Building Services Products Ltd



When sizing a Side Stream Filter which is to be installed into a Heating or Cooling System there are certain factors which need to be observed and implemented to make ensure the Side Stream Filter complies with the relevant Governing Standards.

The Building Services Research and Information Association BSRIA who are recognised by the Chartered Institute of Building Services Engineers CIBSE produce and regularly update the Industry Standards for the treatment of water in Closed Heating and Cooling Systems. These Standards are based on research and consultation with industry specialists and are widely used and adhered to within the Construction Industry. The latest BSRIA Standards which have an influence on sizing and the use of Side Stream Filtration are:

  • BG 29/2020 Pre-Commission Cleaning of Pipework Systems
  • BG 50/2021 Water Treatment for Closed Heating and Cooling Systems.

Below are extracts from the BSRIA Standards (BG29 & BG50) which identify where and how to size a Side Stream Filter correctly.

BSRIA BG 29/2020 – Section 2.3.8 – Side-stream filtration / separation
“Side-stream filtration is suitable for most systems and strongly recommended for systems over 2,500 litres.”

BSRIA BG 50/2021 – Section 3.4.2 – Side-stream filtration
“Side-stream filtration can be used to help maintain a low concentration of suspended solids in the circulating water and reduce the risk of solids deposition and under-deposit corrosion in areas of low flow.”

Ideally, side-stream filters should be located next to either the system flow or return pipe with their own circulating pump to provide flow or on either side of a system pump.”

“If suitably located, a small proportion of the circulating flow is passed through the filter on a continuous basis. If the filter becomes blocked, then the risk of corrosion can be increased due to bacterial colonisation. Filters should therefore be fitted with pressure differential indicators, frequently checked and properly maintained.”

BSRIA BG 50/2021 – Section 5.3 – Physical Water Treatment

“In side-stream filtration a small proportion of the circulating flow is passed through the devise on a continuous basis as discussed in section 3.4.2. The side-stream is sized so the total system volume should be able to pass through it in a 24-hour period.”

BSRIA BG 50/2021 – Section 3.4.1 – Filtration

“Mains water is not sterile but contains a wide range of bacteria that are harmless to people but can become implicated in corrosion processes. Low pressure membrane filtration (with a mesh size of less than 0.5µm) is increasingly being used to provide a physical barrier to bacteria and fungi entering the system from the water supply. If effective and maintained, these fine filters can reduce the need for biocides for subsequent microbiological control.”

BSRIA BG 50/2021 – Section 6.4.1 – Filtering

“Removable bag and filter cartridges do not have backflush requirements and precautions must be taken to ensure that the media does not become microbiologically contaminated.”