Hi @jamespa and a nod to @grahamf for his excellent old/large house explanations. I am still going around in a vortex of different suggestions without a boiler and struggling to find an installer to help me make the right decision and winter is approaching and my wife is getting nervous!.

A reminder - 1840s 300m2 vicarage - no 3 phase possible - 16.9kW heat loss - new extension with UFH - PHE/bronze pump on the old Joule 300ltr - fam of 4, 2 teens.

My new plan was the 17kW Grant Aerona supplemented by a 4 kW or 7kW Slim Jim electric boiler (dialled down) as a (hopefully) unused back up and try to wrangle the BUS on the basis that it was just that - a back up. Going this route on the basis that no one seems to really know how an old house and an ASHP is going to work until you've tried (!) and for c £550 it seems a good insurance policy (and will reassure my wife).

At this stage we want to stick with the 13 rads we have and change them over time (but also see how the system works effectively bolted on to what's here already - and yes we'll initially need to run at 50/55 degs I'm guessing). Many of the rads are newish cast iron, 4 upstairs are never used and we'll increasingly use the new kitchen extension.

Shoot me down and tell me what I should really be doing!....

The Aerona stats are below:

Please read to the very end of this post, I shift my position a bit half way through but I have preserved the trail so you can see the thought process.

I think the problem you are having comes down almost entirely to the uncertainty in the loss being too large to make a comfortable decision.  Can I suggest you review this post and see if there is any way to narrow it down.  If I were you I think I would seriously consider getting the air change measurement done, as this is probably the major uncertainty and should be possible quite quickly.

My concern is that the installer may want to fit a buffer tank so that he (thinks he) doesn't have to worry about this too much (except he is wrong, he does have to worry), particularly as you have both rads and ufh I believe.  This will likely cripple the system efficiency and performance so I would avoid like the plague unless there is a very solid engineering description of why its necessary and how mixing will be avoided.  There are better ways to couple in supplementary heaters, either inline or using a PHE plumbed in a way that does not interrupt the flow from heat pump to emitters.  .

Personally if you must jump now I would probably go for a system that is all from the same manufacturer, such as the Vaillant plus backup heater you first proposed.  This is an inline heater designed to be controlled by the Vaillant and Vaillant controls are generally good.  It's a bit underpowered relative to the Grant however so not ideal, depending on your loss which is so key.  You might take a view on the longer term potential to insulate, possibly sacrificing some efficiency for a couple of years with a plan to upgrade insulation later ( I think you said there were some options).  I would interrogate the installer very carefully about the system design, in particular buffer tanks etc and how he intends combining rads and ufh (I believe you have ufh.

What I would do out of preference is get a better handle on load if there is any way to do this and defer until I had a robust solution with which I was more comfortable.  However if that's not possible then my comments in the paragraph above apply.

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PS don't assume you will be able to leave the rads switched off.  The rooms they are in still lose heat even if they aren't actively heated and that must be replaced. 

PPS I just spotted that Grant do a volumiser with integral immersion heater

GRANTUK.COM "https://www.grantuk.com/media/sozfdbg0/grant-uk-internal-50l-volumiser-uk-doc-0207-rev-11-august-2025.pdf"

  This is designed to be controlled by the heat pump and plumbed inline.  Grant have a bit of a history of advising you/your installer to plumb heat pumps in a way which is well sub-optimal which does ring some alarm bells (depending on what your installer actually does), but if you are considering the Grant (and I can see why you would) I would suggest that the inline volumiser might be a good supplementary heater as its use in the way you need to use it for efficiency seems to be sanctioned by Grant.  This would also defeat any argument about the BUS grant!  

PPPS Having read about the Slim Jim it seems to be designed as an inline device with adjustable output.  It could presumably be plumbed inline and triggered by the Supplementary Heater control on the Grant heat pump (for instructions see the grant volumiser manual) and the output adjusted to whatever the deficit turns out to be.  If this is what your installer intends to do, and provided he doesn't intend to put in a buffer tank, phe or llh between heat pump and emitters, then I would say its actually as good a choice as the Grant volumiser (unless of course you also need a volumiser!).  I would still suggest to get more info on loss if you have the time, but if you dont then I would suggest that this is a viable option.

Disclaimer - Im not an installer nor a qualified heat pump expert, just someone with a degree in physics and a background in engineering (and local government) who has been studying this stuff for 3 years.

Hi @jamespa, thanks again for all your thoughts, very useful. Yes I'm afraid time is now of the essence (and yes you're correct we have new ASHP-ready UFH).

To the BUS question - it seems to me that there's enough ambiguity here if I get the SlimJim fitted now (oil boiler in a heap in the garden), which might buy me some time as it could gently heat the house, and then ask the installer to install the Grant 17kW....see 2(c) below as you correctly identified above.

The govt legislation reads as follows:

(2) A heat pump meets the suitability criteria in relation to the eligible property, or a property to which regulation 14(1)(b) applies, for which it is installed where—

(a) it provides heating—

(i) solely to that property, or to both that property and any related property, and

(ii) for the purpose of both space heating and hot water heating, using liquid as a medium for delivering that heat,

(b) it is capable of meeting the full space heating and hot water heating demands of that property, and

(c) it replaces the heat generating components of the original heating system installed in that property (where applicable), other than any—

(i) supplementary electric heater, including any immersion heater,

(ii) circulation pump, or

(iii) solar thermal collector.

To gild the lily - this is from Ofgem's BUS property owner guide V4 ( https://www.ofgem.gov.uk/sites/default/files/2025-02/BUS-Guidance-for-Property-Owners-V4.1.pdf)

4.37. The installation of some low-carbon technologies alongside the installation of the BUS
funded plant is permitted so long as the heat pump or biomass boiler is capable of meeting
the full space heating and hot water heating demands of the property (our demands are low, we are all jumper-wearers and have 3 woodburners....sigh). Other low-carbon technologies that may be permitted are supplementary electric heaters, including immersion heaters, circulation pumps or solar thermal collectors. 

Then presumably I need the installer to redo our heat loss estimate to fit with the Grant at -1.88 degs (approx 14.9 at 55 degs) ensure SCOP of 2.8 and then I can claim BUS....or not and keep the SlimJim as a hopefully unused back up...

Or am I missing something??

@guydeb The guidelines are possibly written to be definitive and clear, however, though our Daikin heat pump would be capable of supplying sufficient heat for the DHW system, we opted for the separate Sunamp Thermino system as we lacked any sensible site for the whacking great 210+ litre tank MCS required for a ‘4 bedroomed house’. Nevertheless, we received the BUS grant of £5000 and the whole system has served us well ever since. Regards, Toodles.

Posted by: @guydeb

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Or am I missing something??

No I dont think you are missing anything.  There is definitely some ambiguity, the key issue is - is the installer willing to do it.

Before you relax (a bit) and conclude that you have found your solution I would carefully check his intentions as regards the plumbing arrangement.  If he will give you a schematic (which he should) before signing up then thats the easiest way, otherwise question him about

• buffer tanks/PHE/LLHs between heat pump and emitters (there should be none unless he has a really good engineering reason why - because we always do it that way or because Grant insist on one is not a really good engineering reason),
• glycol vs antifreeze valves (the latter preferably, the former tends to tempt installers to insert a buffer!),
• how the supplementary boiler will be plumbed in and controlled and
• whether he intends to fit/use a mixer for the UFH.

Grant in some of their leaflets issue warnings that householders should not touch the ASHP controls (and should instead use TRVs and thermostats).  Its not unknown for Grant installers to put them somewhere inaccessible and/or where they cant measure the room temperature.  This is absolute Bow Locks, the controller needs to be accessible and in a representative room and you will need to touch it.

Im presuming you are looking at the R32 model on the basis that their website doesn't list a 17kW R290.  FWIW this is the Chofu heat pump badge engineered, sometimes people have found (useful) things in the Chofu manual that aren't in the Grant manual, but that may have changed.

thanks @toodles and @jamespa (again) - I shall have that convo when I find an installer (the chap I was going to use isn't experienced enough and has ruled himself out saying that he thinks it's going to cost a fortune to run and I should stick to oil....).

@grahamf I'd be interested to know what you estimate your annual electricity cost might be and if any other members in older homes running bigger ASHPs have some real world elec costs they can share.

WLTM forward thinking, open-minded ASHP installer in Somerset for cosy chats about buffer tanks/PHE/LLHs valves and TRVs....

PS does anyone know how the VAT-free installation works if I wanted to buy the Grant + assorted packs through a plumbing friend's trade account...? 

Possibly not the answer you might want, but if it were me I wouldn't be in any rush at all and I would wait to see how the BUS scheme and the MCS rules develop over the next few years, along with improved high-temperature heat pump models becoming available.

Your situation is far from unique, there's millions of homes where the current BUS/MCS requirements are too rigid and restrictive to make heat pumps simple or even viable. In their current form there's no chance of the UK heat pump roll out succeeding, so they're going to have to change. To make heat pumps practical for these homes is going to need more flexibility - more flexible rules and opening up support to technologies like hybrid solutions. 

I think there's a danger of false urgency, better to wait and get a workable solution than rush to get a heat pump solution which might be costly to operate - the mention of a sizeable flow boiler would worry me, the electricity to oil/gas price differential would make them seriously expensive to run. Surely a hybrid solution would be the more practical option in the long run?

Posted by: @guydeb

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PS does anyone know how the VAT-free installation works if I wanted to buy the Grant + assorted packs through a plumbing friend's trade account...? 

Vat free applies to supply and install only no DIY.

Posted by: @guydeb

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WLTM forward thinking, open-minded ASHP installer in Somerset for cosy chats about buffer tanks/PHE/LLHs valves and TRVs....

 

Get a list of recommended installers from Grant, email them all with the same requirement.  Ditto a few local MCS non-Grant installers.

Posted by: @temperature_gradient

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Possibly not the answer you might want, but if it were me I wouldn't be in any rush at all and I would wait to see how the BUS scheme and the MCS rules develop over the next few years, along with improved high-temperature heat pump models becoming available.

I find the logic in the second part of this difficult to agree with.  Perfectly good high temperature heat pumps are already available, namely many of the R290 ones.  However the basic thermodynamics isn't going to change, so running any heat pump at high temperature is never going to be efficient (nor as comfortable) so 'high temperature' is largely irrelevant.  More to the point that is, very sensibly, not what you intend to do. 

I do take the point about hybrids, although that's actually what you are planning anyway albeit with electricity as the secondary fuel.  You already dismissed an oil assisted hybrid and you dont have gas, so that's the only option left.  Provided the heat pump itself is well set up (no buffers etc) then you should still achieve a reasonable efficiency because you should hardly ever switch on the element.   MCS rules already allow hybrids and I cant see that BUS ever will because it is directly contrary to the policy objective and technically unnecessary other than in cases where a 7.5K grant is wholly irrelevant.  BUS will be scrapped in 4 years time if the lunatics take over the asylum, which currently seems quite plausible.  If you want a heat pump then get a heat pump, just be aware that you may not save money, indeed it may cost a bit more to run at present unless you get the right tariff, given the current ratio of oil price to electricity price (which is historically volatile and currently at more or less an all time high).

Where I do agree with @temperature_gradient is to try to take the pressure off somehow and not rush.  Is there a solution you could deploy for this season only whilst you get the information you really need, namely a better handle on the loss?   The 16kW Vaillant should be available soon which will have better controls than the Grant and, if Vaillant are true to form, will actually do more than 16kW so even if your loss does turn out to be 16kW will do the job unaided.

If you do wish to proceed with a heat pump I really cant stress enough how important it is to get a better handle on the loss.  This is crippling your ability to make a comfortable decision.   You really do need this IMHO given your situation, I would definitely get the ACH measurement done ASAP, as it will likely either open up several possibilities, or close them down.  When I was looking at 16kW loss for my house (based on what two 'professional' surveys said I needed) I was struggling, once I had determined that my loss was only 7kW, the decision became easy.  You wont get down to 7kW, but you may well get down to 12kW and that would then be an easy choice.

thanks guys - I hear you on the not rushing and plainly we need to sort this heat loss, which is crucial (obvs) but also a bit like nailing jelly to the proverbial ceiling reading through contributors' posts.

It matters to us because we're on the tipping point between single 17kW or the dreaded 2 x 10kW, but will we REALLY know until we actually do it and run it? I am tempted to buy a SlimJim for the winter (say the 10kW) and run it at different temps through the winter and see how we fare. As previously mentioned we do have 3 wood burners as well in the mix.

Would this SlimJim idea work (and how would it work with the UFH at 35 degs and rads at (say) 50?)??

And would running 2 x 10kW be very much more expensive than a single 17kW (we spend £2500-2800 annually on oil). 

I am not trying necessarily to save money with the ASHP, I'd even spend a little more annually, I just want to stop burning oil (and yes we do have only electric cars!).

Keep the advice coming, it's much appreciated.

Posted by: @guydeb

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thanks guys - I hear you on the not rushing and plainly we need to sort this heat loss, which is crucial (obvs) but also a bit like nailing jelly to the proverbial ceiling reading through contributors' posts.

It matters to us because we're on the tipping point between single 17kW or the dreaded 2 x 10kW, but will we REALLY know until we actually do it and run it? I am tempted to buy a SlimJim for the winter (say the 10kW) and run it at different temps through the winter and see how we fare. As previously mentioned we do have 3 wood burners as well in the mix.

Would this SlimJim idea work (and how would it work with the UFH at 35 degs and rads at (say) 50?)??

Honestly I doubt, based on your oil consumption** you are on the tipping point between 17kW and 2*10kW, much more likely you are on the tipping point between 17kW and 12-14kW.  However only a real measurement will tell for certain.

An electric heater will work of course, correctly plumbed, just bear in mind the running cost.  If you want two different temps you must have a mixer, thats true whatever the heating as there is no boiler or ASHP available that can output 2 flow temps at the same time!.  An electric heater is also a fantastic way (frankly the best way) to determine your heat loss assuming you have a smart meter and can get half hourly meter readings.  You wound need to take into account your log burners of course (you might have to weigh the logs you burn each day for a couple of weeks), but with the right 'experiment' its the most accurate determination available.  Even the best survey is partly speculation!

Posted by: @guydeb

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And would running 2 x 10kW be very much more expensive than a single 17kW (we spend £2500-2800 annually on oil). 

Correctly set up and controlled almost certainly cheaper to run than a single unit, because you will get much better modulation depth.  You could, with the right plumbing, play lots of games, eg arranging them to supply two separate flow temps simultaneously, arranging so that one is a fall back when one fails.  Its actually a jolly flexible system BUT makes plumbing and control more challenging, so needs a sound installer and a make/model that has flexible control for cascades.  That said for the 4K or less that the additional unit costs its a pretty fail safe way to go

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* one question occurs which might affect this assessment.  Was your quoted oil consumption for years when you were making extensive use of the log burners?

ah that's interesting @jamespa - so could one 10kW (say) supply the HW and the UFH at 35 degs and the other supply the CH at (say) 45 degs? Then presumably in the warmer months we'd only run one anyway for the HW...

Posted by: @guydeb

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ah that's interesting @jamespa - so could one 10kW (say) supply the HW and the UFH at 35 degs and the other supply the CH at (say) 45 degs? Then presumably in the warmer months we'd only run one anyway for the HW...

Yes assuming the pipework permits this.  Not sure this is the most efficient way to run even in the shoulder season though, probably better off just one of them running closer to full chat.  Yet again the answer will depends on what your actual loss is, as well as which make you choose because they play games differently with the compressor efficiency curves.  There again it might just work out most efficient year-round, because most of your heating is at the lower temperature. 

As I say there are plenty of games you could play with two heat pumps, assuming the controller permits it (or you are willing to tinker in Home Assistant or similar).