New Standard Published – Update to BS EN 303-5

In 2021, BSI published details of the updates to the requirements, testing and marking of independent boiler appliances standard, BS EN 303-5 Heating boilers for solid fuels, manually and automatically stoke, nominal output of up to 500kW.

Independent boiler appliances are regulated under the Machinery Directive 2006/42/EC, which is transposed into UK law under the previously recognised Supply of Machinery (Safety) Regulations, which require appliances to carry the applicable CE (UKCA) mark.

The changes to BS EN 303-5 see new technologies brought into the scope of the requirements, and it is important for approved manufacturers to be aware of the changes should any innovative products fall within the new categorisation. These include;

  • Condensing boilers with a heat output of below 500kW – these are boilers that under normal operating conditions partially condense the water vapour from combustion products to make use of latent heat for heating purposes
  • Boilers connected to an outside combustion air supply at heat output below 100kW – these are appliances that take all or partial amounts of air directly from outside the building using a pipe directly connected to the boiler air inlet
  • Considerations for boilers meeting the essential requirements of the Machinery Directive, Ecodesign Regulations and Energy Labelling Regulations

New safety requirements have been proposed to ensure condensing boilers are designed in such a way that the flue gases cannot pass into the atmosphere of the room through the condensate discharge system and that any blockage of the drainage system to prevent this shall not cause increased risk under operation.

Additional requirements for external air appliances include tests for leakage rates and ensuring that those technologies which take a partial amount of air from the room in which the product is installed operate at a minimum 20Pa differential pressure within the combustion chamber in comparison to the room of installation. New provisions within the Ecodesign legislation are transposed into the new standard, requiring that solid fuel boiler products meet the maximum emission limits stated within Table 8 below;

 Further information and details on the updates to the standard can be found by contacting the HETAS product approval team at or by calling 01684 278196.

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