@jamespa - how is that the Vaillant data-logging is showing a COP below 1.0 on occasions?

Separately... Do any of these manufacturer's data-logs show when the ASHP was operating in defrosting mode?

That information is required by NESO and the DNOs because it's needed to plan where grid upgrades are required.
Unlike other forms of demand-surge, all ASHPs within the same area are likely to enter defrost mode together.

@jamespa they didn't provide any contract, or design proposals. I had to ask what the design temp was and scop, and they just replied to the email. The only thing they did provide which was vaguely approaching a contract was the invoice that laid out the payment terms at the bottom.

I knew they were operating fast and loose, shall we say, but trusted them up to the point where it became clear that they had a strong propensity for dishonesty during the installation process and the various things they tried to get away with doing, or not doing.

The way you lay it out is pretty much exactly how I feel and how I have worded my grievance to them. It is working 'mostly' as intended. If they came to us from the start and said they could shave a couple of design degrees off what everyone else was proposing i.e. 47/48, there would be no issue really. I'll have a further think about reconciliation and what you say about what might be reasonable, is useful.

@benson

Strange no contract as it protects them more than it protects you IMHO, ie the absence of a contract puts you in a stronger position.  

Perhaps, in addition to the above, give some thought to (a) what would put you 'at peace' and (b) given what you paid for what you got, and the reality of FT being determined by radiators, was it a reasonable deal.  Somewhere in this there is possibly a sweet spot that you could feel OK with and they could agree to even if its not exactly what you originally wanted.  Bear in mind that their labour probably costs them less than materials, hence their offer of 3 years free servicing.

Posted by: @transparent

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@jamespa - how is that the Vaillant data-logging is showing a COP below 1.0 on occasions?

I don't know

I checked the data resolution and its hourly.  However I think this is in fact an hourly sample (by home assistant - which limits its API calls to the Vaillant online monitoring server) of data that is more frequently updated at source, so subject to sampling noise.  If a sample, for example, corresponds with a defrost more or less anything could happen. 

I don't take a lot of notice of this particular plot TBH, I just observe general trends, but now you have pointed this out I may look a bit more so I am certain what's going on.  What Im more interested in (for now) is cost vs what my gas boiler would have cost over the same period, which I calculate from energy in, degree days and my former measured gas consumption vs degree days

Posted by: @transparent

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Separately... Do any of these manufacturer's data-logs show when the ASHP was operating in defrosting mode?

Its easy to tell from various plots yes for example here on 11th April Flow Temp drops suddenly three times which is defrost.  If you look on heatpumpmonitor its easy to identify defrosts from the compressor modulation, when LWT < RWT etc.

Posted by: @transparent

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That information is required by NESO and the DNOs because it's needed to plan where grid upgrades are required.
Unlike other forms of demand-surge, all ASHPs within the same area are likely to enter defrost mode together.

They wont all occur at exactly the same time.  Defrost will typically occur perhaps once per hour and the compressor ramp up afterwards will perhaps last 5-10 mins.  However I can see that this may represent a higher load than 'normal' running in cold weather