Boiler sizing does not conform to rigid formula. Experience and insight, taking into account, demand and expectation of client is necessary. The old method of looking at heat loss of property and adding a few Kw for DHW will prove inadequate for well insulated homes if the fast recycle times of cylinders is to be achieved. Quoted recycle times, raising 70% of water through 45 C can also be misleading. If thought necessary to install 210 litre DHW cylinder I would size the boiler to heat all 210 litres in 30 minutes ie 22Kw and assume that during those 30 minutes the ambient temperature of the rooms would not be grossly effected. In less well insulated property or commercial setting with high DHW demand further consideration is necessary.