Providing hot water safely: getting the balance right

Specifying hot water systems in healthcare environments needs to navigate the tricky balance of providing enough heat to destroy Legionella bacteria, while mitigating the risk of scalding, says Carol Armstrong of Delabie UK

ntil recently, thermostatic mixing valves were installed as a matter of course in healthcare facilities to prevent scalding incidents. However, the Health and Safety Executive’s guidelines HSG274 Part 2 say “The use and fitting of TMVs should be informed by a comparative risk assessment of scalding risk versus the risk of infection from legionella.” As a result, there has been marked inter- est in specifying other technologies to deliver safe, temperature-controlled water. The risk of scalding depends on the use and the user. A low-level of risk could be a habitual user who is familiar with the outlet and the type of mixer, for example, healthcare workers using a particular basin for regular hand washing.


An intermediate risk level could be hospital visitors who use the public facilities when visiting patients. Although unfamiliar with the facilities, they have no sensory or mobility issues and are able to use the washbasins safely, especially if there are visual warnings about hot water.


Where there is high frequency usage by visi- tors, pressure-balancing mixers can provide an intermediate anti-scalding solution (see panel overleaf for details).

The ceramic cartridge is sensitive to changes in pressure. An internal shuttle continually adjusts to the incoming hot and cold water supply pressures to ensure a constant temperature at the outlet. If the pressure suddenly drops at the cold

inlet, the shuttle reacts immediately and the hot water flow is reduced to a trickle. Likewise, if the hot water pressure drops, cold water is reduced to a trickle. The pressure-balancing cartridge also features a maximum temperature limiter so the visitor is protected from temperature

spikes due to pressure drops or pressure differences in the system.

Thermostatic technology

HSG 274 part 2 states: “Where a scalding risk is considered significant…then type 3 TMVs [TMV3] that are pre-set and failsafe should be provided.” To fail-safe, the hot water must shut off if the cold water fails and vice versa. This is where pressure-balancing cartridges and thermostatic cells differ. The latter is able to provide a complete failsafe to TMV3 standards.

The most serious scalding risk is where the user is fully immersed in either a shower or a bath, predominantly in non-critical patients’ accommodation and residents’ accommodation in care homes. Where the scalding risk is significant – for example the very young or elderly, those with sensory loss, the infirm or significantly mentally or physically disabled people – the guidelines recommend the use of TMVs for sinks or hand washbasins.

Other considerations

A key factor in the development of Legionella bacteria is standing water between 20°C and 46°C. Pressure-balanc- ing mixers feature a single mechanism which controls both flow and temperature. The water circulates from the inlet to the mechanism inside narrow copper tubes. These have two benefits – standing water is minimised and water flow is accelerated. Both these factors minimise the develop- ment of biofilm, which provides shelter and a source of nutrients for bacteria. Many mixers used in public and commercial environments are cast in brass. The moulds are made from sand and although the external surfaces are plated and polished, the interiors are rough, with niches where bacteria can adhere.


Composition of a typical pressure-balancing mixer, showing the cartridge (blue) and temperature limiter (red)


Narrow copper tubes minimise the development of biofilm



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