I would not use a SAP calculation as the basis for boiler size,

I would also add more insulation during the reverb while it is easy.

Thanks both.

We're putting in the maximum thickness of Pavadentro for IWI and 200mm of graphite EPS EWI where possible. We're not expecting all of our hot water to be solar, especially with visitors, but maybe a 300l store would be enough for the 3 of us most of the time?

The peak monthly requirements have been calculated as 2065 kWh/month for space heating and 270 for hot water (although I wonder if 270 for HW is only for 3 people and might therefore need to be increased).

You can use SAP if you get access to the full 'SAP Worksheet' - I use Stroma's Free FSAP 2012 which generates a number of reports including the 'SAP Worksheet' which contains the detailed energy calculations. You may be able to get a copy of the worksheet from whoever did you SAP calculations - I am not sure whether all rdSAP calculators provide this detail though.

Calculation result 39 from the full 'SAP Worksheet' (standard SAP 2013 nomenclature) gives you the heat loss parameter in Watts per Kelvin (both monthly and annually). So for example if you want to calculate the maximum heat requirement, and your heat loss parameter is 300W/K , then multiply this by the maximum difference you might expect between inside and outside temperatures for your region, for example (20C - -5C) = 25C temperature difference. So 300W/K x 25C = 7500W or 7.5kW maximum heating requirement - sometimes people add a margin on for this, but you generally ignore all other heat gains in a home (solar, appliances, people) - for the purposes of this calculation. If you have a very energy efficient home, then your hot water requirements might override and determine this maximum (not an issue in your case?).

My gut feel, given the information you provided is that you boiler and hot water specifications are about right. If you are concerned I would just pay for a more powerful boiler, the cost increment is generally negligible, and it would allow you to refill the HW cylinder quicker if there are 6 of you in residence.

The calculations outlined above using SAP (or the also free MCS Heat Pump Calculator spreadsheet) are more critical for air source heat pumps where the incremental costs of higher capacity heaters/boilers are much greater. There are also a number of other online calculators for this type of calculations, but they are often crude.

You are right, oversizing the boiler might impact the efficiency, depends on the total output of your emitters (radiators), flow rates, and what the capacity boiler can modulate down to. You sometimes find that within a range of boilers the minimum modulation power is the same despite the boilers have different maximum outputs. You really need to look at the specs, but boilers generally modulate down to 15% to 25% of max power, and some will adjust flow rates, cycle to try to ensure low enough return temperatures for efficient condensing to occur. Unfortunately I don't have enough knowledge to be able to help with this detail, perhaps someone on the forum can help, or perhaps better most manufacturers technical departments would be able to help you decide?

Something's a bit fishy with the figures. 2065 kWh for the worst month gives an average power for that month of about 2065/(30*24) = 2.86 kW whilst 310 W/K gives 7.75 kW for a 25°delta T or 9.3 kW for a 30° delta T. You don't say where you are but -10°C is perhaps more realistic for a minimum temperature, IMHO. Anyway, an 18 kW boiler sounds plenty big enough. Lots of insulation on the thermal store is a good idea, and buying from somebody who understands that the world is not flat will also help

One point I forgot to mention. Our house is all-electric and our thermal store has two immersions, one at the bottom and one half way up. The lower one is connected to my solar diverter and the immersion's thermostat is set to 90°C to maximise stored PV energy. The centre immersion has its thermostat left at the default of 65°C and is connected to a mains timeswitch. During summer, the centre immersion is never switched on. During winter, it's programmed to come on for a couple of hours in the E7 cheap period. So we generally always have hot water available and the PV always has somewhere to add at least some heat, whilst we minimise standing losses from paid-for electricity. If we ever seem likely to run out, I just press boost on the timeswitch to make some more.

In your situation, I'd suggest you make sure you have an immersion at the bottom for your PV input and try to ensure the heating coil for the boiler (or boiler flow and return connections) isn't quite at the bottom

Posted By: katesOur SAP predicts the house will need a total of 17660 kWh/yr input

That seems high to me, try working it out again.

Space heating at DHW do different things, and different temperatures and at different times, the 3Ds. It is best to split them up totally in my opinion.

Water usage could be worked out from your current usage, just needs a thermometer, a jug and a stopwatch, then a bit of time logging the data and working out what you are really using.
Reduce it down to a person/year usage and then multiply by the number of people, then you have the expected range of usage.
If your 'guests' use it all up, tell them to stop!

Any storage of thermal energy will have losses, the current methodology assumes that the store is heated up, then the stored water is used immediately. This is far from most peoples usage patterns.
You can calculate the real losses from the thickness of the store insulation, the temperature difference between water and air and the surface area of the store, just like you work it out for the house walls.

Posted By: katesOne of us has a medical condition that requires plenty of mains pressure HW for decent showers.

As you mention a vented system, you will be using a pumped shower, so mains pressure (and flow), is irrelevant here.
I like vented systems, they are generally easy to install, are reliable and cheap. Others may disagree.
Depending on your water hardness you may want to fit a water softener.

Realistically, using PV to heat the water is the best local usage for it. You can do a calculation as to the expected proportion that the PV (from PVGIS) will contribute and then decide if you really need to plumb the store into the gas system.
So say that each person uses 750 kWh of hot water a year and your PV can supply 80% of that for 8 months of the year and 20% for the remaining 4 months.
If electricity costs 15p/kWh then:
750 [kWh/year] x 0.66 [fraction of year] x 0.2 [electricity from mains] x 0.15 [p/kWh import) = £14.85
750 [kWh/year] x 0.33 [fraction of year] x 0.8 [electricity from mains] x 0.15 [p/kWh import) = £29.7
Total per person £44.55

If my arithmetic is right (and it is often wrong, so needs checking), is it really worth plumbing into the gas boiler?

It all comes down to the numbers in the end, so until they are sorted, it is all guess work.

+1

Yes SteamyTea, what were you thinking, you must always trust the supplier of goods and services, they know best!