Management of Hot and Cold Water Systems
||The ACOP says risks from legionella should be identified and managed. The following section on operation and maintenance of hot and cold water systems offers guidance on some of the issues which need to be addressed in order to do this.
Commissioning and re-commissioning
Following the commissioning of a new hot water system, the water temperature should be measured continuously at the bottom and the outlet of the calorifier over a typical day. If the storage vessel is big enough to deal with the demand, the outlet temperature will be constant throughout the day. If the calorifier is too small, the outlet temperature will fall during use and remedial action may be required, particularly if temperature is used as a control method. If the system changes from the original specification, this procedure will need to be repeated.
If a calorifier or any substantial part of a hot water system is on standby use or has been taken out of service for longer than 1 week, the water in the calorifier should be brought up to 60ºC for 1 hour before being used; this should be measured with normal circulating pumps operating and not with the system in a stagnant state. If there are standby recirculating pumps on the hot water circuits, they should be used at least once per week. If the system is to be treated with biocides as a means of controlling legionella, the biocide concentration in the system should reach normal operational levels for at least 3 hours throughout the system, before being used.
Cold water Cold water from the water utility is usually delivered to consumer buildings with a trace of active chlorine disinfectant and fit for drinking. However, users should not rely on this to treat the hot water system. Where water comes from rivers, lakes, boreholes or other sources, it needs to be pre-treated so that it is of the same quality as the mains supply. The Water Supply (Water Quality) Regulations require designers and maintainers of premises to maintain the wholesome nature of the water.
The Water Supply (Water Quality) Regulations permit water utilities to supply water to premises at temperatures up to 25ºC. In practice, the water temperature is likely to be well below this maximum value (in the order of 5-10ºC in winter and up to 20ºC in summer). However, during a prolonged hot summer, the incoming water temperature at home sites can become abnormally warm. If the incoming water is above 20ºC, the water undertaker should be allowed to see if the cause of the high temperature can be found and removed. If this is not possible, the risk assessment should reflect this increased risk and appropriate action taken if necessary.
Hot water The water can be heated by hot water or steam from a boiler which is passed through a coiled heat exchanger sited inside the hot-water storage vessel, the calorifier. Calorifiers heated directly by gas or oil flame have been shown to have the lowest incidence of colonisation by legionella. The calorifier can also be heated by electricity or by means of an electric immersion heater within the vessel.
In a hot water system, cold water enters at the base of the calorifier with hot water being drawn off from the top for distribution to user points throughout the building. A control thermostat to regulate the supply of heat to the calorifier should be fitted to the calorifier near the top and adjusted so that the outlet water temperature is constant. The water temperature at the base of the calorifier (i.e. under the heating coil) will usually be much cooler than the water temperature at the top. Arrangements should therefore be made to heat the whole water content of the calorifier, including that at the base, to a temperature of 60ºC for one hour each day. This period needs to coincide with the operation of the boiler plant (or other calorifier heat source) and is usually arranged during a period of low demand eg during the early hours of the morning. A shunt pump to move hot water from the top of the calorifier to the base is one way of achieving this. However, it should not be used continuously except for about one hour each day (see above). In all cases, the operation of the pump should be controlled by a time-clock.
Alternatively, some calorifiers are fitted with coils extending to the base to promote convective mixing during heating. This mixing may not be required if using alternative treatment methodologies.
Ideally, the calorifier will have specific connections for the shunt pump return as low down on the calorifier as possible. For existing calorifiers without suitable connections, the drain point may sometimes be used. This should not be done before cleaning and descaling of the calorifier, otherwise the operation of the pump may disturb sludge or sediment.
Some form of scale control is desirable in hard water areas. This is because there is a risk of calcium being deposited at the base of the calorifier at temperatures greater than 60ºC. It is recommended that an inspection port is fitted in the side of the calorifier so that the cleanliness of the base can be checked and cleaned when needed. Where one has not been fitted, any debris in the water at the base of the calorifier should be purged to a suitable drain on an annual basis. The presence of scale makes it more difficult to generate hot water efficiently in the calorifier or water heater and reduces the effectiveness of any treatment or disinfection measures. Corrosion control may be required if low-corrosion materials (copper, plastic, stainless steel etc) have not been used in the system.
Whenever hot taps are no longer required for use they should be removed and cut back to the recirculating loop. Where standby units are provided, there should be procedures in place to enable these units to be incorporated into routine use. Standby pumps should be changed over and
used each week to avoid water stagnation. Standby calorifiers should be emptied of water and there should be specified procedures in place to be followed before they are brought back into use.
Keeping water softeners and filters clean is important and best done by following the manufacturers’ recommendations. Coarse filters and strainers should be checked and cleaned regularly to prevent the build-up of organic contaminants.
Regular flushing of showers and taps
Before carrying out the following procedures, consideration should be given to removing infrequently-used showers and taps. If they are removed, the redundant supply pipework should be cut back, as far as possible, to a common supply, (for example, to the recirculating pipework or the pipework supplying a more frequently used upstream fitting).
The risk from legionella growing in peripheral parts of the domestic water system, such as deadlegs off the recirculating hot water, may be minimised by regular use of these outlets. When outlets are not in regular use, weekly flushing of these devices for several minutes can significantly reduce the number 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, e.g. hospitals, nursing homes, etc.
Where it is difficult to carry out weekly flushing, the stagnant and potentially contaminated water from within the shower/tap and associated dead-leg needs to be purged to drain before the appliance is used. It is important that this procedure is carried out with minimum production of aerosols, e.g. additional piping may be used to purge contaminated water to drain.
Automatic drain valves fitted to showers to drain the mixer valve and shower hose after use can produce conditions within the shower that support the growth of legionella, and are not recommended as a method for controlling the risk of exposure to legionella.