Posted by: @andrewj

↑

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

↑

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!

@jamespa Thanks James, very helpful.  From the proposal and some email threads I've seen, the installer comes over as pretty competent.  Obviously the buffer is a thing at the moment but at least it's not an automatic install for them.  I think the background for it, from the little I can see, is that some of the rooms have microbore and that the pump in the Vaillant may not be sufficient to reach a necessary flow rate across the whole house given the restriction.  I don't know, I'm not an expert, but it would interpret it that if it's marginal then the Vaillant's pump would need supplementing.

Posted by: @andrewj

↑

 I think the background for it, from the little I can see, is that some of the rooms have microbore and that the pump in the Vaillant may not be sufficient to reach a necessary flow rate across the whole house given the restriction

Really flow rate is a pipework limitation not a water pump limitation, you can always fit a pump with a greater head or a supplementary pump, but there is only so much flow you can get through a pipe of any size before it starts corroding the pipe or making too much noise.

It sounds like some heat transport calculations need to be done for the rads in question.  Basically can you get enough heat to them at a water velocity that is acceptable (normally taken as ~1m/s or less) with a deltaT that is acceptable (ideally < 5C but if it has to increase a bit thats probably better than fitting a buffer).  Do the calculation for the rad with microbore that needs the greatest output, if that one is OK then the others will be also.

Fortunately heat geek have already done the flow calculations here, these just need comparing against the required output at design OAT.

There is another estimator that you could use, a formula from Michael De Podesta's , which is simply annual gas usage divided by 57.3 multiplied by the temperature difference that you want to support (e.g. x24 for -3C outside to +21C inside).

For 21,056 kWh of annual gas consumption that would give 8820W = 8.82kW maximum heat demand

There is more information about this and the divide by 2900 rules in Michael De Podesta's video: