperature space heating only allows it to run more efficiently than if it has to supply DHW too, thereby improving SCOP.

So with a heat battery for DHW, and an ASHP for space heating, I get a simpler install (maybe) and slightly lower running costs (maybe) than if I paired the pump with a cylinder. Maybe.

Is this set-up viable? Any gotchas? Thanks for reading, and all thoughts very much appreciated.

*MS Copilot. Often just right enough to trick you into missing the whoppers. Very polite though.

QrizB

A couple of thoughts:

- Your COP doesn't care about your property. You could run a heat pump in an open field and it'll deliver the COP appropriate to the ambient air temperature.

- Your oil boiler will have a "COP" of about 0.8 so your current heat cost is more like 8.7p/kWh.

- Heat batteries (Sunamps and the like) only make sense if space is at a premium, due to the cost. It sounds as though that applies to your cottage so it could be a good choice.

NedS

As @QrizB states, the (S)COP will not be affected by the type of property you live in. You have a good idea of how much energy your property requires to heat, and it doesn't really matter whether that heat is provided by an oil boiler, gas boiler or heat pump - what you are primarily concerned with is running costs, and hence the price of the fuel per kWh and the efficiency.

Fink_Nottle said:

To break even, I’d need a SCOP of 3, yes? I have underfloor heating downstairs, and some big rads upstairs (and can easily change the rest myself). BUT I live in an old stone (solid wall) house, 360m up a windy hill in the North Pennines, and I wonder if a SCOP of 3 is ambitious in the cold months (Sept-May!).

Yes, your understanding is correct.

This massively acts in your favour. Heat pumps are low temperature heating systems. The larger the emitters, the lower the flow temps you can run, and having UFH and large radiators may allow you to run low flow temperatures which will enhance the SCOP. I do not see a SCOP of 3 at all challenging for a well designed system.

The two things that you can control that will most affect your running costs are (a) make those emitters as big as possible to keep the flow temps as low as possible, and (b) reduce your unit price as much as possible (here a battery would massively help allowing more of that winter usage to occur at 7p).

How much space do you have for a cylinder? What about alternative locations such as in the loft, or my neighbour has his sighted outside in a purpose built, well insulated lean-to of timber construction, with pipes routed straight through the wall (not ideal, but think outside the box).

Your heating SCOP and DHW SCOP will be very different, as the heating can hopefully run at fairly constant low flow temps whereas the flow temps of 55-60C are typically required to heat the DHW cylinder to an acceptable temperature, and of course that is less efficient. Although it will drag down your overall SCOP figure, the cost of heating your house has not changed (and this is what is important). Even with a SCOP of 2.5 for heating the DHW, if you schedule that to run overnight, it's still only costing you 2.8p/kWh so less than half the cost of oil and 2.5 times less than a heat store /immersion type system.

For reference, we replaced an oil boiler (8 years old?) with a heat pump last year, and our first year of running cost around half what we were spending on oil. We did have some insulation upgrades at the same time and we also upgraded all of our radiators to the biggest we could reasonably accommodate and ran at a flow temp of 32-35C all winter which massively helped our efficiency. Overall we used less energy and what we did use cost us less per kWh. In Summer we get COPs of ~3 as we're just running the hot water, in Autumn/Spring we see COPs of ~5 and in the coldest 3 months we see COPs of around 4.

Fink_Nottle

Many thanks @QrizB and @NedS, It helps to get ideas and opinions and experiences from real people rather than AI bots... @NedS you seem to be getting excellent results which is very encouraging to hear.

I could probably shoehorn a cylinder in somehow but it's very awkward, hence trying to look at alternatives. But then in an old house where even the internal walls are two feet thick, it's all awkward. e.g. I can put the ASHP where the boiler was - easy for connecting water, but horrible for connecting power (given that the pump needs rather more of it than an oil boiler). I'm still trying to work out how to disguise the living room's new surface mounted 6mm cable going to the car charger (paint it green and pretend it's an exotic plant?).

So the idea of slipping a battery under a utility room worktop, rather than re-routing copper all over the house to a cylinder, has its appeals. And at c £2k inc VAT, I think the purchase price is "only" £500 to £1000 more than a cylinder, and I might even recover this on installation costs.

Nevertheless, as a natural sceptic, I'm more than open to hearing any other downsides to the battery option.
(I'm also a little uneasy at 1kWh in = c.1kWh out being less green than it could be, but. that's all I've got so far).

NedS

We have a Joule pre-plumbed slimline DHW tank. I think they retail around the £2k mark (not cheap), so I don't think your alternative is going to cost that much more, and if a cylinder is not an option then the cost is kind of irrelevant (as are your green concerns)

I have a friend who has a Sunamp Thermino heat store, and I know he's very pleased with it - like you, he has no room for a conventional DHW cylinder.

Our old oil boiler was in the corner of a wide hallway and the slimline tank occupies pretty much the same footprint (around 700x900mm). It would likely fit into 700mm square, but I think you'd need a non-pre-plumbed solution and a decent plumber to get a DWW tank into a 600x600mm space (I think the slimline tanks are 475mm diameter, then you have to allow for plumbing connections). Obviously all pipework was already in place, and it's located close to bathrooms and kitchen so overall a good location. I do need to build a cupboard around to enclose it, but for now it's on full show for everyone to admire!

Without doubt, your first main decision is how you solve the hot water problem.

QrizB

NedS said:

We have a Joule pre-plumbed slimline DHW tank. I think they retail around the £2k mark (not cheap), ... I think you'd need a non-pre-plumbed solution and a decent plumber to get a DWW tank into a 600x600mm space 

We've got a 20-year-okd 450mm diameter foam-insulated indirect cylinder. Something like this would be the modern equivalent:

https://www.heatershop.co.uk/gledhill-replacyl-stainless-steel-vented-cylinder-1050mm-x-450mm-indirect-connection-140-litres?cat=9097

£200, not £2000, and fits comfortably in a 600x600mm space. I'll measure our airing cupboard later and confirm the footprint.

This style or tank includes an immersion heater boss so you can still heat water on your overnight cheap rate if you choose.

Update: my airing cupboard is 660mm wide and 510mm deep.

Fink_Nottle

Thanks again for these details, much appreciated. Space is only part of my issue - although it is an issue - re-routing plumbing and electricity supply to viable spaces is the uglier bit. The one place where I have both appropriate plumbing and a suitable electrical connection is where the cylinder used to live in the olden days before we moved in - but that space is in the corner of the bathroom, which is in the eaves and is cramped enough as it is...

Regarding cylinder price, perhaps we're not quite comparing like for like anyway. @NedS refers to a pre-plumbed, ASHP-ready solution, whereas the Gledhill is a bare cylinder. That specific model is actually marked as not compatible with heat pumps, but I do get the point that cheaper options may be available.

I think if I lived in a less wonky house, I'd have been perfectly happy to put in a cylinder, but I think the heat battery does seem to be a good fit for my needs. It could be a little while before I press the button on this project, but I'll let you know how I get on.

And I hope I'm not straying into politics here but the other thing I've learned from this process is that much of the (ahem) 'informed comment' I've read in the media about how all this woke green technology can't be made to work with old housing stock appears to be motivated by...something other than a desire to look for solutions.

NedS

QrizB said:

NedS said:

We have a Joule pre-plumbed slimline DHW tank. I think they retail around the £2k mark (not cheap), ... I think you'd need a non-pre-plumbed solution and a decent plumber to get a DWW tank into a 600x600mm space 

We've got a 20-year-okd 450mm diameter foam-insulated indirect cylinder. Something like this would be the modern equivalent:

https://www.heatershop.co.uk/gledhill-replacyl-stainless-steel-vented-cylinder-1050mm-x-450mm-indirect-connection-140-litres?cat=9097

£200, not £2000, and fits comfortably in a 600x600mm space. I'll measure our airing cupboard later and confirm the footprint.

For reference, the Joule tank to which I referred is pre-plumbed as part of a Samsung/Joule kit. The tank comes pre-plumbed with motorised diverter valves for DWH and two heating zones (UFH + Rads), a 3kW immersion fitted, temp sensor and two flow meters (electric and mechanical), isolation valves, mag filter, plus all the control circuitry and Samsung controller for their heat pumps (which I believe retails separately for around £470). This makes it very easy (and quick) to install and wire up, saving on plumber and electrician time/costs which is why I guess some companies use them. But due to everything being attached to the tank, they do require more clearance whereas with a DIY tank, valves and control panels can be tucked into corners or on walls reducing the required footprint. An equivalent bare indirect cylinder (Joule) costs around £600-£1000, so once you've added in all the attached bits plus labour saving, but are a completely different proposition from a simple direct cylinder heated by a 3kW immersion.

My main concern with going the direct (immersion) route is what will happen if cheap overnight EV tariffs disappear? Heating hot water suddenly becomes very expensive, or we are back to solar diverters.

NedS

Fink_Nottle said:

And I hope I'm not straying into politics here but the other thing I've learned from this process is that much of the (ahem) 'informed comment' I've read in the media about how all this woke green technology can't be made to work with old housing stock appears to be motivated by...something other than a desire to look for solutions.

Absolutely. There are certain areas of the press that are clearly negative, and so many myths around like you can't have a heat pump in an old building which are just complete nonsense.

I do accept that getting a heat pump instal right can be a complete minefield, but it shouldn't be as it's not rocket science, but there is only so much a homeowner can know before the event that it's impossible/extremely difficult to filter out good vs bad installers beforehand.

There are some huge red flags you can watch out for when getting quotes from prospective companies which should filter out the complete non-starters, but you really need a solid understanding to know when an installers answers indicate concern.

NedS

Fink_Nottle said:

Many thanks @QrizB and @NedS, It helps to get ideas and opinions and experiences from real people rather than AI bots... @NedS you seem to be getting excellent results which is very encouraging to hear.

Yesterday we didn't do a DHW run, so it was heating only and outside ambient temps were mild at 12-15C, but I thought you'd be interested to hear we saw a COP of 6.12 for the day for the heating in our old 1700's cottage with conventional radiators. Flow temps were around 32-33C, which is the lowest we can achieve, no underfloor heating which would be even more efficient. And this is with a massively oversized heat pump with the heating on for four 45min periods yesterday, so far from ideal.

There is no (technical) reason why you cannot achieve excellent (S)COP figures with UFH on ground floor and large radiators upstairs. If those radiators are large enough, and enough heat can rise from downstairs, you may even be able to run the whole system at one very low flow temp of around 30C rather than blending down from a higher flow temp for the UFH, which has the potential to be extremely efficient with some careful system design.

Fink_Nottle

It's a valid point to wonder what happens if really cheap overnight electricity disappears. My feeling is that so long the UK keeps adding inherently unpredictable renewables to the grid, it should make sense for suppliers to offer low costs when there is more supply than demand. If the grid ever gets to the point where there's enough persistent energy storage to smooth this out, things might change of course, but I think I'll chance it that this may take a while.

And further down the line I'm likely to add more of my own renewables (I inherited a 3Kw turbine with the property  but am struggling to get it repaired, so it's currently just a decoration) and this should help mitigate.

COP of > 6? Magnificent!