Safe and sanitary

Rada, showers, DHW, domestic hot water, legionella, Legionnaires' Disease
Advanced thermostatic mixing valves, such as Rada’s digital TMV, can do much more than deliver water at a safe temperature. They can also ensure that water is passed at appropriate intervals to prevent the pipe to an outlet becoming a dead-leg and also pass water at a higher temperature to disinfect that length of pipe.

Water that is hot enough to prevent the growth of legionella can be dangerous to use. Roy Minett highlights how sensible precautions and best practice can ensure the delivery of safe hot water, while reducing legionella-related concerns in buildings.

Hot bath, shower and tap water plainly has the potential to be dangerous. The facts speak for themselves, with over 600 people in the UK being severely scalded each year. Three quarters of these seriously injured victims are under the age of five, many of them bearing the scars of their accident for the rest of their lives. Older people are also vulnerable and it is estimated that 15 pensioners die each year as a result of a hot bath-water accident.

The problem lies in how quickly hot water causes injury.

• Bath water can be delivered to taps at dangerously high temperatures — as much as 70°C in many cases.

• Scalding can conceivably occur at water temperatures above 45°C.

• At 50°C, adults can suffer third-degree burns after five minutes and children after less than two minutes.

• At 60°C, the time drops to 5.4 seconds for adults and 1.5 seconds for children.

• At 70°C, it takes just 1 second for adults to suffer third degree burns and 0.27 seconds for children.

The challenge is, however, that best practice dictates that hot water is generated and stored at not less than 60°C to prevent growth of micro-organisms such as legionella.

While infections caused by the legionella bacterium (Legionnaires’ Disease) are relatively uncommon, vigilance has to be maintained because of the relative virulence (and lethality) of the condition and the ease with which outbreaks could occur.

There are several hundred cases of Legionnaires’ Disease every year. The mortality rate is 10 to 15% and may be higher in some population groups. The vast majority of infections occur in the community (57%), with a smaller number (3%) being hospital acquired (the remainder being generally acquired overseas).

Legionella bacteria can multiply rapidly in water at 20 to 45°C. As the temperature rises above 60°C, most bacteria will be killed within five minutes. Below 20°C they remain dormant. Maintaining water temperatures above 50°C and below 20°C is therefore the main method of controlling legionella in water systems recommended by the Department of Health, and the UK Health and Safety Commission.

Facilities managers and engineers have access to the guidelines for preventing legionella multiplying, including the Health & Safety Commission’s Approved Code of Practice & Guidance for the Control of Legionella Bacteria in Water Systems L8 (ACoP L8), which is an acknowledged source of best practice

It recommends a number of measures to assist in the control of legionella bacteria in hot and cold water systems. They include supplying water to hot taps at over 50°C and to cold taps at below 20°C.

Regular system maintenance is essential by flushing under-used outlets for several minutes on a weekly basis. Likewise, disinfection is vital by routine thermal disinfection of the hot-water circuit to a temperature at which legionella will not survive.

The water-delivery system and equipment such as correctly designed and installed thermostatic mixing valves (TMVs), shower heads and basin installations also play a key part in the system maintenance regime. On the temperature regime, ACoP L8 Para 169 recommends that hot water should be stored at 60°C and distributed so that it reaches a temperature of 50°C within one minute at outlets. Care is needed to avoid much higher temperatures because of the risk of scalding; where a significant scalding risk has been identified, the use of TMVs on baths and showers must be considered to reduce temperature.

Hot-water facilities in most commercial establishments are covered by legislation or authoritative guidance designed to protect users — the more vulnerable by TMV3 which covers hospitals, care homes and schools. This type of protection was extended further this year into domestic dwellings with the introduction of Part G of the Building Regulations. Part G now requires the fitting of thermostatic mixing valves (TMV) in all new bathrooms.

Why then is it that TMV2 remains only suggested best practice in hotels — which after all are domestic settings in commercial establishments? In my opinion, guidance should go further to protect users of showers and baths in hotels from scalding risks.

Leading proprietary commercial TMVs now incorporate digital intelligence which makes them smarter, safer and more hygienic than lesser valves on the market. Not only do such TMVs allow accurate temperature control, they are also equipped with features that facilitate the implementation of the measures for controlling legionella bacteria that are highlighted in ACoP L8.

Digital, intelligent valves will automatically compensate for changes in supply pressures and temperatures. This means that a shower, for example, is factory pre-set at 41°C, delivers 41°C during commissioning and continues to deliver 41°C throughout its life. That same digital intelligence also carries out a number of safety checks, many times per second. Where errors are detected, the valve ‘locks-out’ and the nature of the error is communicated to the service engineer via either the display or by interrogation with a hand-held computer.

The best valves also include a second temperature sensor to shut off the flow independently of the mixing valve if the outlet temperature exceeds safe levels — for any reason. This ensures that every user continues to receive the necessary protection all the time.

L8 Para 165 deals with regular flushing of the appliance. With regard to legionella, it is well known fact that the risk of breeding also rises when peripheral outlets are not in regular use and where ‘dead legs’ occur. ACoP L8 advises that flushing through these outlets for several minutes can significantly reduce the numbers of legionella discharged from the outlet.

Once started, this procedure has to be sustained and logged, as lapses can result in a critical increase in legionella at the outlet. Risk assessment may indicate the need for more frequent flushing where there is a more susceptible population present. In practice, when conducted manually, flushing the appliance not only consumes a lot of resource but also wastes water because frequently used outlets are flushed unnecessarily as there is no way to tell which are used regularly and which are not.

Again, leading proprietary commercial digital TMVs incorporate an intelligent duty flush feature that monitors the amount of time since the outlet was last used. An outlet that is not used for a pre-determined period automatically flushes at a safe temperature. The non-use period, flush temperature and duration of the duty flush should be fully programmable. This is important because flushing for a few seconds is rarely sufficient to flush the whole dead-leg.

Dead-legs are a credible threat even in commercial establishments that use recirculating systems; under-used areas, rooms or junctions of the main system all can become dead-legs. An unused room in a hotel would be an example of a possible threat.

Biocide, chemical or ionisation treatments used to treat the water system are usually prevented from reaching under-used outlets by dead-legs of water that build up before to the outlet. A duty flush will ensure that such treatments pass through the TMV and the outlet pipe-work on a regular basis.

Finally, ACoP L8 also recommends routine thermal disinfection as a means of preventing or controlling the bacteria. The most advanced valves also deliver a thermal disinfection cycle which — under supervision — allows hot water at at least 60°C to be flushed through the digital mixing valve and all outlet pipework. A hand-held computer accurately records both the temperatures reached and the duration of the disinfection process and produces a log that can be downloaded and stored on a PC as a permanent record.

Though flushing/thermal disinfection could be considered environmentally unfriendly because it wastes water, organisations must consider the wider costs of unmanaged water systems, especially in light of the recent corporate manslaughter act.

Constant vigilance for the factors that support the growth of legionella and the implementation of effective preventative measures, can successfully reduce the incidence of infection. Implementing best practice and using the best, state-of-the-art equipment can be a winning combination in the ever-present battle to prevent waterborne infections such as legionella.

Roy Minett is marketing manager of Rada.

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