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).