Posted by: @mattc

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From earlier comments, it looks like some folks on here feel that the 11.2kW Mitsubishi may be oversized, apart from a few of the coldest days in the year.  How difficult would it be to achieve a decent COP when the weather is milder and it would only be running at, say 5kW for most of the day?  To match the running cost of the current gas system, I'd need to get this up to at least 3.5 on average.

You might find it helpful to have a look at another 11.2kW Ecodan on heatpumpmonitor.org.

The URL is https://emoncms.org/app/view?name=MyHeatpump&readkey=712fd396480df6bda505ea65f70be870

They have a mixture of new and old radiators and UFH. Despite the addition of a buffer, their SCOP is 4.0 for the last 365 days

@mike-h 

That's great.  Thanks.  I'll take a look.

Does anyone have any thoughts about question 1 (installation cost)?

You would think/hope that energy used in heating a building would be an ideal benchmark to build a system round, but unfortunately it doesn’t seem to form a part of the assessment that is undertaken by a system designer. Instead the heat loss calculation is used, and in the case of eligibility for grants it is enshrined as a cornerstone of the process. Whilst this is meant to provide an “accurate” assessment of the thermal performance of your house a great many people’s experience of heat loss calculations, especially the one provided(?) by MCS, is that they overestimate the heat loss by quite a large margin due to inbuilt conservatism. So it is no surprise that your three heat loss calcs are noticeably at variance with each other.

It might benefit you to understand what measurements or categorisations are driving the differences in the calcs, you have paid for them you should have been given detailed workings, not just a final figure. I knew there were errors in the one my installer did, but I just looked at it again and it looks even worse to me now. I could say its just wishful thinking for you to pick the lowest of the three if it weren’t for the fact that the figures you have mentioned for your actual usage do seem to imply a heat requirement that does not match those suggested in the quotes. But those figures are quite vague and if I were you I would try to make a more complete history of your daily energy use for, say, this year so far and then plot it against the degree day values (a figure based on the daily average temp expressed as a difference between a base temp and the actual temp so you have a scale that increases in line with the increase in heat required), degree days.net is a good site to start from.

Whichever supplier you go with that would be a solid bit of evidence to put in front of them if you ask for the size of the heat pump to be justified.

Supplier 2 has given you a heat loss of 8.15kW but not recommended the 8.5kW unit, which I might query as the Mitsubishis are meant to be able to produce 9.5kW at -2°C at maximum power. They clearly have planned for your heat to be provided at a lower flow temp as they have recommended 9 radiators be upgraded. If you heat requirements are indeed lower than this estimate, then in order to improve COP you have to be able to deliver that heat at an even lower temperature, which may require even slightly bigger radiators.

I am not sure having an overly oversized heat pump is a great problem, I look at heatpumpmonitor.org and it is really random as to which systems are running efficiently. The effectiveness of the delivery-side design in not bringing down the COP is key. In warmer weather the COP will be better, that is the strongest correlation, but you will get into cycling patterns at lower temperatures with larger pumps – I find my Mitsubishi is more erratic in the quantity of heat delivered when this happens, so not a COP penalty but an energy used penalty.

FWIW my installation with 4 new rads cost £14000 a couple of years ago. Some of the figures quoted for install costs are highly optimistic, achievable if positioning and pipe runs are simple and the installers are plumbers who don’t give a ****, or the company’s main business is selling electricity and they don’t mind have no margin on the install as they expect to make it up on the energy.

@harriup Thanks for the useful insights and for taking the time to reply.

Your similar installation cost with the 8.5kW Mitsubishi does at least help to put my quotes in context.  I'd seen figures around this before but it's always hard to compare like-for-like.  In our case, there should be little or no new plumbing required, so the main part of the cost is just the supply/install of system itself, a new cylinder and the radiators. Do you recall what the calculated heat loss was for your property and how this compares to reality?

I do have the details from all of the surveys but I'll need to go through them to pull out the comparable information as they all present the information differently. From memory, they were all working with an outside temperature of about -3C and the design temps for each room varied between 18 (bedrooms, hallways) and 21 (living rooms), with 22 for bathrooms.  The individual room heat losses and radiator output figures varied a lot.

The real-time results on heatpumpmonitor.org are really interesting - I hadn't come across this before @mike-h sent the link to his.  I've only looked at a few of the Mitsubishis but it looks like the 11.2kW Ecodan is not capable of running at much below 6kW, which was my main concern. I've asked the supplier whether they would be prepared to consider installing an 8.5kW instead so we'll see what they say.

I just realised I didn't comment on the price, @mattc. Our installation was near as makes no odds £15k before RHI and £4k afterwards. We didn't need any rads changed and our living room (easily the largest room in the house) already had UFH. We did, however, need a new hot water cylinder and a whole load of pipework swapped for larger bore. The oil boiler we had was on its way out anyway so it ended up being just the expensive side of a normal boiler replacement. I don't know how much the BUS grant is, but if your pre-grant quote is in the same region then it gives you a reasonable comparison.

@majordennisbloodnok Thanks.  Similar installation cost again.  Looks like RHI was a really good deal.  The new BUS grant is £7500 (formerly £5000) which I thought was was pretty good but it seems small in comparison (not complaining - free money is free money).

I paid £14,500 2 years ago for my Samsung 12kW and this included 8 new radiators. The most important thing to get right is to have a really good installer, so have you researched the installers that have given you quotes? Our installer claimed to be the UK's No 1 installer. They certainly installed a lot of heat pumps, but were short on quality and have the dubious distinction of having a Facebook page created by disgruntled clients to share horror stories. They have since gone into liquidation.

It is also important to get the right radiators. If you need 5kW for a lot of the time, it is so much more efficient to have radiators that can provide 5kW of heat at 30degC rather than 45degC. Sadly, there is recent strong evidence that many radiators being sold today have inflated claims for their heat outputs, so you can't even trust these figures. My set up cannot go below 32-34degC as my radiators will only produce around 3.6kW at these flow temperatures, which is the minimum heat output for the 12kW Samsung. Going below this leads to horrible cycling. Like @majordennisbloodnok, we have setbacks at night and aim to have the house warm by 7am rather than the 24 hour low and slow technique.

Since mid October our COP is 3.14 which is poor compared to most on heatpumpmonitor.org, but heating for 10 hours per day means our electricity consumption is a lot less than many others. Aiming for an average indoor temp of 17degC also helps! We are on Agile which has cost us £321 for heating and DHW for the period 17/10/23 to 16/04/24 (182 days) compared with £518 for equivalent heat production using a 90% efficient gas boiler.

@mike-h 

Thanks again.  The installer I'm leaning towards (Puraflow) seems to have consistently good reviews on TrustPilot and I've been generally impressed by their professionalism throughout, and no-one on here has so far expressed any reservations about them, so I have reasonably high expectations.  The one I rejected first (Greener Living) also made the "we're the biggest" claim but just came across as a classic double-glazing sales operation. 

I've deliberately oversized the most important radiators, increasing some from K1 to K2 and K2 to K3, so I think I've done as much as I need to with these.  Worst case, it's not too difficult/expensive to upgrade one or two at a later stage.  I've been successfully running the gas heating at a flow temp of 50 for the last 3 months and I'm currently stepping it down towards 35 one day at a time to see what happens (45 today, 40 tomorrow).  I'd like to have done this over a longer period to get more data but the weather is warming up now, so needs must.

For completeness and for anyone who's interested, here are the design and performance parameters, as described by each supplier.

@mattc I'm in a similiar position to you - evaluating a few quotes/suppliers. 

We're making some internal layout changes so we have a kitchen/dining space opening into our garden (rather than the utility room as it is today) and the wall on which the existing 15 year old boiler sits will no longer exist. My first thought was to just get a new boiler installed in our new utility space - this would involve some pipework alterations and a new gas feed from the meter. Given the increased grant and alterations we'd have to do anyway it made sense to look at an ASHP

The property is reasonably well insulated - 300mm loft insulation, cavity walls filled (though back in the 80s I think). Half our upstairs walls may well be solid block given what we've seen of neighbours who recently had their rotten timber cladding off and EWI installed. This has lead to some ambiguity on heat loss - my own attempts put a best case at ~7.5-8kW @- 2c, worst cases could be closer to 9.5kW. 

So far I've had four quotes - costs are after the grant:

• One from Octopus which is £5800 but no survey yet (the cost refundable so I thought why not)
• A desktop survey recommending either a 8.5kW Ecodan or a 10kW Vaillant (which would need planning, ugh). £6350 and £9099 respectively with radiator changes an extra cost. Up front deposit for a heat loss survey required.
•  An verbal, indicative quote after a site visit of £4-6000 for an 11.2kW Ecodan and including at least 4 radiators changed for a 45c flow, reckons a SCOP of 3.5-3.7 is achievable.
• A detailed quote after a heat loss & site survey for an Ideal 10kw, no radiator changes as running a flow temp of 50 of £7700. I have to query this as I've been quoted a SCOP of 4.46 at 50c which doesn't match the MCS data - I suspect a genuine mistake rather than malice.

For comparison replacement boiler quotes came in between £4800 and £9300 (that last one blew my mind and was the first I received).

So from what I am getting back I don't think the costs you are seeing seem wildly off the charts.

Posted by: @mattc

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The one I rejected first (Greener Living) also made the "we're the biggest" claim but just came across as a classic double-glazing sales operation.

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This is a very important point.

One of the things I can see retrospectively that we did right was not to judge the prospective installers on what they said as much as what they didn’t. The saying goes that “God gave us two ears and one mouth so we may listen twice as much as we speak” and the people we went with did just that; they listened to what my wife and I were looking for and then specced out something to meet that.

If Puraflow are listening to you, that’s a good sign. If you can see evidence they have translated what you asked for into their quote, that’s a better sign. If they can then justify their choices when you pick up on details, that’s better still. As you have done, treat any double-glazing sales impersonators to a “no” with an “off” in it.