Closed System Water Hygiene & Maintenance - BSRIA BG50 2021

Implementing a properly designed water treatment program is the first step towards maintaining acceptable water conditions and minimising maintenance and repair costs.

The objectives of a water treatment programme in closed heating and cooling systems are to:

  • maintain the system in a clean condition
  • assist in maintaining system efficiency
  • prolong system life through the control of corrosion and fouling.

A correct and appropriate water treatment programme will include:

  • Control of corrosion:
  • Control of scale: closed heating and cooling systems
  • Control of biofouling/biofilms:
  • Control of sedimentation:
bg50 screenshot

Corrosion damage in closed heating and cooling water systems

This is a common problem and facilities managers and maintenance engineers often ignore them. They don’t attract much attention until they spring a leaks or irreversible damage has occurred to expensive system component such as boilers etc. It is therefore important that closed systems are carefully monitored and regularly maintained to prevent corrosion and scaling occurring. Good corrosion protection is extremely important as the insoluble products of corrosion cannot be eliminated since there is no bleed off. These insoluble products form deposits in the system plugging lines and valves. Deposits also impede heat transfer thus increasing energy costs.

Closed systems are also prone to microbiological contamination which can result in severe waterside problems including degradation of inhibitors such as nitrite, local corrosion; gas generation and fouling by deposits.

Maintenance should continue throughout the life of the building and should involve BACTERIOLOGICAL AND CHEMISTRY sample taking at a maximum interval of 3 months. The BS8552 Sampling and monitoring of water from building services closed systems offers advice on frequency of sampling and locations.


Pseudomonas bacteria has the ability to cause corrosion to any open or closed water system. Once the bacteria is established it forms a jelly like film which clogs valves and reduces heat transfer. The anaerobic conditions created between this film and pipe work provide a breeding environment for other bacteria including SRB’s and NRB’s. It also creates a barrier to debris preventing a successful chemical clean if not treated properly.

As it is a living organism it will reappear if not completely destroyed. The results of pseudomonas within the system are often shown by changing commissioning results, greasy films on strainers and often as muddy reddy brown colouring to the water.

In simple terms these have been linked to corrosion.

Sulphate Reducing Bacteria (SRB’s)

This is one of the most destructive groups of biological foulants because they cause corrosion resulting in severe localised pitting of pipework surfaces. They have been implicated in severe corrosion problems for copper, iron, steel and aluminium pipe work. As a result of their metabolism, they produce hydrogen sulphide which attacks iron and steel and forms the end of produce ferrous sulphide. Steel thus becomes pitted, and cast iron becomes ‘graphitised’.

Nitrite Reducing Bacteria (NRB’s)

This bacteria gasses to produce ammonia. This poses a risk of stress corrosion cracking to brass/copper components such as valves and fittings. As with SRBs above these are anaerobic bacteria which tend to produce acid (corrosive) by-products. These as with SRBs dwell beneath deposits or in systems with limited oxygen.

N.B. Low/Medium NRB levels detected in isolation of any other Heterotrophic plate count is unlikely to be of concern and system(s) will be deemed under microbiological control.

Total Viable Count (TVCs)

The Total amount of bacteria expressed in colony forming units. High levels are often an indication of a dirty system. The dirt providing a hide out for the bacteria and ideal breeding conditions.