Monitoring and Inspection

To comply with their legal duties, employers and those responsible for controlling commercial premises should implement, manage and monitor the precautions taken to avoid or reduce the risks from legionella bacteria.

Tables 1–3 and Checklists 1-3 within the ‘Approved Code of Practice & Guidance’ document (L8) published by the Health & Safety Commission (HSC) recommends suitable routine monitoring and inspection programmes for each type of asset. Recommended time intervals between checks are also listed.

Domestic Water Systems

One of the most important elements of the monitoring scheme for domestic water systems are hot and cold water temperature checks. Avoiding temperatures at which legionella can multiply is the single most effective control measure. Sentinel outlets (those closest to and furthest from each mains water entry point, water storage tank and calorifier) should be checked every month. Other outlets should be checked at least once a year. Many sites find that the most practical solution is to check sentinel outlets plus a selection of other outlets each month, so as to cover all outlets over the course of each year.

Where thermal mixing valves (TMV) are fitted, the temperature of water supplying the valves should be measured, rather than the temperature of the hot water at the outlet they supply.

The use of digital infrared thermometers can make temperature reading more convenient. However, shiny surfaces can give misleading readings, so it is advisable to place matt adhesive labels at any points where water temperatures can only be read through pipes (e.g. TMV supply pipework).


Cold water storage tanks should be checked six-monthly. The ambient temperature, supply water temperature and stored water temperature remote from the ball-valve should all be recorded. Each tank should also be inspected internally on an annual basis, for signs of corrosion, scale and other contamination.

Calorifiers (hot water cylinders / tanks) should be checked monthly. Flow and return water temperatures should be measured. Annually, each calorifier should be inspected internally and/or blow-down (water from a drain point) checked for signs of corrosion, scale and other contamination.

Conventional showers (not self-purging), due to the increasing number of Legionnaires’ disease incidents associated with these devices, special attention should be made to their actual use. Those not in regular weekly use should be flushed at least once a week and more frequently where highly susceptible occupancies are present (hospitals, nursing homes etc). L8 describes how this procedure has to be sustained and logged as lapses can result in a critical increase in legionella at the shower.

Domestic systems should be tested for legionella if:

  • 'At risk’ groups are exposed to the water (e.g. hospital patients with weakened immune systems)
  • Water temperatures and/or water treatment regimes cannot be consistently maintained within appropriate ranges
  • An outbreak is identified or suspected

Cooling Towers & Evaporative Condensers

As a result of the increased risks posed by these systems, several checks are advisable on a regular basis.

There are two main sets of tests: those aimed at ensuring the effectiveness of the chosen water treatment system and those to check for biological activity. Make-up water and cooling water should each be checked.

Weekly tower / condenser monitoring should include biocide concentration, conductivity (total dissolved solids) and pH tests, together with biological analysis (typically using dipslides). Several electronic probes are available to aid chemical / biological testing (many capable of multiple tests in a single operation).

Monthly monitoring should include hardness and corrosion / scale inhibitor concentration tests. On a quarterly basis, alkalinity, sulphate, suspended solids, soluble iron, water temperature and legionella tests should also be conducted.

L8 advises that records of all monitoring, inspections, tests and checks carried out to control a reasonably foreseeable risk to health from legionella, together with details of the results, should be retained for at least five years.

To conclude, a well designed, implemented and documented monitoring and inspection programme will assist in showing ‘due diligence’ when audited by inspectors. It can also form part of a strong defence against negligence claims or prosecution.

Article by:
Steve Brown
EuroScience Environmental Consultancy