Running an ASHP on battery?

Does anyone have experience running their air source heat pump (and rest of house) from battery on a winter’s day? How big is your heat pump, what’s your indoor temperature and what kWh capacity do you need / use? I have just had a heat loss survey done and am considering switching to a 14KW ASHP from an old gas boiler but I think it would consume a whopping 60KWh on a cold winter’s day. That’s very much a worst case calculation, and I think there’s a lot of caution built into the heat loss survey, but it would help enormously to hear others’ experience that have actually done it, and whether you’re happy with the result. Thank you

Given the energy efficiency of ASHP being 300-400% relative to a modern condensing gas boiler (92%) are you saying your boiler is using 180-240kWh of gas in a cold winter's day (16-21 m3 gas)?

As above. Is that 60KWh from the power supply (battery, mains, whatever) or 60KWh heat usage? Assuming a low COP of 200% you are looking at 30KWh into the heatpump. With some sort of cheap electric overnight you can make all your hot water in that time and also charge the batteries. Then heat the house in the evening on the battery. I think the 60KWh is very conservative.

that 60KWh is from the power supply… and based on the heat pump running 24/7 without changing the thermostat. i know it’s a huge amount but that’s why i’m keen to hear what other have actually used from experience (the usual variables notwithstanding)

Usamagpie 60kWh/day can happen for a few days a year. It's not worth sizing the battery to cover these cases. I have a 16kW ASHP, EV, 5kWp of PV and 2x5.2kWh batteries which work well for a good part of the year and then load shift the ASHP to cheap rate as much as possible.

Usamagpie I have had our 18kW ASHP installed for 20 months now. Summer usage for hot water only is less than 2kW. Winter is typically around 20kW a day, and I have seen daily consumption as high as 50-60kW a day in bitterly cold extended periods.
This is just the heat pump. House load on top.
I only had the batteries installed earlier this year so this be the first winter with both. I’m not expecting to be able to support the entire load on the 14kW of batteries, neither enough solar to fully charge them or enough time overnight either. Like stevelewis I’m expecting to load shift with the battery so at least I’m never paying peak grid rates

Overall annual running costs for the ASHP were similar to the oil boiler it replaced in year 1.

I have a Bosch hybrid that combines an undersized ASHP and a gas boiler allowing me to set the price ratio and it also analyses the outdoor temp and water temp so that it can use whatever is most efficient price wise.

All hot water is delivered by the gas boiler, but you can get a tank if you want as well.

If you want to take a look, Bosch 7001iAW

Thanks for the info so far.

Agree 100% with shifting the load so right now I have a 14kWh battery which will cover the house excl. ASHP in winter from overnight low cost grid. That time of year I don’t believe there will be much battery excess to help with a heat pump but will find out a couple of months from now (battery is new). I’m on Intelligent Octopus so one 6hr low tarif window per 24hr period meaning one battery recharge cycle per 24hrs - albeit powering the heat pump for 6hrs at the low cost rate. Nonetheless my thinking is that without more battery storage this means 18hrs of heat pump at the higher rate, which could potentially be very pricey if we have a cold winter and the 60KWh order of magnitude comes to pass.

Am I right to think that switching to a ASHP is really a carbon footprint decision and not to expect too much in the way of cost savings? Have those of you with a ASHP saved much on your heating bills? Just want to go in with realistic expectations and also this will help me decide how far I would want to take the battery side of things.

Midlandterrier Yes, picked a day in Dec and consumed 183 kwh of gas. Also, i turn the thermostat down from aprox. 11pm to 11am but as I understand it with a heat pump the idea is to keep it constant. Which I think is why the heat loss survey looks at house build etc rather than using prior years gas consumption. With a heat pump the avg temp would be higher

geoffreycoan are you planning to add battery capacity?

NotoriousPyro this sounds amazing and sounds like it’s designed to solve the problem i’m trying to get to the bottom of… but sadly i think it would be out of my league as even with the gov grant i’m finding the whole ashp thing to be £££ 🙁

Usamagpie Have you considered the benefits of the Octopus Cosy tariff which gives you two charge slots? We’re working through the same questions as you and the 9.5 battery will not be quite enough to get over the peak rate 3hrs. We will need a 10-12kW pump and to get a design point cop of 4 we are considering some seriously big radiators. We do have a log burner for peak cold days and you might consider something like that too.

We have an 8kW heat pump heating a 175sqm detached house, generally keep the indoor temp at 21 degrees.
Our average daily ASHP usage was around 10-12kWh last peak heating season (Dec-March) but there were some days where it was 30-40kWh. We retrofitted underfloor heating in about 40% of the downstairs area and that makes a big difference to running costs as we can heat it at the cheap overnight rate and it normally doesn't come again until the late evening if at all.
We are waiting on an AIO install so will see how that impacts running costs with cheap overnight electricity.

I’m seriously looking at new types of HT heat pumps like RED/ Octo Cosy as we can get a £9k grant in rural Scotland!!! ( plus they claim minimal radiator changes) Obviously running cost is a key consideration though plus persuading SWMBO. I put in a 4kw PV array and a 9.5kwh GE battery, worked great since March averaging 10kwh a day from solar. Max was 23 kWh. From June I installed a 2nd hand 10kwh Sunamp heat battery and did away with a hot water cylinder. So had free hot water and free household electricity from June to mid Sept.

Plenty spare summer solar to charge this Sunamp directly ( only cost £550 ) saved 600 litres of heating oil. Still got the oil boiler just now for space heating.

Our house is 2 story stone built pre 1900 600mm whinstone walls with lathe and plaster linings , no insulation there or floors sadly, but it’s South facing and I’m sure the long south facing wall catches and stores any heat that’s going.

I was hoping that 20 kwh a day at off peak rates ( or special tariffs , more on this later) would be sufficient for a HT HP with a cop of 3 = 60 kwh as we use normally about 5-6 litres of oil per day in colder months ( space & water) = 60 kwh at 10 kwh / litre of oil equiv? On extra cold days we can fired up either dining room coal fire😊 or the living room log burner 🔥

Cost comparison 6 litres of oil at 90p = £5.40
20kwh of off peak eco 7 at 15p = £3.00

But eco 7 needs time shifted?

Figured out that Octopus Cosy 2 x 3 hours let’s me also charge the GE at the same time letting me spread the offpeak across a wider time period😊 some folk use OCTO GO this way too and now OVO have brought out an anytime 15p rate for heat pumps but you must buy their heat pump ( delivered by a Heat Geek installer ) They are also doing the same with an Anytime Car charger rate.

The market is moving : alliances are being forged betweenEnergy companies, installer networks and software developers.

Seeing the Energy Tech advisor from Home Energy Scotland in a week to see if they’ll sanction a HT heat pump linked to a new Sunamp as any proposed system MUST be used for heating space and water. Sunamp do versions for enabling thermal and electrical charging . Can enable 1kwh into the HP to generate 3kwh into the heat battery. Win win? At 15p for all your hotwater in the colder months, seems too good to be true , though a lot of capital invested to do this.. Might not be worth having the HP on at all in the Summer?

No maintenance needed on the Sunamp, no legionella problems , simple install , but very heavy 180 kg but very compact 1/4 the size of a cylinder, and 1/4 the heat loss rate. Check them
Out made 15 miles from where I live , 50,000 manufactured, good for 50,000+ cycles so always consider a 2hand one if you can find one .

Bill G

Usamagpie geoffreycoan are you planning to add battery capacity?

No I’m not. I’ve run the maths lots of different ways and the payback period for what would be a 3rd battery for me is at least 5-6 years. That’s with saving peak rate usage during the winter and charging in cheap or high solar pv times in the summer and then exporting in the flux peak rate.
If I just look at one of those and not the other then it’s 10-22 years payback of the £4k or so for another 9.5 or 8.2 battery.

On the question of rationale for an ASHP and running costs, my oil tank failed and it wasn’t possible to put a new one in the same place- too near the boundary fence and nearby shed. After the government grant the ASHP cost about the same as a new oil tank would have been, about £2500.
In terms of running costs I spent ages analysing it and came to the conclusion that it would be about the same as I was paying for oil (and this was without the solar panels and batteries which came this year and will improve things).
Year 1 running costs on the standard variable rate it cost about the same, slightly better than oil.

I’m currently on octopus flux and I reckon that my batteries will totally see me through the peak rate period with the ASHP so at most it’ll be the same running cost as I was on the standard variable rate. The overnight cheap period will reduce the cost further. Octopus cosy probably similar experience, the batteries will see me through the peak period so I’ll be paying daytime or cheap rates.

My current leaning is towards Octopus Tracker, single rate per day. On average this has been cheaper than the variable rate and will free me from worrying too much about when the heating is on

Usamagpie Depends on what you are switching from. We replaced an old oil boiler 8 years ago and the savings have been substantial plus there's the hassle factor with oil deliveries, leaking tanks, smells, etc. Remember that the cost of all Smart Charging with IO (and any other consumption during the same period) is at the off-peak rate, even if it's in the middle of the day, just don't abuse it!

I'm in new build with ecodan 14kw ASHP. Have an 4kw fit solar, AIO and as of two weeks ago an EV and moved to IO. Downstairs of 5 bed house is underfloor and upstairs is rads.
Last winter on eco7 ran ASHP over night. Set rooms stats so that they stayed on all off peak period, house stayed warm all day (19 degrees) and even warmed upstairs. There were a couple of days when upstairs came on but with underfloor the response time is so slow, I find it's best to consider the floor as a heat store and max it as far as you can each off peak and with modern insulation the house will stay warm.

All I changed was the compensation curve to increase flow temp when weather got colder (setting on the ecodan).

Planning on the same this winter except now I'm on IO it should be cheaper again.

Typical usage in winter is 20kwh for heating and 10 for house (now covered by AIO) so hoping for minimal peak consumption. All working together for cost effective energy

redzer_irl Thanks so much for your post. Underfloor heating makes a whole lot of sense now. i hadn’t considered that but it definitely gives me something to think about if i find my average consumption is way high. Really reassuring too to know your peak period average, and indoor temp. i would want to be at 21 degrees. I bought the AIO in anticipation of it becoming modular and having two but GivEnergy are not there yet. Hopefully not too far off but they advised just buying one for now. Actually that’s the main reason for waiting to spring to make the ashp leap, plus and being absolutely sure of the numbers and what I can expect. Thanks again

stevelewis interesting… I need to look into IO a bit more then. I just rely on 11:30-4:30 at the moment

geoffreycoan interesting, thank you. after the gov grants of £7,500 ASHP cost here is still coming in at around £10k plus the cost of replacement rads so it feels like a massive decision and one i’ll regret if i don’t have the right expectations going i to it. hence this post and thanks to you and others that have responded. your experience has been very helpful

billG I’ve been considering a Sunamp too - think they look like a very smart piece of kit and as you say versatile in terms of their power source(s), but had it in mind more for if I end up going the route of replacing my old gas boiler with a more efficient one. The rationale being that I could at least then turn the boiler off completely in the warmer months (hoping 6-7 months of the year) and cut some of my remaining carbon footprint that way. interesting though to hear that you have paired it with a heat pump. do you still have the ASHP water cylinder too?

Judith Hi Judith, I’ve worked out that IO is cheaper overall for me even with the single charge window; the ASHP will be running from the Grid during the 6hr low tarif window where the rate is much lower than Cosy, and also my EV consumes quite a bit as it has a 78kwh battery. But otherwise yes I imagine cosy could work better. Yes definitely to log burner! Love it anyway but great way to bolster the heat - even in the shoulder months like now to avoid turning on the boiler! hehe

s_r_thomas I'd echo that on the UFH. You can buy a lot of concrete as thermal mass for the cost of a battery and it won't wear out.

I'm interested in the sizes of these ASHPs people have fitted. We had an 8.5kW Ecodan fitted earlier in the year, along with a bunch of new radiator upgrades, in our early 70s 4/5 bed with standard cavity wall and loft insulation and slightly aged double glazing. This was after a full heat loss survey etc. We were also advised not to oversize the pump as that could lead to cycling (on/off/on/off) behaviour which lowers the COP greatly apparently vs running continuously at a low rate.

Does everyone else live in giant mansions needing 14kW+ pumps or have I got a very undersized pump fitted?! Not had a chance to actually try out the heating in anger yet as it's not got cold enough since it was fitted.

Usamagpie

Usamagpie actually the scenario you propose is my current situation. I’m only look at heat pumps at the moment. Started in March With a 4kw PV array and a 9.5kwh GE battery, delivering up to 23 kWh at the height of summer , in early June I added a 10 kWh. 210 litre equiv ) Sunamp ( 2nd hand £550!) a version that is only charged by electricity.

So as you hope for , in early June I switched off the oil boiler over the entire summer. Saved about 500 litres of oil. Result ALL my household electricity and hot water was free using only solar and the Sunamp.

But as I can get a £9000 grant to put in ASHP that heats house and water, looking get a new Sunamp that can be charged both thermally and electrically.

Sunamps are made just 15 miles away from where I live , and I know their MD Andrew Bissell who invented it.

Usamagpie

Usamagpie this sounds amazing and sounds like it’s designed to solve the problem i’m trying to get to the bottom of… but sadly i think it would be out of my league as even with the gov grant i’m finding the whole ashp thing to be £££

Hybrid systems do not qualify for the Boiler Upgrade Scheme
https://www.ofgem.gov.uk/environmental-and-social-schemes/boiler-upgrade-scheme-bus/property-owners

Usamagpie I scrapped my hot water cylinder, the Sunamp replaced it. All my hot water is now at mains pressure. Sadly the installer who was great and v cheap decided that all the old copper was his now😊

Barns wow that’s interesting! my house is not huge by any standard. 150m2. Daikin spec’d 18kwh. An independent company (found via Vaillant) spec’d 14kwh. Interestingly Vaillant don’t make one that big… their design team is reviewing my heat loss survey. Am waiting to hear what they think

Barns there are many factors to consider: building construction, location (both region and town/out in the country), SCOP and available sizes for each manufacturer). If it works for you, I wouldn't worry about it.

Zakalwe

Zakalwe thank for this. def rules this out for me then

stevelewis I guess I'll find out over the next month or two whether it works or not! Been too warm so far to turn it on for heating at all - lounge sensor never drops below 18-19C even at 3am. Fingers crossed. The total heat loss for the house calculated to about 8kW I think in their "not quite worst case but when it's pretty cold" simulation so 8.5kW pump was what they recommended. They said they very rarely ever fit anything larger than that to a normal domestic property assuming it's got sensible insulation etc (and you really shouldn't be fitting a heat pump until you've got reasonable insulation anyway). Underfloor heating was discussed but it would've been a bit of a nightmare plus cost to retrofit and they didn't think it was a huge deal assuming enough radiator area was specced.

Usamagpie Unfortunately Hybrid Systems such as the Bosch 7001iAW don't qualify for the BUS Grant.

The grant specifically says that you must remove the gas boiler from the heating/HW system.

Usamagpie Have you checked out Octopus Cosy tariff 2x 3 off peak slots 6 hours apart ? With a 14kwh battery to can bridge the gaps between 7 and 1 pm and 4 to 9 pm? Not sure how much inpu energy in kWh you need to trickle the HP along 1.5, or 2 kWh perhaps ? Remembering you’d get 3- 3-5 x output , depends on COP. Could work for your EV as well?

Also OVO now have an ANYTIME HP tariffs for heat pumps and another for EV chargers but details scarce but rate is 16p ? Other electricity charged at appropriate rates.