Legionella risk in hot and cold water systems

In hot water supplies, the use of use of unvented cylinders in hot water supplies is very common, and expansion tanks are often the preferred method to take up thermal expansion. One potential problem that can arise, however, is when the expansion tank is sited incorrectly, resulting in a ‘deadleg’, and a possible microbial problem.

The HSE guidelines are very clear with respect to these installations. Indeed HSG 274 pt2 states, in its ‘Introduction’:

‘There is a reasonably foreseeable Legionella risk in your water system if:

•  Water is stored or recirculated as part of your system;

•  The water temperature in all or some part of the system may be between 20 and 45 °C;

•  There are deposits that can support bacterial growth, such as rust, sludge, scale, and organic matter;

• It is possible for water droplets to be produced and, if so, if they can be dispersed;

• It is likely that any of your employees, contractors, visitors etc, could be exposed to any contaminated water droplets.’

Within the document itself, HSG 274, pt2 states:

Section 1.38: ‘Expansion vessels in systems operating at steady temperature and pressure may have long periods without exchanging any significant amount of water and therefore can be at risk of aiding microbial growth.’

Section 1.39: ‘To minimise the risk of microbial growth, expansion vessels should be installed:

•  In cool areas on cold flowing pipes;

•  Mounted as close to the incoming water supply as possible;

•  Mounted vertically on pipework to minimise any trapping of debris;

•  With an isolation and drain valve to aid flushing and sampling;

•  To minimise the volume retained within them;

•  Designed to stimulate flow within the vessel.’

Graph showing Pressure and Temperature measurements over a week, from an expansion tank fitted adjacent to the unvented cylinder. The temperature will support microbial activity. Temperature measurements taken using a calibrated data logger and a pt1000 probe.

Graph showing Pressure and Temperature measurements over 11 days, from an expansion tank fitted adjacent to the unvented cylinder. The temperature will support microbial activity. Temperature measurements taken using a calibrated data logger and a pt1000 probe.

These tanks should be fitted with a valve, to:

a) Isolate and drain to aid flushing and sampling;

b) Stimulate flow within the tank.

The correct FlowThru valve will do all these tasks.

The set-up with and without FlowThru

Without FlowThru no water is going in or out of the tank. With FlowThru particles are being forced into the tank.

 

Now you can see the complete mixing of the water inside the water chamber.

The FlowThru has to physically direct a proportion of water into the expansion tank to be effective.

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