Sizing a Boiler for a House on the Ridge
The usual sign that an exposed Hartshead house has the wrong boiler, or a tired one, is a system that copes in October and falls behind in January. When it comes to replacing it, the instinct is to buy something bigger. Sometimes that is right. Often the old boiler was adequate and the shortfall lay elsewhere: radiators that have lost output to sludge, a circuit that sends most of the heat to the rooms nearest the boiler, or both at once.
So the survey looks at the whole house. We go through the rooms and radiators, note which walls face the weather, and check the state of the water in the system. The quote then recommends a boiler that suits the building, and if a flush or rebalancing would make more difference than extra output, it says that too. A condensing boiler also works best when it is not oversized, because it spends more of its time running efficiently at low output instead of firing up and shutting off.
Hot water is a separate question from heating. A farmhouse with a big family and two bathrooms can need a system boiler with a cylinder even where the heating load alone would suit a combi, and the survey weighs both before a model is chosen.