Hi

I have just posted the below in response to your intro text.

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In terms of getting quotes, be prepared to get a fair few to find a good installer who will do what you want and do ask lots of questions once you narrow it down.  Key things to bear in mind are:

1. System design matters more than choice of ASHP, but some ASHPs have more user friendly/flexible controls
2. Don't accept any quotation with a buffer, low loss header or plate heat exchanger between ASHP and emitters (and do ask if its not clear from the quote).  A 2 port volumiser is OK.
3. If you have gas meter readings DO sense check the surveyed house loss against your gas consumption.  Surveys can be wildly out and sizing does matter, gas meter readings can help avoid potentially expensive mistakes 
4. DO design for the lowest reasonable flow temperature.  As a general guide 45C or less for radiators, 35C or less for UFH, unless space constraints mean that these are impractical
5. Don't seek/allow the installation of third party add on controllers, room thermostats or smart devices like Hive etc.  Heat pumps should generally be run 24*7  controlled by their native controller, all or most TRVs and thermostats set a couple of degrees above the desired temperature so they are acting as limiters not controllers.  The exceptions to the advice against third party controller hardware are those designed specifically for heat pumps namely Homely or Adia. There is also Havenwise who do remote third party control again specifically for heat pumps.

Of course there are always some exceptions to the above, but they are rare and, if they apply to your case, there will be an identifiable reason which you may wish to test here.

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My first observation is that 11kW is big and I note that 7kW is also quoted.  Its important not to oversize (its also important not to undersize), you want to get as good a handle on this if you can and surveys can be a case of GIGO.  As an extreme example I had two 3 hour surveys, both of which came out with 16kW, whereas my actual measured loss is 7kW!   By way of getting a handle whats your floor area and house construction.  Do you have smart meter readings, or indeed any information on gas consumption?  11kW Daikin is actually a larger one (14-16kW) downrated so not an ideal choice if your loss is in fact 11kW

I dont know anything about BOXT, others might, do check the questions above.  If they want to put in any 4 port device between emitters and ASHP then look elsewhere, if they don't then as you say worth considering.  The thing about heat pumps is that simple is best!  You can find some reviews of some makes of heat pump (including Vaillant - written by me) here.

Thanks for the replies to both forum posts.

I'm due to speak to the Octopus sales team tomorrow, so will ask for more info about the heat loss calculations, as I've only had the updated quote email from them which didn't provide huge detail.  From what the surveyor was saying during the installation, I think the calculated heat loss was approaching 10kW so we're kind of in the gap between the two heat pump solutions they offer. Cosy (which I don't like the appearance of so am happy to avoid) is currently available in 6kW and 9kW sizes, and the Daikin is the bigger option.  Interesting to read that it's a downsized bigger unit, as I believe that impacts the ability to which it can ramp down in warmer temperatures, making it less efficient.

The house is a 4-bed detached property, built in 1980, with 116m2 of floorspace.  We've got cavity wall insulation which we believe is also in the conservatory too, but can't easily evidence this.  I think I did enough to persuade the surveyor to reflect this in the EPC assessment.  The ground floor is solid tile construction.  One of the pre-reqs for an Octopus installation will be to remove the (unused, but still working) gas fire in the living room and cap the chimney (as they would then cap off and remove the gas meter as part of the installation process).  I'd look to do the same, regardless of choice of installer as this is an obvious heat loss exposure area within the property.

Our gas supply is still on a dumb meter, but I'll be able to get consumption data going back for at least a couple of years, so I can do a bit of digging on that.

I suppose my main doubts about BOXT are that they are primarily gas boiler installers, albeit one of the bigger names doing this (they sponsor Leeds United, but I won't let that cloud my judgement!)  Finding information on their heat pump installations is difficult as the searches primarily throw up gas boiler testimonies.  It's the reason that I haven't sought a quote from British Gas at this stage.  On the plus side, they offer the full 7 year Vaillant warranty, which I believe requires that their installers have done additional training with Vaillant.  From what I can see, there's no obvious indication that they're including a buffer in their quote too, which appears to be a positive sign based on reading various posts on here.  The full payment up front is an annoyance, but not necessarily a deal-breaker with them and the difference in price quoted by them will mean they'll remain under serious consideration, but will probably be the final option I use in terms of committing to a survey while I collate comparative data from other companies.  It feels like using them will be something of a leap of faith, so I'll spend more time doing due diligence on them.

@sheriff-fatman, given you're based in Yorkshire, I have three excellent installers I can recommend that are not in the mass volume game. Let me know if you'd like me to connect you with them.

Posted by: @editor

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@sheriff-fatman, given you're based in Yorkshire, I have three excellent installers I can recommend that are not in the mass volume game. Let me know if you'd like me to connect you with them.

Thanks.  I'd be happy with this as a way to explore further options.

Posted by: @sheriff-fatman

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The house is a 4-bed detached property, built in 1980, with 116m2 of floorspace. 

Is 116 sqm both floors, or just the footprint?  If the former then 11kW sounds possibly a bit high to me.  Your conservatory may be dominating however and older double glazing isn't necessarily too good.  I replaced the glass in mine (but not the frames) with low e argon filled, it made a noticeable difference!

For comparison my house is 200sq m (sum of both floors) 1930s build.  Originally solid 9" wall; most but not all walls now upgraded with internal wall insulation.  Double glazing and 300mm loft insulation.  Uninsulated floors (either concrete or suspended) throughout.  7kW verified by measurement both of the gas and the ASHP.  

There is a tendency for surveys to overestimate ventilation loss.  2-3ACH per hour is typically assumed for older houses but there is at least some evidence that 0.5-1ACH is more likely unless there are obvious draughts.  Also some surveyors will ignore any fabric upgrades that they cant see.  There is room for debate how much oversizing matters and it quite possibly varies from manufacturer to manufacturer (or even model to model).  There are good reasons why it might matter but smart control algorithms can go some way to addressing this, hence the possibility it varies from manufacturer to manufacturer.  There is some, extremely limited, evidence (no more than a suggestion really) that both Daikin (M series) and Vaillant deal well with oversizing, albeit by very different control means.  This doesn't mean that others don't.  

@jamespa The 116m2 is the total floor area as per the EPC, so I assume its the total usable area.  It will be interesting to see what the other surveys come up with in terms of their own calculations.

I also should have mentioned previously that we have 15mm pipework throughout, so no complications arising from microbore, which was another reason why the surveyor viewed it as a fairly simple installation, from a technical perspective.

The conservatory was obviously a challenge for him to consider.  I think it has a heat loss of close 3-4kW in isolation.  Within a couple of minutes of entering the property he said that it might have to be excluded from heating to meet their criteria, but the subsequent calculations and assessment of the radiators meant this wasn't necessary, although I think he's planning to add a 3rd radiator in there due to the lower temperature flow with a heat pump.  Ironically, it's a room that we spend more time of the year trying to keep cool rather than warm, but he's done the assessment based on it being a regularly habitable room, which is the appropriate usage for it.

Whether or not this is a factor in the sizing of the heat pump, I'm not sure.  It was potentially more of a factor into whether or not the job would fit within Octopus' installation criteria or was a job that they ultimately turned down after doing the survey.  I guess we passed that test, but perhaps were somewhere close to the cusp in that regard.

Posted by: @sheriff-fatman

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The 116m2 is the total floor area as per the EPC, so I assume its the total usable area.  It will be interesting to see what the other surveys come up with in terms of their own calculations.

11kW does sound way over in that case!  As you say it will be interesting to see what others come up with, maybe try to get a copy of the calculations and dig out your gas consumption.   You would be well advised if possible to avoid getting a 10-12kW model if a 7kW one will do.

Posted by: @sheriff-fatman

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The conservatory was obviously a challenge for him to consider.  I think it has a heat loss of close 3-4kW in isolation. 

I can see why he is thinking of fitting another radiator!  Finding a radiator even to do 1.5kW at say 45C is a challenge!

Posted by: @sheriff-fatman

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Ironically, it's a room that we spend more time of the year trying to keep cool rather than warm

Bear in mind that most heat pumps can cool as well as heat, but for cooling you need fan radiators or underfloor heating at least in the rooms you want to cool.  It adds another level of complexity to the purchase decision and strictly speaking requires planning consent.  That said the only feasible enforcement action, so far as I can see, is to tell you not to use it for cooling!. 

Some heat pumps, including Daikin and Vaillant have cooling disabled.  In the case of Vaillant you can buy a 'coding plug' (AKA a 4K resistor) for either £500 (with the correct part number) or £5 (same part, different part number) which enables it, I don't know about Daikin.  Lots more to consider if you go down this route, the likes of Octopus wont support it and converting it probably invalidates the guarantee.

I've done some calculations based on our gas usage, for which I have 'dumb meter' data within Octopus going back to October 2022, when we switched to them.  Applying the m3 conversion on the latest bill, the total Gas kWh usage in 2023 was 14,100kWh, split roughly as 11,100 heating, 3,000 hot water.  In 2024 the figure is higher, being 17,800kWh in total, with a higher hot water estimate of 4,500kWh, which feels a bit on the high side (I've assumed that the quarter from 1st July to 1st October is all hot water, with no heating being used in those months, which is an over-simplification).  The increased usage might be a factor of both a colder overall year in 2024 (solar generation was well down compared to 2023) and the gas boiler inefficiencies that are prompting the decision to replace it.

By comparison, the EPC carried out last week estimates 11,200kWh and 2,300kWh respectively, so is quite close to the 2023 usage.

Not sure how this equates overall to an estimate of heat pump size.  A google search suggested dividing the total usage by 2900, which gives an answer of 5-6kW, depending on which figures are used, but I'm not sure how good a calculation this is.  If anything like, then it suggests the 7kW Vaillant unit auto-quoted by BOXT might actually do the job, albeit this is without the benefit of any survey done by them.

Crudely dividing by somewhere between 2000 and 2900 gives an estimate dependant on assumptions/who you believe.  My total gas consumption for a 7kW house was 18-20MWh/yr.

For my money, assuming you heat the house to a sensible temperature most of the time, this puts to bed 10-12kW and puts it firmly in the 5-7kW bracket. Even that's quite a lot for 110sqm but the conservatory sounds like it may be close to uninsulated.

It would be good if you could rationalise this back to the fabric calculations, I found I could by assuming ACH = 0.5.  More than one installer has said publicly that 0.5-1 is more realistic than 2-3 typically assumed by MCS.

Please note I'm just a homeowner with a physics/engineering background who has spent two years studying this stuff for 'fun'/to inform their own installation.  I am not an expert, so you will need to form your own judgement (that's the disclaimer!).

Posted by: @jamespa

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Please note I'm just a homeowner with a physics/engineering background who has spent two years studying this stuff for 'fun'/to inform their own installation.  I am not an expert, so you will need to form your own judgement (that's the disclaimer!).

I did wonder where all the detailed knowledge came from, and had assumed you were involved in installation in some form or another.  Caveat duly noted!!

Posted by: @sheriff-fatman

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Posted by: @jamespa

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Please note I'm just a homeowner with a physics/engineering background who has spent two years studying this stuff for 'fun'/to inform their own installation.  I am not an expert, so you will need to form your own judgement (that's the disclaimer!).

I did wonder where all the detailed knowledge came from, and had assumed you were involved in installation in some form or another.  Caveat duly noted!!

 

Lots of reading and questioning which the background makes easier.  I started when I first thought about getting a heat pump for myself and it became a bit of an obsession. 

For me the process of getting a heat pump took two years, due largely to planning issues (long story), giving me plenty of time to understand the technical side.  I like to think this is, at least in part, why my installation was straightforward and gave good performance from day 1.