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For houses and flats


Replacement boilers are rarely sized correctly. Oversized boilers cost more to purchase and generally operate less efficiently resulting in higher running costs and increased emissions to atmosphere. This 'whole house' procedure provides the busy heating installer with a simple but reasonably accurate method of sizing which is both quick and easy to use.

This method is aimed at typical dwelling types found in the UK as indicated by the U-values and other factors used (see TABLES below). Where the dwelling is untypical, then a more detailed procedure should be used. The method should only be used for gas, oil and LPG boilers up to 25 kW and should not be used for combination boilers or solid fuel heating systems. It is based on the assumptions shown below.



  • a design internal temperature of 21°C (included in location factor).
  • design external temperatures, dependant on the location of the property (included in the location factor).
  • an allowance of 10% for intermittent heating (included in the location factor).
  • an allowance of 5% for pipe losses (included in the location factor).
  • a ventilation rate of 0.7 air changes per hour (included in the 0.25 ventilation factor).
  • an allowance of 2 kW for heating hot water.


Factors used

Window Factors

Detached 0.17
Semi-detached 0.2
Mid terrace 0.25
Flat 0.25

Window U-values

Double glazed wood/plastic 3.0
Double glazed metal frames 4.2
Single glazed wood/plastic 4.7
Single glazed metal frames 5.8

Wall U-values

Filled cavity wall 0.45
Unfilled cavity wall 1.6
Solid wall 220mm 2.1

Roof U-values

Pitched < 50mm insulation 2.6
Pitched 50-75mm insulation 0.99
Pitched > 75mm insulation 0.44
Flat uninsulated 2.0
Flat 50mm insulation 0.54

Location Factors

     North & Midlands 29
Northern Ireland 26.5
Scotland 28.5
South East & Wales 27
South West 25