Health and Safety Executive’s guidelines HSG274 part 2 state that: “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 interest in other technologies to deliver safe, temperature controlled water at the point-of-use.
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.
Pressure-balancing technology
In public areas within healthcare facilities, the risk of scalding increases when users are unfamiliar with the facilities. Where there is high frequency usage by visitors, pressure-balancing mixers can provide an intermediate anti-scalding solution. 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.
For example, Figure 1 shows a ceramic cartridge where both the cold and hot water supply pressure is 3 bar, and the outlet temperature is pre-set to 41°C. If the cold water pressure drops to 2.5 bar and the hot water pressure remains at 3 bar, the higher pressure at the hot water inlet causes the shuttle to displace, restricting the hot water flow into the mixing chamber while simultaneously allowing more cold water to flow. As a result, the mixed water at the outlet remains at 41°C (see Figure 2). Similarly, if the pressure at the hot water inlet drops to 2.5 bar and the cold water pressure remains at 3 bar, the shuttle adjusts the apertures to increase the hot water and reduce cold water flow into the mixing chamber. (See Figure 3).
Figure 1
Hot and cold water pressure is 3 bar, outlet temperature is 41°C
Figure 2
Cold water pressure falls to 2.5 bar, the cartridge compensates and outlet temperature remains at 41°C
Figure 3
Hot water pressure falls to 2.5 bar, the cartridge compensates and outlet temperature remains at 41°C
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 supply fails, the loss in pressure causes the shuttle to close the cold water inlet 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.
Inside a mixer with a pressure-balancing mechanism
Thermostatic technology for high level scalding risks
HSG 274 part 2 states that: “Where a scalding risk is considered significant … then type 3 TMVs [TMV3] that are pre-set and failsafe should be provided.” ⱡ
To failsafe, 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 TMV type 3 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. There are also key cohorts 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. Here the guidelines recommend the use of TMVs for sinks or hand washbasins.
Other considerations for infection control
A key factor in the development of Legionella bacteria is standing water between 20 and 46°C. Pressure-balancing mixers feature a single mechanism which controls both flow and temperature. The water circulates from the inlet to the mechanism inside narrow copper tubes.
Narrow tubes reduce standing water volumes in mixers
These have two benefits, the volume of standing water is minimised, and water flow is accelerated. Both these factors minimise the development 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. Tapware with smooth interiors has fewer niches where bacteria can adhere, significantly slowing biofilm development. Moreover, as a preventative measure, cleaning mixers in a de-scaling solution will further reduce biofilm build-up. If those mixers can be removed easily, they can be regularly cleaned to prevent biofilm development or disinfected if a contamination episode occurs.
Advances in anti-scalding technology and infection control have led to a multitude of products that offer different solutions for patient, staff and visitor safety in healthcare environments. Certainly the technologies are not mutually exclusive and not always complex. It is possible to control Legionella and provide scalding safety for most situations, to accommodate the comparative risk levels.
Notes:
ⱡ All quotes taken from the Health and Safety Executive guidelines HSG 274 Part 2: The control of legionella bacteria in hot and cold water systems. Published 2014.
* At the point-of-use the maximum recommended temperature for washbasins and showers is 41°C, and 44°C for baths (46°C for assisted bathing). Maximum temperature guidelines are based on NHS Estates Health Guidance Note, BRE IP14/03, and Preventing Hot Water scalding in bathrooms and TMVA Code of Practice for Safe Water Temperature.
