It sounds like you have cracked it to me. You may need to factor in a small additional amount for domestic hot water heating. 

Given you are using weather compensation already you will also know the potential flow temperatures needed and have a fair idea of the COP you will obtain too. 

Many thanks @bontwoody. I had thought I needn't worry about the need for hot water heating, but on reflection you're right because if it takes (say) 60min to heat the water, that's 60min when the house will have been losing heat, which in principle I'll then need to replenish on top of the steady-state requirement. Also, that's a good point about being able to predict my COP - I shall have a look for the relevant technical specifications.

Posted by: @jamesw

↑

Would there be anything to gain from calculating the heat transfer coefficient?

You say that you have measured your hourly gas usage during cold days and so have a good idea of your heat load. In which case you already know your heat transfer coefficient from the sounds of it. For example if you were needing 4kW when it was 0C outside and 20C inside, then your HTC is 4000/20 = 200 W.

Is this what you are asking or have I misunderstood?

The excellent article that you have referred to suggests that to get a more accurate figure for the HTC, you need to collect your hourly gas data on multiple days, but it sounds as if you have already done that.

One thing the article doesn't mention is that some radiator outputs and some heat pump COP claims are a bit like car manufacturers' mpg figures. Otherwise, it is one of the best ASHP articles that I have read, especially as he didn't even have a heat pump when he wrote it.

It would be worth doing your own heat loss survey as they are not difficult to do and will give you an idea of what a professional survey might suggest. These are frequently quite different from those calculated from gas usage, but unfortunately have to be used to size the heat pump due to MCS rules.

Hi @mike-h, many thanks for (unlike me) spotting the blindingly obvious. You are right that my measurements lead very directly to the HTC. However, I think (although I may be misunderstanding) that HTC is just a means to an end, namely that it can be used to calculate the heat load in the coldest conditions for which the system is designed. Since I am able to measure that directly, the HTC is a bit of a sideshow.

Although I knew that MCS installers have to do a heat loss survey, I hadn't realised that the rules oblige sizing of the heat pump to be done according to that survey. If the survey comes out with a suggestion that differs from my empirical evidence, I will take a lot of convincing to install a bigger or smaller heat pump than I know to be what is required in practice. My experience with some fairly sophisticated thermal modelling before I started retrofitting my house leads me to be sceptical of its accuracy compared to actual measurements.

I will therefore look at doing my own survey as you suggest in order to be forewarned.

@jamesw The MCS surveys can vary greatly so stick to your guns and find an installer who is willing to fit what you want. You will probably find some will be horrendously oversized. Similarly the size of the DHW tank can be an issue if you have a big house with low occupancy.

@bontwoody If retrofitting an ASHP after the brood have flown the nest, would it not be sensible to take into account the next property owner's likely DHW requirements when specifying the cylinder size? I imagine that many people plan to spend their offspring's inheritance on improving their home rather than downsize.

@jamesw

It is very unlikely, if not almost impossible, to achieve an accurate heat loss calculation, because some of the factors upon which the calculations are based are estimates and/or assumptions, while other factors are not included at all.

If my understanding is correct a heat loss calculation involves measuring the walls, floors, ceilings, windows and doors of each room and then applying a factor dependent upon the level of insulation that each material provides. While the dimensional measurements may be quite accurate, the actual insulation factor is unlikely to be so.

A further point of error will be the estimated rate of air change each hour, which can easily be overestimated or underestimated. The actual value may also vary dependent upon the wind speed and direction.

Factors that are not taken into account are solar gain, wind chill, rain affect and human activity, which can have a positive or negative effect on the true heat loss.

I would be more inclined to believe the figures obtained from your gas consumption, rather than a heat loss survey that does not consider the above mentioned factors.

@ianmk13 It just depends on individual circumstances. It’s not uncommon for a single person to live in a 3 bed house with no intention of moving and get stuck with an oversized cylinder. 

In addition to potential siting issues and capital costs, depending on the cylinder a larger amount of water may need to be heated every day with its associated losses and costs. 

I have heard that some installers do give a bit of wriggle round with this. 

@bontwoody I think the system we have (Sunamp Thermino) is a move in the right direction to reduce this wastage. Though the capacity is the equivalent of 210 litres, if unused, the heat lost to the environment is low (less than 0.75 kWh per day). The unit is in our airing cupboard and thus any losses are ‘always useful’; usage in our case is ~3.5 kwH per day in summer inc. any losses but we have the potential of approx. 11 kWh stored  at any one time, should we need it. Regards, Toodles.

@toodles Certainly another viable option!

@toodles May I ask about the Thermino? I understand that it uses phase change of the chemical but does this occur all at once? Does it sit there charged up waiting to be triggered and if so what triggers it? The videos I watched did not really explain this.