The whole house boiler sizing method is only suitable for gas ,oil ,and LPG boilers upto 25kw not combis ect.
25kW should be mean it is suitable for the majority of properties. If you need anything larger, you will probably be rich enough to employ a consulting engineer who will do all the heatloss calcul;ations for you and present you with a typewritten report and specifiaction so youcan go out to tender for the system.
It may not be directly suitable for combis, but if you size a combi boiler just on the hot water requirement there is a danger that it will be oversized for heating. For example a Vaillant 937 will produce 37kW for HW but 12-28kW for heating. Many modern houses only need 12-15kW for heating. The boiler is unable to modulate below 12kW, so when the weather is warmer it will revert to on/off control.
Internal design temp of 21c - seems reasonable -the maths is not difficult for a different temperature
External design temp dependant on location - sensible; winters in Scotland are colder than Devon
Pipe losses of 5% may be a bit low
Air changes of .7 per hour presumably an average
Allowance of 2kw for heating the hot water. that's the norm - can easily be altered
When i did the course i was advised NOT to include the 2k for the hot water as you want the condensing boiler to be working hard at nearly full load all of the time due to the fact that it is only efficient when it is in condensing mode . At all other times it is no more efficient than a non condensing boiler of the same capacity.
The boiler is in condensing mode if the return temperature is below 55°C. It has nothing to do with how "hard" the boiler is working.
Remeha system and heat-only boilers with weather compensation handle the Hot water in an interesting way - effectively using W Plan (HW priority). A diverter valve (not midposition) is used but installed the "wrong" way round, port A to cylinder, Port B to Heating. Port B is normally open so the heating circuit is open all the time. The boiler is controlled by the weather compensator and room thermostat. When hot water is called for the valve operates, closing the CH circuit and opening the HW circuit. At the same time the boiler disconnect from the weather compensator and automatically increases output to max. You therefore have the full boiler output available for heating the water, which will reduce the time required considerably.
The best method is to use a proper calculator like the Stars one or one from a radiator manufacturer. I have a Myson one which i got in a copy of a Central Heating Guide.
Agreed, but it may be too complicated for some users.
I have use Stars, Kermi and the Barlo one (no loger available) and they all give similar results and are close to the "whole house" calculator. The only problem with the detailed calculators is there are so many choices of materials that the average user will spend most of their time worrying that they have chosen the wrong U value for the floor or wall etc.
