@windymill, what’s Octopus Energy’s justification for retaining £1,500?

Thats crazy and I would say you are probably due a refund of your £1500! 

I have often argued (and nearly as often got shot down) that real-world practicality is an important part of working out a retrofit solution.  There are a fair few installers who would rip up floorboards to replace pipework that they deem marginal (often it isn't marginal at all), in preference to finding a way to do it without that disruption.  This is nonsense to my mind, because there are always alternative solutions.  Similarly a 250l DHW tank is lovely, but only if it will fit.  Otherwise you need another solution.

Before we got our ASHP, which btw is excellent, I spent a lot of time thinking about pipe routes, cylinder configuration, location of controls etc.  Its my house, I know it better than any installer ever can, and I also know how I use it, which no installer ever will.  A few tweaks at design time or when its installed can make a big difference to the usability of the outcome.  

And apparently it can modulate down better than most other brands, so oversizing should be less of a concern?

The table here does indeed show a good min output for its size, 3.3kW @ 15C/28C, actually very slightly less than my Vaillant 7kW (although based on fewer results both from the same system).  Goodness knows why they don't sing from the rooftops about this if its correct!

Its notable that the system in question is apparently running at a fairly low SCOP (3.3); its possible that the output is underestimated due to sensor mismatch.  The deltaT when the measurements were done was (unsurprisingly) only 2C, so any sensor error would have a significant effect, but the min output figure is still very encouraging.

The current installer I'm in talks with would be including the Grant volumiser 

As mentioned already make sure its plumbed as a volumiser not a buffer!

We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.

Of course you can get the lower flow temps without it being oversized by just putting in bigger radiators!

Indeed. My point was that if you design for a flow temp of 50C at -3C based on a 10kW heat loss, and in reality the heat loss is more like 7kW then you will be able to run at lower flow temperatures to achieve that lower heat loss figure, all else being equal.

It's a detached house with a fairly large split-level entrance/hallway/landing area that's all under a vaulted ceiling with velux windows. the kitchen also has 2 windows, one of which is 2x1m, as well as an external door. Then there's an extension with french doors, a decent sized window, and 4 velux windows. I think these 3 areas alone make up a significant amount of the heat loss. 

@petch just wondering how your averaged heat loss survey data compares to actual usage data. Is your existing boiler gas, and if so do you have access to half hour data (or daily data) for gas usage from a smart meter?

Maybe you could look back to the coldest day this winter to see what the actual usage was, as a sense check for how that 10kW heat loss figure stacks up. Estimating the efficiency of the old boiler may be a bit of a guess, depending on it's age and whether it was being run in condensing mode or not, could be anywhere from 60-90% efficient. But it's one more data point to satisfy yourself.

We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.

I don't really have accurate usage, our gas meter hasn't been working over winter. We've got it replaced now and today, to keep the house at 15c all day whilst it was around 6c outside, has used 40kWh of gas (including DHW). I still don't know how useful that is as the boiler as old as the house (25 years) and if my boiler engineer is correct it's massively oversized for the house, so is probably quite inefficient.

I've no idea what the DWH is doing because it's a Hive unit but I've never managed to get through to them on the phone to transfer the physical hardware to my Hive account as this was all installed by the previous owner, and there's seemingly no way to access the web portal without having their support do something first, but that's another problem and at this point one I'm not really bothered about solving since all that will be going. All I know is the water coming from the cylinder is scolding hot all the time!\

I'm pushing for a Grant R290 unit for a few reasons; one is that their manual states it can go next to air bricks which is basically a requirement for us because there are 3 in close proximity to the most convenient install location. On paper it's very quiet and is one of 2 12kW units I've found that would pass the sound check for a neighbouring window. And apparently it can modulate down better than most other brands, so oversizing should be less of a concern? The current installer I'm in talks with would be including the Grant volumiser 

We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.

Of course you can get the lower flow temps without it being oversized by just putting in bigger radiators!

@petch just wondering how your averaged heat loss survey data compares to actual usage data. Is your existing boiler gas, and if so do you have access to half hour data (or daily data) for gas usage from a smart meter?

100% agree with this, but perhaps I am a bit paranoid about oversizing.

Maybe you could look back to the coldest day this winter to see what the actual usage was, as a sense check for how that 10kW heat loss figure stacks up. Estimating the efficiency of the old boiler may be a bit of a guess, depending on it's age and whether it was being run in condensing mode or not, could be anywhere from 60-90% efficient. But it's one more data point to satisfy yourself.

We overestimated our heat loss (mainly due to overly high ACH figure used in older MCS calculations). Besides our heat pump being oversized, it has mainly resulted in allowing lower flow temps which of course is no bad thing.

The house is around 180m² and probably somewhere around 10kW heat loss if we average out the heat loss surveys

This positions your house as a serious outlier on the chart below from heatpumpmonitor, which depicts measured, not surveyed, loss vs floor area.  The consequences, if it turns out that that the loss is in fact lower, can be managed, and certainly (high) system volume - referring to the discussion above - is part of managing it.  Thus suggestions of a sizeable volumiser (not a buffer) should probably be taken seriously unless you can somehow narrow the loss down.