Welcome to the IDHEE on-line calculator for the Crown Copyright document "WHOLE HOUSE BOILER SIZING METHOD".
It is offered by the IDHEE as a service to members, and for consumers to check the boiler size offered by a non-member.

Whilst the IDHEE can accept no responsibility for the factors used in the calculation, it recommends its use when changing a boiler to ensure unnecessary over-sizing does not occur.
As the resulting boiler size may be significantly smaller than the unit to be replaced, it is important to realise that the recommended boiler will operate far more efficiently than the over-sized boiler being replaced. (This will be in addition to the higher efficiency of the newer boiler). It may be useful to show the customer a copy of the method work sheet, along with a printout of the result of the calculation.

The method work sheet is available from the BRE on 01923 664258.

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 -1C although more recently -3C 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.......

Dwelling Type: Detached:      

If a Flat, which floor is it on?
Bottom floor:      
Middle floor:      
Top floor:      
Select type of dwelling.
Internal Measurements
(in metres)

and Number of outside walls:

and Number of floors:
Front to back:        metres
Number of walls this long  

Side to side:        metres
Number of walls this long  

Room Height:        metres
Number of Floors:      
These are the outside walls only, i.e. through which heat will be lost to the outside.
e.g. In a mid-terrace dwelling the number of front to back outside walls will be set to 0 as they will be adjoining a neighbouring dwelling, so there is no heat loss to the outside.
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.
Outside walls type: Filled cavity wall:      
Unfilled cavity wall:      
Solid 220mm wall:      
Window Type: Double glazed                                          
wood/plastic frames:      
metal frames:      

Single glazed                                          
wood/plastic frames:      
metal frames:      
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
Roof Type:
Pitched, less than 50mm insulation:      
Pitched, 50 to 75mm of insulation:      
Pitched, more than 75mm insulation:      
Flat and Uninsulated:      
Flat with 50mm insulation:      
If there is an extension....
....add any radiators output in Watts.        
Allowance for
Domestic Hot Water:
Change hot water allowance if necessary
Enter Watts:      
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.
And finally....

Where is the dwelling
located in the UK?
North or Midlands:      
Northern Ireland:      
South East or Wales:      
South West: