STAR LETTER: Rules of thumb

Studying table 1 the writer proposes to increase the boiler capacity by 20%, if you want to increase the design outside air temperature from -4°C to -1°C. First of all, a temperature difference of 3°C does not at all relate to a heating load increase of 20%, it would be 13% only. But what's the point of it anyway? Is it usually the boiler that is undersized when the internal design air temperature is not reached? Isn't it most of the time that the distribution pipework or even more likely the radiators have been undersized? When will the building ever need as much as the undiversified total design heating load anyway?

It's obviously up to the relevant engineer to define where redundancy is appropriate. But please do it at the right place. I recommend you invest the money saved by a smaller boiler into the heat recovery of the ahu, which will make the boiler even smaller again.