Biocides for closed systems

Choosing Biocides for Closed Systems – Closed Circuit Heating & Cooling Systems

Most organisations are keen to take steps to control the growth of micro-organisms which can flourish inside closed circuit water systems, including closed heating and cooling systems. Controlling the growth of bacteria in such closed circuit systems can also help to prevent the build-up of biofilm on the inside of pipe surfaces. It will also help limit the bacterial breakdown of any corrosion inhibitor chemicals that have been added to the water in the system.

Preventing the Growth of Bacteria & Biofilm in Closed Systems

So how do we stop bacteria growing in a closed system? This is usually done by adding a carefully selected biocide into the water.

However, adding biocide into the water isn’t the end of the process. It’s also essential to consider a number of other elements to ensure you achieve the best protection for your closed systems:

  • Pre-commission cleaning & flushing

    Before you begin to treat the water make sure that the system is clean and free of any debris or significant contamination. It’s good to start by cleaning the system before it comes into use. Details on pre-commission cleaning and flushing of pipework can be found in the BSRIA guide BG 29/2012. This sets out an excellent process for cleaning and flushing of closed circuit systems with appropriate biocides.

  • Types of biocide & optimum dose rates

    Think about both the type of biocide being used and the concentration of the biocide which is added when the system is initially filled.

  • Monitor your closed system

    Ensure you have a procedure to monitor your water system regularly, adding biocide as required. It’s not realistic to expect that one dose of biocide is going to be enough to treat a closed system which is heavily contaminated with micro-organisms, bacteria and biofilm. Sometimes, bacteria can become resistant to one type of water treatment chemical, and you may need to switch to another to achieve the desired level of protection. Laboratory testing of the water in the closed circuit may be required to establish which bacteria are present. This information can then be used to determine the most appropriate biocide to treat it.

  • Use a bio-dispersant with a biocide

    Think about using a both a bio-dispersant and biocide to combat both bacteria and remove any biofilm. Consider also where the biofilm is going to end up once you remove it; will it clog up your strainers and filters? Is there a potential for blockage? Bio-dispersants might not be suitable for all systems, so take specialist advice.

  • Hot water temperatures

    Look at the water temperatures achieved in your closed system. If the water in your system is consistently at temperatures of over 60oC, then it’s unlikely a biocide will be required as the water will be too hot for bacteria to survive and multiply. However, if you are only using your system at certain times of year (such as an air conditioning system in summer only) then add biocide to the system and allow it to circulate before you switch it off. It’s good practice to switch the system on and let it run for an hour a day even if you don’t need it, just to allow the water and biocide to circulate more effectively, and so provide a degree of protection.

Choosing Biocides for Closed Circuit Water Systems

There’s lots to think about when choosing a biocide to use in your closed system, whether you’re using it for pre-commission cleaning, a flush, system maintenance or to provide a “shock dose” to tackle a problem. Here are a number of important issues for you to consider when choosing a closed system biocide:

  • Contact time

    Think about how long the pipework flushing process is going to take, and choose a biocide which has time to work effectively in that period. There’s little point in selecting a biocide that requires a long contact time to work if the flushing process is completed quickly.

  • Correct product concentration and dose rates

    Get your calculator out and make sure you’re using the correct concentration of water treatment product given the system volume. Remember, you’ll use different concentrations of biocide depending on whether you are working to a maintenance dose to prevent bacteria growing, or a shock dose of biocide to kill off a major bacterial problem.

  • Understanding operating temperatures

    Make sure you know the operating water temperatures of your system. Some biocides work better than others at certain temperatures, so choose one which performs well given the temperature of the water in your closed system.

  • Water pH

    Use a biocide suitable for the pH of your water; if the water is too alkaline or acidic, the biocide may degrade and become less effective over time.

  • Other closed system water treatment chemicals

    Check for any other water treatment chemicals which are present in the system and that may have an effect on the performance of the biocides you are considering.

  • Broad spectrum biocides

    Broad spectrum biocides can often be a good choice. It’s ideal to choose a biocide which can kill off a wide range of bacteria.

  • Biocide – bio-dispersant compatibility

    If your closed circuit system is heavily colonised with biofilm you may also need to consider the use of a suitable bio-dispersant. If a layer of biofilm has started to build up on the insides of the pipes, then you should choose a biocide which can work in conjunction with a bio-dispersant.

  • Cost effective

    You should also consider cost-effectiveness as some biocides are lower in cost than others and may perform just as well.

  • Handle biocides with care

    Remember that biocides should be handled with care, and anyone coming into contact with biocides should have appropriate protective clothing. Always read the manufacturers safety data sheet (SDS) before starting work with biocides and take all recommended precautions.

Key Points when Choosing Biocides for Closed Systems


    If your water system is running at over 60oC, you probably don’t need a biocide.


    Think about seasonal or weekend shutdowns of your closed heating and cooling systems and how the risks of this type of intermittent use pattern will be managed.


    Choose the right biocide to treat high levels of bacteria in a closed system, and use it at the correct dose rate and continue to monitor.


    If there’s a problem with biofilm, use a low-foam bio-dispersant in conjunction with your selected biocide. Incorporate strainers into the closed system to trap biofilm and stop blockages.


    Laboratory testing can help you work out the most effective biocide for your system.

Expert Management of Closed Heating & Cooling Systems

Water Treatment Services provide expert advice and support covering the management of closed heating and chilled water systems ncluding pre-commisson cleaning and flushing in accordance with BSRIA guidance (BG 29/2012) during the final commissioning stages for all newly installed and refurbished closed water systems.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained engineers, in-field water treatment service specialists and technicians we can offer cost effective management of closed circuit systems and engineering support services throughout the UK and Ireland.


Further Reading…

More information about closed heating systems … here →

Choosing Biocides for Closed System Heating & Cooling Systems
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Choosing Biocides for Closed System Heating & Cooling Systems
Learn to choose biocides for closed system heating & cooling systems. Closed circuit systems are vulnerable to bacteria & biofilm causing reduced heat transfer efficiency, increased costs & reduced reliability. How to select the best biocides for closed circuit heating & chilled water systems.
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Water Treatment Services
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