Posted by: @davesoa

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@jamespa My actual gas usage is 16500kwh & electricity 3500kwh. My survey assumed a heat demand of 19200kwh. So a heat loss of 14300w? It can’t be. I watched the Heat Geek video mentioned by @toodles above. Fascinating (and worrying) at the same time. I also visited the Heat Geek site which suggests a heat loss of 8.2kw

No your demand cant be 14.3kW.  My annual gas usage is 18-20MWh and my measured consumption 7.5, max 8kW.

Sadly this appears to be 'situation normal'.  Have you got the spreadsheet calculations from those quoting or is this based solely on eyeballing the house?

The 7kW Vaillant (eg) should do nicely in your case provided you can run (say) at FT45, no pp required assuming you can meet the noise and other misc pd requirements.  11.2kW mitsi is likely overkill, but wouldn't be a complete disaster because it has a decent turndown capability, the 8.5kW Mitsi is most probably a better choice than the 11.2.  A genuine 14kW pump will be well oversized and qill therefore cycle at quite low temperatures, leading quite probably to an efficiency loss of perhaps 10% (or more if its badly set up).

Have you by any chance got half hourly meter readings to you can prove to yourself and others that 14kW is a load rowlocks?  Probably an irrelevant question because it seems like the industry has effectively turned its back on you.

Posted by: @jamespa

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Probably an irrelevant question because it seems like the industry has effectively turned its back on you.
 

@ivanopinion and @jamespa. I do have the half hour readings (but then so do Octopus). However I’m going to have one last go. Today I called a local firm, who have done most of the electrical work in my house and who know the builder and plumbers who built the extension. They have been fitting HPs, they say, for years. I’ve told them of my journey so far, and failure to get anywhere. I have the heat loss calculations for all three previous companies (but not the detailed Octopus results which they will not release). I suspect that you are correct about the original point - they will look expensive compared to Octopus. They, incidentally, fit the Mitsi unit you mention.

To give yourself more confidence, look back over the past 3 years and work out the rolling average consumption over (eg) 3 hrs, 6hrs and 24hrs, for each half hourly reading.  Then plot the frequency in 'bands' on a graph, or look for how often a number above (say) 8.5kW occurs.  If >8.5kW (or some other chosen limit) only occurs after it has been switched off overnight, then you can pretty much disregard it, because its just your heating catching up with a setback and you simply shrink back the setback when operated with an ASHP.

I get (or rather got) a few 9.5kW readings (on a 3 hr average) when I do this, but only in the shoulder season when I had switched off overnight. 

This winter I turned my gas boiler (I too am struggling with PP) down to the lowest it would go (8.5kW), never was I cold and again only in the shoulder season when I had turned off overnight was 8.5kW reached. 

Case proven as far as I am concerned, the 7kW Vaillant (which does 8.5+ at 45C) or the 8.5kW Mitsi will be fine, once I get pp (by fair means or foul) and assuming I can find an installer to play ball!

Another benefit of doing the actual consumption analysis is that it gives you a figure for the typical/mode consumption (probably about half the peak).  Ideally you want your ASHP to turn down at least this far (at the corresponding OAT and flow temp), otherwise it will be cycling most of the time.  In my case its 4kW which is my consumption (+/- 0.5kW) for probably 60% of the season +.  in the cold shoulder this is the 24x7 consumption, in the warm shoulder this is the consumption on the basis that I turn it off overnight so it does some catching up during the day.  Only in the coldest and warmest part of the season does it depart materially from this figure.  So another constraint, for my case, is that the ASHP must turn down to 4kW, preferably a bit less, at 35C flow temp (my design flow is 45) and OAT= (say) 10C.  The Mitsi 8.5kW and Vaillant 7kW both do this, the next sizes up don't quite and the 16kW that two recommended wouldn't have come close.

Obviously all this calculation remains theory (but based on experiment), so could yet be proven disastrously wrong.  However that's still, at least for me, a whole lot more comfortable than overengineering based on guesswork.

Octopus do seem to be doing a good job of keeping prices down but only for the type of install they do.  That's fair enough really.  The real problem is the price gouging and over-'engineering' by the grant harvesters which make up so much of the industry many of whom will doubtless vanish once the grants cease.

@davesoa your gas usage is similar to mine and I have a 130m2 detached property build in 1979. Insulated cavities, 300mm loft insulation and double glazing. I've had my heat loss estimated today by a Heat Geek at 8.3kW @ -2.8C with the 7kW Vaillant unit specified so if your property sounds anything like mine 14kW sounds crazy.
I have Octopus coming next week as I've been in the survey queue and got lucky with an appointment so it will be interesting to see what they say. I have had some other numbers ranging from 9kW (believable if you assume some of my walls are uninsulated but not backed up by gas usage) to 10.8kW (crazy).
The Heat Geek survey was very close to what I estimated myself and my coldest day assessment.
Obviously doesn't help with your current struggles if Octopus stick to their guns but hopefully gives you some confidence in your view that something isn't right with their assessment.

Posted by: @jamespa

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To give yourself more confidence, look back over the past 3 years and work out the rolling average consumption over (eg) 3 hrs, 6hrs and 24hrs, for each half hourly reading.  Then plot the frequency in 'bands' on a graph, or look for how often a number above (say) 8.5kW occurs.

@jamespa - great idea. I’ll download the data. 

@gunboatdiplomat - yes similar house. Mine is detached, 160sqmtr, 300mm plus insulation, double glazed through, cavity wall insulation, south central England. It is a hybrid though and I think that’s throwing the heat loss calculation. Part was built in 1911, part 1970s and part 2000s. I’d be interested to hear the Octopus assessment. 

I find increasingly staggering that the MCS rules don't require that the heat loss estimated by GIGO spreadsheet be compared with actual consumption, and that any discrepancy of (say) 30% or more either way be reconciled (with reference to the customer) before proceeding, and the reconcilliation documented.  It’s such a simple check to do yet it’s not required or, by most installers, accepted.  Why?

A cynic might suggest that the regulations protect the installers more than they protect the customer and perhaps this is not surprising because they were designed by the industry.  If the installer follows the rules then they are pretty much bullet proof.  The phrase 'contractually compliant but unfit for purpose' comes to mind.

Of course I'm not a cynic so I wouldn't for a moment make such an allegation.

@jamespa I’ve challenged Octopus’s figures using the very helpful comments from this forum. They’ve come back and admitted that the heat loss might not be correct. However if I’d like to install cavity wall insulation then they will re-look at the heat loss at that point. The EPC confirms that the house does have cavity wall insulation (where it can), the extension built in 2004 was insulated according to building regs at that time and their surveyor was told at the time of the survey. I’ve sent them the certificate of cavity wall insulation (for what it’s worth) but it’s clear they have a process from which they cannot/will not deviate from despite considerable evidence to the contrary.

Posted by: @davesoa

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@jamespa I’ve challenged Octopus’s figures using the very helpful comments from this forum. They’ve come back and admitted that the heat loss might not be correct. However if I’d like to install cavity wall insulation then they will re-look at the heat loss at that point. The EPC confirms that the house does have cavity wall insulation (where it can), the extension built in 2004 was insulated according to building regs at that time and their surveyor was told at the time of the survey. I’ve sent them the certificate of cavity wall insulation (for what it’s worth) but it’s clear they have a process from which they cannot/will not deviate from despite considerable evidence to the contrary.

Posted by: @jamespa

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My understanding is that they are concentrating on a specific subset of properties with a loss of 6 (8?)kW or less.  

@ivanopinion Or could it be that they are staying below the application for PP level? Regards, Toodles.

As far as I’ve been able to glean, Octopus seem able to offer lower installation costs by adopting a standardised process that can cover the majority of customers and concentrating on the “low hanging fruit”. It’s an eminently rational approach so in theory has my support.
In practice, situations as you have outlined, @davesoa, show the standardised process Octopus are using still needs a lot of work; there are some significant flaws. Given how well Octopus have treated me as a customer with their energy supplier hat on, I have a fair amount of confidence they will listen to feedback and address those flaws head on. Only time will tell, however, if my confidence is justified.