Posted by: @majordennisbloodnok

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However, people don't seem to talk much about the fact that fossil fuel boilers don't HAVE to be run in an on/off way, and indeed in some European countries there is an obligation to install them with weather compensation. Even on this forum, we have bemoaned several times why the UK doesn't set up boilers that way, but I can't remember anyone here ever actually suggesting someone try doing that as a stepping stone towards getting a heat pump.

Our Vaillant boiler was working with weather comp, but operating with a low set back different temperature overnight. We limited the boiler flow temperature and found which rads were struggling to keep rooms warm.  Very useful to process the heat loss report advice and decide which rads to upgrade. 

Hopefully it will all start paying back this winter and we will manage to see further drops in our winter energy bill!

Posted by: @majordennisbloodnok

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However, people don't seem to talk much about the fact that fossil fuel boilers don't HAVE to be run in an on/off way, and indeed in some European countries there is an obligation to install them with weather compensation. Even on this forum, we have bemoaned several times why the UK doesn't set up boilers that way, but I can't remember anyone here ever actually suggesting someone try doing that as a stepping stone towards getting a heat pump.

For a year I did the closest my boiler would permit.  I ran 24*7 at it's lowest fixed ft of 50C (the boiler has an add on WC module, but I didn't possess this and it was 15 years old so not obtainable).  I manually raised the ft to 55 in the coldest weather. 

Result - lower fuel bills and greater comfort, surpassed only when I got a heat pump!

Like @jamespa we ran our gas boiler at 55-60C for a year with 24hr operation  and an overnight lower temperature. That was about the radiator limit and we couldn’t go down further in shoulder season because the hot water temperature couldn’t be set separately.

So @majordennisbloodnok we may be a rare species but we do exist

Agreed, @jamespa and @judith. That’s just it. A fair number of us have reduced the flow temp as far as possible as a springboard to heat pump system design but @batpred’s the only one I know who actually ran a boiler with WC.

Posted by: @majordennisbloodnok

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Agreed, @jamespa and @judith. That’s just it. A fair number of us have reduced the flow temp as far as possible as a springboard to heat pump system design but @batpred’s the only one I know who actually ran a boiler with WC.

An engineer friend of mine runs a boiler with WC.  Because the plumber didn't fit the boiler's internal diverter valve, he had to implement a circuit to fool the boiler that the outside temp is sub zero whenever he does a dhw run.  It's a mad boiler design which my old boiler, of a different make, also shared.  Thank goodness heat pump manufacturers didn't follow suit and made provision for external diverters.

@majordennisbloodnok could I ask a quick question as I see you have battery storage, how much does your battery cover the heat pump usage especially during the winter months. I have 10kW of storage (Growatt ARK) so was wondering if I should add a little more as my V1 batteries are fairly cheap as V2 are now out.

Cheers 

Ian

@ian-s I’m sure Major will answer anyway but perhaps I can also answer, even if you didn’t ask me!

I have 27 kWh’s of Tesla Powerwall battery and am on OE’s Cosy. The Daikin heat pump is an 8 kW model and we only ever use the cheapest 8 hours to keep our whole house running from day to day. Our winter daily consumption is ~40 - 45 kWh’s all in (we are all-electric, no gas). We are able to import at 10 kW so have no problems recharging during any of the Cosy 3 plus 3 plus 2 hour cheapest slots. Solar energy contribution is minimal Nov. - Feb. Hope this may help, Regards, Toodles.

By all means, @ian-s. As it happens, I have a spreadsheet that I have maintained since installation, which predates my Home Assistant installation and so is invaluable as a complete record. As a result, I can comfortably say that I can't answer your question directly. I can, however, give you a slightly different take on it all.

Whether or not you have solar panels to go with that battery of yours, I'm sure you're well aware that the generation over the winter is not likely to be enough to cover a heat pump working hard. It's the battery itself that's far more relevant and in particular its ability to let you timeshift; purchasing electricity at a different time to when you use it, thereby effectively extending or even eliminating your "peak rate". I only have a 6.5 kWh battery, but I also use Octopus' Agile import tariff and I have the means (in my case, Home Assistant) to use it properly.

What this means is that when it's sunny, my solar PV will be charging the battery up, then, if there's still more generation, topping up the car battery and finally any surplus being exported at a decent price. Any home usage is therefore coming from solar PV first, then the battery and only being bought in when absolutely necessary. If the import price goes negative (not an every-day occurrence but does happen surprisingly often - 9 times for me in the last 2 months) then a forced import from grid will immediately fill up the battery and car. When there's little solar PV, the system is set up to recognise 4-7pm as the expensive import time and so will do its normal thing until about 2pm before deciding if there's enough in the battery to tide us over that peak period. If not, it'll charge the battery from grid ready to be full by 4pm, after which the home is running from battery until the battery's discharged. That may or may not be after 7pm but if not then we were never going to manage it that day.

All of this has the effect of lowering the average unit price of any electricity we buy from Octopus. Once we take everything into account - home-grown energy, timeshifting, export and so on - this last year has shown a net price per kWh consumed of just under 10p, and that would not be possible without the battery. We are strongly considering upgrading to an inverter capable of greater throughput and a much bigger battery, at which point I'm certain our net unit price will go down significantly.

The flip side to that, of course, is consumption and you asked about our heat pump. It's an 8.5 kW unit and can be quite a big consumer when it's worked hard. Our very worst day was 6th January this year when it went through just under 71 kWh (including DHW). That's the aspect that causes most new or potential heat pump owners the greatest anxiety; heat pumps are much more efficient over the course of a year but around 75-80% of the cost of running one is centred around just three months and people aren't initially tuned to accepting "it'll be fine in the long run" when they see daily usage of 30, 40 or 50 kWh. However, by all the efforts of reducing the average price per unit of electricity it means my heat pump, even going full chat, only needs to achieve a relatively modest efficiency before it is cheaper to run than gas or, in my case, oil. That is why I'm saving around £850 per year running my heat pump instead of my old oil boiler and another £1,000 per year running my EV in preference to my ICE car.

Of course, in order to use Octopus' Agile tariff you really need to make good use of some fairly comprehensive intelligent smart home system and a lot of people aren't prepared to go down that rabbit hole quite that far. However, many members here report being able to achieve something pretty similar with more predictable time of use tariffs like Cosy, Flux or Go, albeit helped with larger batteries than mine. In that case, it's more a matter of setting up a simple schedule for charging from grid rather than getting software making on-the-fly decisions.

As you can see, the simple answer to your question is that a 6.5 kWh battery does not offset a heat pump that much on an expensive day but even a modest 6.5 kWh battery matched with intelligent control and a reasonable time of use tariff can make a very big difference over the whole year.

@ian-s I too can provide some insight.  We have a Cosy 9 along with 10kW PV and 16kW of battery, 8kW inverter plus an EV.  The batteries are charged up overnight and then provide load to the house until they run out.  Last year we used £108 of peak rate electricity mostly in December, January and February otherwise only off-peak.  If we’d been able to run purely on off-peak, I.e. the cost to charge batteries, it would have saved around £70 over that £108 spend.  My electricity bill last year was -£7 in the end (including EV charging).  We have no gas so a fully electric household.  There are only 2 of us and after a rather difficult start to the season (we suffered from overheating believe it or not!) the house was kept between 22c and 23c.  I would only recommend increasing battery capacity after you know what you need, not before.  It would have cost us £5k to provide capacity to ensure we only ever used off-peak electricity.  The most important thing to have is an inverter capable of running the heat pump and other high power devices such as ovens, irons, kettles etc without recourse to the grid.  In my view an 8kW inverter is the minimum needed for this but it will depend on how the household consumes energy.

@ian-s 

Unless your aim is backup as opposed to tariff optimisation it might be worth doing the math.  All indications are that your house loss is 5kW or less, possibly as low as 3.5kW.  On the basis that it's 5kW this means that most of the season you will be operating at 2.5kW.  With a cop of 4 that's an input power of 600W IE about 14.5kWh per day.  Add 400W baseload gives a most of the season consumption of 24kWh.  If you have 6hrs cheap rate that's 18kWh during peak rate.  What's the payback time on the additional batteries for this, and what's the payback time if you try to cover the peak days as well?  This will tell you where the sweet spot is.

@jamespa I have been looking at battery prices for my system and as Growatt has brought out a V2 of our setup, which is backward compatible, companies are selling off V1 with large discounts so I can add 5kW for around £1000 but add 10kW for £1700. This would cover a fair chunk of usage for not bad money but don’t want to oversize my storage for the sake of it.

First time poster here but I can share my experience as it may help the OP as I see similarities...

I have a 1906, 3 bed detached house that up until 2.5yrs ago was heated with an ageing gas boiler.  The house always felt chilly despite good levels of loft insulation, cavity wall insulation, and good double glazed windows.  I suspect I was just too stingy with the heating. I was using ~10,500kWh of gas per year.

I had a heat loss survey done in readiness for installation of a heat pump.  Figure was 6.5kW, maximum designed flow temperature of 45°C. 

This left one question – to go for the Vaillant Arotherm 5 or 7.  The 5 was chosen based on its PEAK output ability.  At the time everyone was saying ‘do not oversize the heat pump’.  They were right, but also do not undersize!  Manufacturer figures can be optimistic or just not what they seem (a whole other discussion).

All seemed good initially.  Then we had a few very cold days (below freezing) and the heat pump was struggling – the house was just not warm enough and the heat pump was working hard – I had to light the log burner (which I enjoy). 

Long story short, the heat pump was switched for the Arotherm 7.  Problem solved, warm house, and the electricity usage barely changed with the larger heat pump.  I normally love to light the log burner but have not bothered to use it since the larger heat pump was put in (I’ve got 2 tonnes of logs and 1 tonne of coal unused).  The house remains at a steady temperature and is very comfortable.

See for yourself here:  https://emoncms.org/app/view?name=MyHeatpump&readkey=0e6f9cc1b9855446fdd166a64868c441

SCOP of 4.2 is not bad.

Electricity usage for first 12 months: 2651 kWh

Electricity usage for second 12 months: 2601kWh

Electricity usage since installation in May 2024: 5443

I have a 13.5kW battery and am on the Octopus Cosy tariff (3 off peak periods spread over 24hrs).  99% of my electricity import is at the cheapest rate.  The battery gets me between the cheap time of use slots on all but the very coldest of days.  I also have an EV and this tariff is not the cheapest but overall is OK.

Heat pumps are not for everyone or every house but I wouldn’t change back – its transformed this house and with the battery, is cost effective to run.

I hope there is something from my experience that you find useful.

And if you want a great installer, perhaps Szymon of Urban Plumbers ( https://www.urbanplumbers.co.uk/ ) will venture as far as you (he’s based in Frimley Green GU16 ).  If not, his YouTube channel is worth a watch: https://www.youtube.com/@UrbanPlumbers/featured

Good luck with whatever you decide.