@ivanopinion Octopus have told me they will have a Cosy 16 next year capable of delivering 50c and will cover the alleged 14kw heat loss - if I’d like to wait that is. Be that as it may if their heat loss is wrong (we know it’s wrong) then it would be oversized by some margin.

Posted by: @toodles

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

@ivanopinion I wonder how well that works when attempting to modulate a 16kW unit down to be a 9kW unit which in turn may be required to modulate down to perhaps 25% power? (I’m assuming that the 16 - 9kW derating is accomplished by modulating - perhaps there is another method to do so?) Regards, Toodles.

Posted by: @toodles

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@ivanopinion I wonder how well that works when attempting to modulate a 16kW unit down to be a 9kW unit which in turn may be required to modulate down to perhaps 25% power? (I’m assuming that the 16 - 9kW derating is accomplished by modulating - perhaps there is another method to do so?) Regards, Toodles.

This is a very good question and one I suspect that may be difficult to answer.

One way could be to look closely at the manufacturers data tables (if they are available) for the different models, to see what the minimum output capacity at say a LWT of 35C and an OAT of 12C.

Posted by: @toodles

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@ivanopinion I wonder how well that works when attempting to modulate a 16kW unit down to be a 9kW unit which in turn may be required to modulate down to perhaps 25% power? (I’m assuming that the 16 - 9kW derating is accomplished by modulating - perhaps there is another method to do so?) Regards, Toodles.

@ivanopinion Looking at my Daughters vast Eco Forest 20 Kwh machine it can modulate down to 4 Kwh according to the plate. oops misread 4.2 for 2!.

@ivanopinion I stumbled across this thread Performance of 9kW Daikin - Hardware / Heatpump - OpenEnergyMonitor Community where at least two people managed to get Octopus to swap their 9kW unit after installation as the performance was dire. I have just realised I will likely get specified the 9kW unit by Octopus as my heat loss is above 8 (though it may be closer to 7.5 if I'm more optimistic about the construction of some walls). Between that thread, not wanting a 16kW software-limited unit and the fact that on paper my airing cupboard is too small for the cylinder (650mm wide x 600,mm deep where Octopus specify min 800x800) I'm not sure even at the cheaper price Octopus will be for me.

Posted by: @gunboatdiplomat

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@ivanopinion I stumbled across this thread Performance of 9kW Daikin - Hardware / Heatpump - OpenEnergyMonitor Community where at least two people managed to get Octopus to swap their 9kW unit after installation as the performance was dire. I have just realised I will likely get specified the 9kW unit by Octopus as my heat loss is above 8 (though it may be closer to 7.5 if I'm more optimistic about the construction of some walls). Between that thread, not wanting a 16kW software-limited unit and the fact that on paper my airing cupboard is too small for the cylinder (650mm wide x 600,mm deep where Octopus specify min 800x800) I'm not sure even at the cheaper price Octopus will be for me.

@davesoa I had my survey - seemed to go OK!
The heat loss came out at 7.5kW which is within my "reasonable" estimates (lower than the Heat Geek one I had done but he assumed some uninsulated walls which I now know are insulated after the use of a drill and an inspection camera!). There was some discussion about the cylinder location - the current cupboard housing it is just big enough for their slimline cylinder, though the buffer will have to go in the loft. The snags are:

Design is for 50C - this means fewer radiator changes but also lower performance. The surveyor was unable (not allowed) to design for 45C.
I have a prerequisite to get some artex removed in the cylinder cupboard as it contains asbestos (fair enough, but annoying)
Their survey software recommends the 9kW unit which I do not want - I need to find the data sheet for the 8kW and see if it can in fact cover my heat loss and argue for this

On the positive the surveyor was open to oversizing radiators they did need to replace (ie a P+ would be fine at 50C but I said can I have a K2 please). I just can't get all the necessary radiators upsized as some show as no change required with the higher design temperature.
For reference my current fixed quote is ~£5800 which while cheaper than other quotes is not like some cases where it is £500-1000 so I feel justified in being picky at getting the right system as its still a substantial investment. Maybe this will come down after the survey as the surveyor felt it was a pretty straightforward install. For example no plastic in the system, 28mm primary pipework to the cylinder & pump currently, 22mm under the floors feeding the 15mm to the rads and the positioning of the heat pump means a straight via the loft to the current cylinder location (though I think this could go under the floor but they don't want to lift any flooring..).
Once I've got final proposals from the rest of my quotes I'll lay them out somewhere but thought you might find this interesting. No phantom rooms and the heat loss around where I'd expected.
 

@gunboatdiplomat That’s good news and a good price as well - I’m only faintly jealous. My indicative quote came out at £6200 (including an unvented hot water tank which I already have so would have reduced the cost) though it’s entirely hypothetical as I still have the sound of silence from Octopus. Organisations do not like admitting errors. I’m still looking for my West Wing of 5 rooms but no sign so far.

A final ‘No’ from Octopus. Apparently they accept that I might have a heat loss of 10w but their software is not able to distinguish the subtleties between mixed construction (solid walls/cavity walls) and therefore they cannot be sure what size HP I might need. I am getting a local installer to come and have a look but, like the OP states at the start of the thread, all other quotes look high compared to Octopus. 

Posted by: @davesoa

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A final ‘No’ from Octopus. Apparently they accept that I might have a heat loss of 10w but their software is not able to distinguish the subtleties between mixed construction (solid walls/cavity walls) and therefore they cannot be sure what size HP I might need. 

Interesting.  Thats a big limitation on their software/chosen market, they really are going for the low hanging fruit.  I guess they want to get some solid experience and solid numbers from simple properties before tackling more complex ones.  I bet they are harvesting data from the installs and post-install performance to inform their future design process. 

Having said that its actually quite a sensible strategy, albeit frustrating for those of us, like me, with properties that have a mixed construction.