Posted by: @andrewj

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Brother in law has recently had a survey completed with the heat loss estimated at 9.73kW, a recommended 12kW Arotherm and a flow temp of 50c at -3c for a SCOP of 3.97.  There seems to be some suggestion of requiring a 30% oversize (a new thing recommended within the last 2 months - by whom, Vaillant??) because the units weren't tested with the UKs' high humidity so they struggled on cold days.  Obviously I don't know if that is oversized, I'm always a bit wary of a solution that involves a heat pump > 7kW so I'm trying to sanity check it.  On a separate survey, Octopus ended up recommending a 15kW unit but I don't know what they calculated the heat loss at.  

There is a thread on openenergymonitor which does suggest that the Vaillant Arotherm plus 7kW in particular doesn't deliver the (typically, depending on FT) 8.5kW it claims during defrost, and is more like its sticker capacity of 7kW, with the 5kW likewise delivering closer to its sticker output during defrost than the higher output stated in the capacity tables.

Dividing by 2900 is based on typical degree days in (I think) the South East of the UK.  If the house is allowed to cool a lot for a long time it will be an underestimate, but if it only cools by a degree or two for a few hours a day then the difference that makes is small.  Also factor in boiler efficiency.  If the boiler is fairly new and run at 55C or less then it might be more than 100% 'efficient', up to 110% in fact, sol the calculation based on 2900 may be an underestimate.  If its old and run at 75C cycling madly most of the time it will be a lot less efficient, so the calculation based on 2900 will be an overestimate.   If all the corrections are in the same direction then they can be worth worrying about, if they arent all in the same direction then less so..

Posted by: @andrewj

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The Vaillant pump tends to offer less head room in which case we would include a buffer."

er how does that help?

My total gas usage was 20-22MWh/year when I heated part time at 75C, controlled on thermostat and trvs.  It dropped to just over 18MWh/year when I heated full time at 50C (still controlled on thermostat and TRVs although the TRVs weren't doing much).  The 7kW Vaillant Arotherm plus is just about right for me.  It works fine down to -5 (we dont currently get colder here for any amount of time).  The figures include DHW but we dont use much of that and in summer DHW was heated from solar.

Depending on how your brother in law heats his house he may be a bit more than mine so possibly a bit too marginal for the 7kW unless he is prepared to use a bit of supplementary heating on the coldest days.  The simplest pption to squeeze 7-10% more out is just to heat DHW on the immersion on the coldest days instead of on the heat pump.  That said the Arotherm plus models seem not to lose too much in efficiency if they are oversized, judging by the data I have seen (but I couldn't quantify that statement).

if he has half hourly meter readings plot average daily use against degree days or average OAT and look peaks of usage over 3, 6 and 12 hours like the attached, which is in essence how my heat pump was sized.

Note that the new Arotherm pros have not yet been submitted to the same scrutiny so far as I am aware.

Hope that helps.  Without some more data he is definitely taking a fairy high risk going with the 7kW.  The new 12kW pro is, I think, single fan so its less of a bugbear visually to jump from 7 to 12 than it was with the plus range.

Another question worth asking - is there any opportunity to shave a kW off with insulation?  If so I would be tempted to run the numbers and consider that!