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.......
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Dwelling Type:
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Detached:
Semi-detached:
Mid-terrace:
Flat:
If a Flat, which floor is it on?
Bottom floor:
Middle floor:
Top floor:
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Select type of dwelling.
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Internal Measurements
(in metres)
and Number of outside walls:
and Number of floors:
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Front to back: metres
Number of walls this long
Side to side: metres
Number of walls this long
Room Height: metres
Number of Floors:
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For the front to back, and the side to side dimensions, you could measure outside the dwelling and subtract the thicknesses of the outside walls.
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Outside walls type:
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Filled cavity wall:
Unfilled cavity wall:
Solid 220mm wall:
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Window Type:
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Double glazed
wood/plastic frames:
metal frames:
Single glazed
wood/plastic frames:
metal frames:
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If there is a mixture of double glazed and single glazed, select the single glazed option.
If the window areas are unusually large, enter the actual total area of windows here, to 2 decimal places:
square metres
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Roof Type:
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Pitched, less than 50mm insulation:
Pitched, 50 to 75mm of insulation:
Pitched, more than 75mm insulation:
Flat and Uninsulated:
Flat with 50mm insulation:
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If there is an extension....
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....add any radiators output in Watts.
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Allowance for
Domestic Hot Water:
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Change hot water allowance if necessary
Enter Watts:
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2000 Watts is the basic allowance. However, some of the latest cylinders have coils that can use more heat for a quicker and more efficient heat up.
Also, if the control system gives priority to the hot water, then this number could be reset to zero.
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And finally....
Where is the dwelling
located in the UK?
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North or Midlands:
Northern Ireland:
Scotland:
South East or Wales:
South West:
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