Ordinarily you might expect very well insulated new homes to use smaller boilers. They don’t. Nor does systematic oversizing bring any benefit for baths or showers, because water supplies are inadequate to use the maximum boiler output.
I visited a large development of new homes last month where all of the houses had 35kW combi boilers. (For comparison, the ‘typical’ boiler size for average-sized existing homes and ten radiators is 24-28kW.) Some of the houses had two bedrooms, others three or four. The houses were carefully designed and insulated, and reasonably well built. They were intended to measurably out-perform the energy requirements of current Building Regulations. (I was there to help ensure that the local planners’ high aspirations for good energy and carbon performance were actually achieved on site.)
If the boilers in these homes were sized according to space heating requirements alone, and assuming a heat-loss coefficient of 160 W/K for the larger houses and a temperature difference of 22°C on extreme winter days, total heat loss would be about 3.5 kW (160 x 22 / 1000), so a small boiler of, say, 12 kW would provide sufficient heat to bring the house up to temperature in an acceptable warm-up time.
However, given that there is no hot water tank, the boilers are sized based on hot water demand rather than space heating (following the usual practice). Vaillant’s 35 kW combi-boiler can generate 14.1 litres/minute with a temperature rise of 35°C*. If water entering the boiler starts at 12°C this suggests a hot-water supply temperature of about 47°C.
A typical shower head is designed to provide 8-10 l/m, but remember that this is usually mixed with cold water too. In winter, for the sake of argument, let’s say the mix is 1/4 cold and 3/4 hot water. This means you need roughly 6 to 7.5 l/m of hot water for a shower. Having the 35 kW boiler theoretically allows you to have two good showers simultaneously, or to fill a bath and have a shower at the same time. (A typical modern bath tap has a flow rate of 12 l/m, but this is less significant because if the flow-rate falls you just wait a little longer to fill the bath.)
HOWEVER, the water supply companies are required to supply a minimum of 9 l/m downstairs (at the outside tap), although 10 l/m is usual. The flow rate is likely to be lower upstairs. Arguably, there is no benefit in installing a boiler that can heat more than 7.5 l/m (7.5 l/m or hot water, mixed with 2.5 l/m of cold) unless you know in advance that the water company can supply more than this. Remember too that the pressure of water supplied varies through the day, with lower pressure (=lower flow rate) at peak times, when people are most likely to shower or bathe.
I would argue that unless you know you’ll have a water supply from the utility that is way above the regulated minimum, it is pointless to install a 35 kW boiler. This means the boiler will almost always be modulating down output in space heating mode (because it is three times larger than necessary), which means it will not operate at peak efficiency. Nor will it operate at full output in water heating mode, because the water supply is inadequate to use all the heat from the boiler. Vaillant’s 30 kW boiler (heating 12.2 l/m), or even their 25 kW boiler (heating 10.4 l/m) is quite big enough for a water supply up to about 13 l/m.
At the housing development I visited, considerable efforts to cut heat loss by insulating properly and providing air-tightness are being eroded by boilers so big they cannot run at peak efficiency. It is widely known that supplying hot water is a more significant contributor to carbon emissions in new, low-energy homes than space heating. Less well understood is how systematic over-sizing of boilers hits boiler efficiencies and actually rebounds on carbon emissions from both hot water and space heating – with no gain in most cases because the water flow rate is too low to use the boiler at full whack.
*Average bath temperature is reckoned to be 40-42°C.