Heat loss calculations – OFTEC technical notice 38


This technical notice explains a change to building regulations guidance that requires appropriate detailed heat loss calculations to be undertaken when installing a heating appliance. This notice applies to installations at domestic and non-domestic buildings.

Heat loss calculations – OFTEC technical notice 38

Building regulations guidance

Governments in all regions issue building regulations guidance documents, such as Approved Documents in England and Wales. These documents set out what is reasonable provision for compliance with building regulations. Technicians who follow the guidance in these documents benefit from a presumption of compliance with the building regulations.

Beginning in 2021, building regulations guidance documents in England, Scotland, and Wales have been amended to require that appliances and heating systems should not be significantly oversized, as this reduces energy efficiency. New guidance states that the specification of heating systems (including appliances) should be based on both of the following:

  • An appropriate heat loss calculation for the dwelling.
  • A sizing methodology that takes account of the properties of the dwelling.

For many years, technicians have carried out detailed room-by-room heat loss calculations and used the results to specify new heating systems, including the heating appliance and emitters. However, for appliance replacements, many technicians used a simplified method – “The Energy Saving Trust’s CE54 Domestic Heating Sizing Method” (often referred to as the ‘whole house’ method). As explained below, this method is no longer considered appropriate.

In England, before publishing the final version of Approved Document L Vol 1 2021 (ADL), the Government published a draft version for industry and the public to comment on. The draft version listed the CE54 ‘whole house’ sizing method as one suitable option for boiler sizing. However, this method was not included as a suitable method in the final published version of ADL. Explaining why, government stated: “In response to concerns that CE54 may lead to oversizing, it is no longer referenced as a method”1.

The Welsh and Scottish governments have subsequently published the same guidance for sizing heating systems in their equivalent versions of ADL. Other regions are highly likely to follow suit.

It is therefore evident that regional governments expect more detailed heat loss calculations to be undertaken to ensure that boilers are not oversized.

Appropriate method

There are many room-by-room heat loss calculation tools available to technicians. Some are robust and accurate, while others include many assumptions about the property and are as likely to lead to oversizing as the ‘whole house’ method.

OFTEC is happy to endorse the room-by-room heat loss calculation method found in the Domestic Heating Design Guide2, as this is based on the recognised standard for calculating the heat load of a building – BS EN 12831-1.

However, many prefer the use of digital tools. To ensure that a room-by-room digital heat loss calculator is appropriate, technicians should verify it complies with BS EN 12831-1.


Technicians should keep records to demonstrate that their installation work complies with building regulations. Such records act as a protection against claims of poor workmanship and may be asked for by a building control officer or an OFTEC inspector. As part of OFTEC’s rules of registration, OFTEC registered businesses are required to maintain records of works for a period of no less than six years from the completion of works. Such records include system designs and heat loss calculations.