An over-sized boiler is inefficient.
Boilers should be sized to provide sufficient heat to provide hot water, and warm the dwelling to the required comfort temperatures when the temperature outside is below freezing. (Historically, this has been -1°C although more recently -3°C is being recommended).
This means that for most of the heating season, during the times of day when comfort temperatures are required, the boiler will produce more heat than can be used by the system.
The heat excess to requirements is lost via the flue through convection. The cooler air entering at the base of the boiler cools it so that it fires-up again, even if the system does not need any heat at that time. When the internal thermostat is up to temperature the burner is switched off, but the heat produced is going nowhere except out of the flue. The boiler is doing what is known as short-cycling.
If a boiler is over-sized even when it is freezing outside, the amount of heat wasted is significantly increased throughout all of the heating season.
This Building Research Establishment method for calculating the size of a replacement boiler, will ensure your new boiler will be as efficient as possible.
If the dwelling floor plan is not rectangular, for example T-shape or L-shape, divide the floor plan into two or more rectangular sections and repeat the calculation for each section. Enter the allowance for hot water in one section only, changing the allowance to zero in the other section(s). Add together the results of the sections.
If the dwelling has a single storey extension, add the output from any radiators there in the box provided.
Here we go.......