D Halisham you seem to struggle along with Simond that central heating is not designed to only reach 21 degrees when its minus 1 outside strange you both use the figure minus 2.
I'm not struggling at all. I don't know why Simond used -2C in his example; I just used it to maintain consistency. In any case using -1C is no longer recommended; the temperature varies regionally. See
Whole House Boiler Size Calculator or the latest version of CE54 - Domestic Boiler Sizing Method, published by the Energy Saving Trust.
The rad sizing is to get it to 21 in a set time (this is not the maximum temp the house will get to) otherwise the heating could be on all day to eventually get it there, its in the old books that you wont find in your google search.
Firstly, I don't need to do google searches to find this out; it's simple physics. Yes, if the radiator was sized to match
exactly the heat loss when the room temperature was 21C, it would take longer to reach temperature. That's why radiators are oversized by 10-15%. This is also done to allow TRVs to control the temperature better.
Radiators are sized on the heat loss, which depends on room geometry, U-values of the materials used and the maximum anticipated temperature difference. Time does not come into it. If you can prove otherwise .... .
So using your calculation in the winter of 2010 when it was minus 17 outside even with a 30kw boiler a house would only have got to only 5 degrees eh don't think so . Or in the worst places at minus 26 the houses would not have got above freezing.
The answer is air changes! Heatloss calculations allow for up to three air changes per hour, depending on the room. This can account for up to a third of the heat loss from a room. When it is very cold, the first thing anyone does is to close all windows to prevent the heat escaping. This reduces the air changes, so more heat is available.
In winter of 2010 did your own house heat up to perfectly ok
Yes, but that's because the house has been double glazed and loft and cavity insulated. It still has the original radiators which are now twice as large as necessary, so there is plenty of spare capacity for very cold days.