"slapped in without much thought for design" describes my experience perfectly. I never saw any calculations, and I don't think any were done. I've ended up with an over-sized heat pump, undersized buffer, UFH throughout the house which struggles especially in the bedrooms unless you set the flow temp to foot-melting levels, cabling and pipework which looks like it was done in the dark, no link between the UFH controller and the ASHP, so the system runs all the time whether there is heat demand or not, no attempt to setup the system, everything left on defaults, and no documentation at all. Oh and the guy who did it has disappeared, leaving a string of equally unhappy customers, so even if I had the energy to sue him I couldn't.

Thanks to various YouTube videos and forums such as this one I have got the system running reasonably efficiently, but I'm still a long way from what I expected.

I’d’ve voted flawless if it wasn’t for a couple of necessary return visits to sort out an intermittent tripping of the RCD. Given it was a retrofit and they were dealing with an electrical situation they had no chance of spotting in advance, I’d still rate this a success; I don’t judge companies by what went right as much as how they handled what went wrong, and our installer acted excellently.

Its early days yet, I have yet to experience sub zero conditions, but these are my thoughts and experience to date.

Finding installers was frankly difficult. I approached 5 companies;

Octopus Energy; despite a promising start when it came to a detailed survey they dismissed my property due to it having microbore pipe work

Local renewable installer; Very detailed survey, price acceptable. However they required most of my rads to be upsized, had a very odd location for the outdoor unit, required two heating pumps and everything crammed into my existing airing cupboard.

Global Energy; cancelled my appointment. Paper quote and price was however acceptable.

Vaillant; Despite contacting several approved installers not one came back to me. Even contacting them directly resulted in no call backs.

Abacus energy solutions; The company I chose. Knowledgeable, experienced, an excellent system design. 
Outdoor unit at the back of the house by the sheds, tucked out of the way. Pipes run in a neat insulated ducting along the integral garage wall. All the equipment, tank, pumps and pipework in the back corner of the garage. An absolute master stroke. Compact, accessible, quiet in operation. All the previous loft tanks, hot water cylinder, shower & heating pumps all gone. So much quieter in the house with no tanks filling and gushing. And a bonus of extra storage in the now empty airing cupboard. 

The system works well so far. I’m having a battery fitted soon which will reduce running costs enabling me to maximise my off peak Ev tariff. Actual current running costs seem just slightly less than my previous gas heating. 

I don't think you're that far wide of the mark Mars in your estimate. 7/10 'could do better' would be my honest appraisal.

ASHP: tidily installed work on electrical and mechanical sides, well designed and well thought through install, decent heat load calcs done, by myself and by installer for MCS registration. Making good by the trades including boxing in pipework and fixing up walls opened up for cabling was excellent. The lads were very considerate and tidy working in a family home that was occupied throughout. What let things down was poor system control and rushed commissioning, leaving the whole system running flat out to deliver a constant 50deg CH water supply and primary and secondary flow rates too high. Weather compensation was not enabled. The Grant controller is quite clever but absolutely no handover, tuition or documentation on it given to the system owner. Had I not been in building services I would be wondering by now why my daily electricity consumption was astronomically high. A bit of head scratching with a less than informative installation manual, a few parameters corrected, some temperature logging and it all looks better now and chugs away quite happily.

Solar PV: internal electrical work very tidy. External solar PV panel install was rushed, despite having in the installer's words, 'the best scaffold access we've seen' (arranged by myself via a contact). A full day was required but they tried to cram it into half a day. Result: broken roof tiles, poorly cut to length support rails, utterly useless anti-bird mesh barely attached to the perimeter of the arrays. Installer called back out to rectify and make good, which cost them the better part of another half day to do it right, plus them paying through the nose to buy pan tiles that are no longer manufactured from a reclaim merchant on Ebay.

@allyfish, did the same company do the ASHP and PV?

@editor Yes, both by Thrift Energy

Posted by: @editor

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I'm currently engaged in several 'private conversations' with heat pump installers and association members that have taken exception to me stating that less than 20% of heat pump installations in the UK have been flawlessly designed and installed, bringing the competency of installers into question. I believe that most systems are slapped in without much thought for design, especially when retrofitting a heat pump to replace a boiler. This results in systems that don't adequately heat the house and are expensive to run. So I'd like to see how far off the mark I am. Please vote on the poll, and you're welcome to elaborate on your poll selection but leaving a reply below.

Mars, I think you are very optimistic and have significantly overstated the number of well designed systems. From the amount of systems that we sort out, I would estimate it to be in the sub 10% level. This unfortunately is caused by the industries attitude of "sell a box at any cost and dam the end user and consequences"

@hibbsy did you have to pay beforehand for both surveys you got done?

We had two ASHPs installed, a Stiebel Eltron one for our house and a small Grant one for our office building. The installer was used to putting in Grant ones, but to his credit, said that he thought our house wouldn't be suitable for a single Grant ASHP. He was honest and said he hadn't installed a Stiebel Elton ASHP before, but had just been on the training for it and he thought a single one of those would be more suitable than two Grant units.

The Grant one was installed fine, and he said he would come back when it is really cold and set up weather compensation properly and improve flow rates. I can't get my head around the controller though, so not sure what temperature it is currently operating at!

However, the Stiebel Eltron was a bit of a nightmare and the unit wasn't working properly and he kept coming back to fix it but it still wasn't working properly. In the end, after he had spoken to another Stiebel Eltron installer he found via LinkedIn, he worked out that the Stiebel Eltron plumbing diagram didn't include a valve that we needed. Once that was fitted, it has been working great. He even got the Stiebel Eltron engineer to come out to check everything, who also set up the weather compensation - albeit at 23 degrees which was way too hot, so I've been adjusting it since. The installer had set the heating temperature at 50 degrees which was obviously wrong, but he did say it was very mild in September (it was) and he would come back when it was colder to get it right. He wanted to ensure we had heating at least if the temperature dropped. 

One major problem we've had is the supply of parts. The controller provided was an older model (works okay) as the new WPM4 is not available. Plus the internet service gateway isn't available and we've no idea when we will get it. I know Stiebel Eltron have had issues securing chips, plus there has been huge demand for ASHPs in Germany for obvious reasons.

Our installer was a small company called Orange Energy, who I'd say was straightforward and did keep coming back to try and fix the initial problems (putting off other paid customers to try and sort ours). I do think he could have been a bit better in explaining weather compensation, and especially how to set the Grant unit. The Stiebel Eltron is much more straightforward and I've been adjusting it and getting it to be more efficient - I just wish we'd had the WPM4 unit which we ordered though. 

@knukes Hi, thanks for sharing. The Grant weather compensation parameters are quite straight forward to adjust once you get into the settings in the less-than-intuitive controller. Grant publish very little information in their installation manual about the controller and the features, I think this is intentional, because it could get screwed up quite easily. It factory defaults to give minimum water temperature supply at +20degC and maximum at -4degC. It's all adjustable in the installer 'INST' level.

I'm in the process of 'suck it and see' on a fairly new Grant 10kW ASHP install, to see if the compensation is set about right. I'll know in the depths of winter if I need to adjust it to improve CoP if I can get away with a lower supply temperature.

@allyfish I think I still need to spend some time getting my head around this! Thank you though.