Watts highlights danger of Pseudomonas


Watts Group PLC is highlighting the hidden dangers posed by Pseudomonas following an outbreak of the bacterial infection at a hospital in Northern Ireland.

Mark Rabbett, Watts’ M&E director, has outlined the various ways in which the pathogen can affect buildings, and particularly air conditioning systems, in a recent paper.

He said: “Pseudomonas have been around pretty much for ever but, until relatively recently, were not particularly tough. The family of bacteria has developed and is now strong enough to resist most bactericides/antibiotics.

“It lives almost anywhere there is moisture - it has a starring role in the creation of snowflakes, for example. Most buildings will have the bacteria in their water supplies, in fact it is so common but generally so benign that we don’t even test for it in our drinking and domestic water.”

The main points to consider are:


  • Most of us life with one or more strains of Pseudomonads all the time and it does us no harm.
  • If Pseudomonas can enter the blood stream then some strains of the bacteria can be lethal.  
  • Laboratories routinely test for Pseudomonas Aeruginosa as a key indicator.  We might most commonly meet the results of this bacteria if we know someone with Cystic Fibrosis, where the bacteria can enter through the lungs.

Air conditioning

  • Pseudomonads in low temperature hot water and chilled water systems have been a significant issue for over a decade. 
  • To survive it needs only moisture, relatively cool conditions (below 60°c) and oxygen.
  • In an existing, little modified and well maintained system there is so little oxygen in the water that the bacteria does not develop.
  • The most common source of infection is when the systems are worked on during fit-outs or even plant/equipment replacement. In these circumstances, fresh water (with bacteria and oxygen) is added to the system and the bacteria thrive.
  • The most common problems arise from construction, when systems are filled, drained, refilled and then sit full of impure water for months during testing and commissioning.  

Effect in pipework and equipment

  • Slime formation that coats the pipework, plant and equipment internally. The slime acts as an insulator and reduces system performance.
  • Gas that causes air locks and loss of performance.
  • Increased acidity that itself produces system corrosion.
  • Globules of slime that block small valves, particularly in chilled beam/ceiling systems.
  • The slime that Pseudomonads produce can harbour anaerobic bacteria (which gets oxygen from the water itself), such as sulphate reducing bacteria.  The combination of these has been cited in several buildings Watts has surveyed as the cause of pipework pitting so severe that fan coil units fail in less than three years.
  • Heating systems are normally immune to serious problems because they operate at high temperatures.  However, when off during summer shutdowns, the bacteria can infect a system and get established in those cold pockets such as isolated radiators/fan coil units, air vents and dirt pockets.


  • Fan coil units or other AC equipment that need venting too often.
  • Listen for loud gurgling noises in your ceiling voids caused by gas in the pipework.
  • Ask your maintenance team whether they test for the bacteria in the “closed water systems”.  The majority still don’t.
  • Don’t just test at the pumps.  You may have a thriving colony in a fifth floor fan coil unit that won’t show up at the pumps until it has spread system wide.
  • Test every water system - this includes ground source circuits.


  • Adding dosing chemical just isn’t enough as these cannot penetrate the slime and kill all the bacteria.
  • Pseudomonads are immune to a number of bactericides. In some conditions Pseudomonads are known to live of them not die from them.
  • Step one is to get specialist advice.  Watts has encountered maintenance contractors and water treatment companies that have not grasped the significance of the problem.
  • It is recognised that getting rid of an infection is practically impossible but it can be lived with in small doses.
  • Initial treatment will likely require flushing out, chemical cleaning and heavy dosing.
  • It will be disruptive and it will be expensive (£100,000 for a typical large office is common).

Mark added:  “Pseudomonads are a system killer, especially when teamed up with SRB. We can’t beat them but, with proper care, we can live with them.

“To the best of our knowledge, there is yet to be a case where treatment of Pseudomonads is a dilapidations requirement.  We believe this is just a matter of timing.  The first cases should be appearing in the next few years.”

For further information, please contact:

Kirsty Maclagan (Marketing and Business Development)
D: +44 (0)20 7280 8015  E:

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