Different Types of Water Systems
This type of system is found in many commercial buildings. Cold water enters the building from a rising main and is stored in an intermediate cold water tank. The cold water is fed from the tank by gravity to the points of use without recirculation. Cold water is fed from the storage tank to the calorifier (hot water cylinder) where it is heated. There is a continuous flow of water from the calorifier / cylinder around the distribution circuit and back to the calorifier. This ensures that the hot water is quickly available at any of the taps, independent of their distance from the calorifier. The circulation pump is sized to ensure that the return temperature back to the calorifier is not less than 50°C. The vent pipe at the calorifier should be linked to a separate tundish/drain. This should only discharge water under fault conditions. These design principles also apply where an electrically heated cylinder or direct fired storage heater is used instead of a calorifier.
In a mains pressure hot water system there is no intermediate cold water storage tank. Cold water is fed directly from the mains to the points of use. The rising main is connected directly to the calorifier, water heater or plate heat exchanger. Since the water in the system will expand due to heating, an expansion vessel and a safety temperature and pressure relief valve are required. Hot water distribution from pressurised systems can be used in both recirculation and non-recirculation systems. The latter is commonly found in houses with combination heating and hot water (combi) boilers.
Cold water enters the building from a rising main which directly feeds the hot water calorifier where it is heated. There is a continuous flow of water from the calorifier / cylinder around the distribution circuit and back to the calorifier. This ensures that the hot water is quickly available at any of the taps, independent of their distance from the calorifier. The circulation pump is sized to ensure that the return temperature back to the calorifier is not less than 50°C. The vent pipe at the calorifier should be linked to a separate tundish/drain. This should only discharge water under fault conditions. These design principles also apply where an electrically heated cylinder or direct fired storage heater is used instead of a calorifier.
The rising main is connected directly to the calorifier, water heater or plate heat exchanger. Since the water in the system will expand due to heating, an expansion vessel and a safety temperature and pressure relief valve are required. Hot water distribution from pressurised systems can be used in both recirculation and non-recirculation systems. The latter is commonly found in houses with combination heating and hot water (combi) boilers. Cold water storage may supply water to some or all outlets.
These systems are typically found in smaller buildings such as domestic dwellings and small office buildings where cold water outlets are fed directly from the water supply without storage. Combination boilers or instantaneous water heaters provide hot water directly from the cold water supply by heating the water as it passes through the heater. These units supply continuous hot water at a rate that is usually limited by their power rating. High flow rates through the units can result in warm water leaving the heater before reaching the target temperature.
- Low storage volume POU water heaters are those that store no more than 15 litres of hot water (see Figure 2.2). These systems generally heat water to a set point that is often variable via a simple dial on the unit. These systems deliver a small volume of stored hot water before they need to be left to recover and bring the temperatures back to the set point.
- Combination water heaters store a volume of cold water (ranging from 10–200 litres) above the hot water storage unit (ranging from 15–150 litres). In these units the cold water header tank feeds the hot water storage vessel as hot water is drawn from the system on demand. The cold water header tank is topped up directly from the cold water supply, usually via a float-operated valve. The combination water heater is usually fitted with an expansion pipe so that any expanding hot water returns into the cold water header tank. Expansion may also occur by the cold feed pipe.
- The design of a combination water heater may allow hot water to enter the cold water space. The Water Supply (Water Fittings) Regulations 1999,15 the Scottish Water Byelaws 2004,16 and BS 3198 Specification for copper hot water storage combination units for domestic purposes17 recognise this and permit a maximum cold water storage temperature of 25 °C where it is serving other domestic outlets or 38 °C when serving the hot water vessel only. Careful consideration should be given to managing the risks from these types of systems and this should be reflected in the risk assessment. The thermostat should be set to as close to 60 °C as is practicable without exceeding it and hot water at the outlets should be at a minimum of 50 °C; correct setting of the thermostat and regular water usage is necessary to keep the temperature increase in the cold water to a minimum. Where this is not possible, eg during periods of low usage such as overnight or at weekends, fitting a timer which switches off the immersion heater may prove effective. The timer should be set to switch the immersion heater on again in time to ensure the water is heated sufficiently to achieve microbial control before use.
- Electrical immersion heaters usually heat combination heaters but some units incorporate internal coils for primary boiler heating circuits.
- In some combination units, the header tank is split into two sections: one feeding the water heater below and the other supplying cold water to the closed heating system. Possible cross-contamination and poor temperatures should be considered as part of the risk assessment.
Gravity system without recirculation
Gravity systems without recirculation are generally installed in domestic dwellings and small buildings. Cold water enters the building from a rising main and is stored in a cold water tank. The cold water tank provides backflow protection to the mains supply and a stable pressure and reserve in the system if the mains pressure fails or demand exceeds the capacity of the mains supply. Cold water from the tank is fed to the calorifier (hot water cylinder) where it is heated and drawn via pipes that branch to sinks, washbasins, baths, showers etc. In contrast to recirculating systems, the water only flows when it is being used and is usually allowed to become cool in the pipes after use.
Gravity system with recirculation
Gravity systems with recirculation are typically installed in larger buildings such as commercial premises. Cold water enters the building from a rising main and is stored in a cold water storage tank or tanks. The tank provides backflow protection to the mains supply and a stable pressure in the system; it also provides a reserve if the mains pressure fails or demand exceeds the capacity of the mains supply. Cold water from this storage tank is fed to the calorifier.
There is a continuous circulation of hot water from the calorifier around the distribution circuit and back to the calorifier by means of one or more pumps, usually installed on the return to the calorifier, but it can be on the flow. This is to ensure that hot water is quickly available at any of the taps, independent of their distance from the calorifier and reduces the risk of localised temperature fluctuations. The circulation pump is sized to compensate for the heat losses from the distribution circuit so that the return temperature to the calorifier is not less than 50 °C.
The pump has little effect on the pressure at the tap, which is determined by the relative height of the storage tank. The expansion of water as it is heated within the system is accommodated by a slight rise in the levels of the tank and vent pipe. The vent pipe should be directed into a separate tundish/drain which discharges at a safe and visible point and acts as a warning pipe. Discharge into the cold water storage tank is not advised as this can result in warm storage water temperatures and increase the risk of microbial growth. In the cold water system, water is fed by gravity directly from the cold water storage tank to the points of use without recirculation.
In these systems, water expands when heated, requiring an expansion vessel, safety temperature and pressure relief valve (in a pressurised hot water system there is no open vent to a high level). Hot water distribution can be a recirculating or non-recirculating system.
Larger systems or those that require higher pressures to reach the top of the building often include break tanks and booster pumps, in place of direct mains water, that subsequently feed the water heater.
Hot water heaters are water storage vessels heated by:
■ primary heating circuits of low pressure hot water or steam which is passed through a heat exchanger inside the vessel;
■ gas or oil flame, directly;
■ electricity, normally by means of an electric immersion heater within the vessel; or
■ an external heat exchanger (sometimes returning to a holding ‘buffer’ vessel).
Direct-fired (gas) water heaters
Characteristic of this type of design is heating from below which avoids the reduced temperature areas found in indirect heating calorifiers; they also have lower storage volumes and even temperature distribution. This type of water heater has been shown to have a low incidence of colonisation by legionella.
Indirect heating calorifier vessel
In these vessels, the cold water typically enters at the base of the calorifier, creating an area below the coil where the initial blended water temperature may support microbial growth. Stratification, which may occur in large calorifiers, should be avoided and fitting a timer-controlled shunt pump to circulate the water from the top of the calorifier to the base during the period of least demand should be considered. The shunt pump should be activated when demand is at its lowest and the temperature within the calorifier is likely to be highest, this is often during the early hours of the morning. The boiler plant (or other calorifier heat source) should be heating while the shunt pump is active to ensure a temperature of at least 60 °C is achieved throughout the vessel for at least one continuous hour a day. Pressure relief Hot water return Cold water in Hot water out Inspection hatch Burner Drain Isolation valve Expansion vessel Check valve
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 cold water feed may be used. Shunt pump operation should not be done or any alteration carried out before cleaning and descaling the calorifier, as operating the pump may disturb sludge or sediment. As an alternative to shunt pumps, some calorifiers are fitted with coils extending to the base to promote convective mixing during heating. Particulate matter can accumulate at the base of the calorifier so the design should incorporate an easily accessible drain valve.
Calorifiers attached to solar heating systems
Hot water storage cylinders (calorifiers) attached to solar heating systems or other microgeneration systems often have two heating coils one fed from the conventional heat source (boiler, heat exchanger etc) and one from the solar panels. The solar coil is usually positioned at the bottom of the cylinder and is used to pre-heat the ‘dedicated solar volume’ – the volume of water that can only be heated by the solar input. The boiler coil is fitted above the solar coil to raise the temperature of the water at the top of the vessel to 60 °C .
Calorifiers attached to solar heating systems should be managed, monitored and maintained to achieve the flow temperatures as for conventionally heated calorifiers throughout the year. As with conventional calorifiers, there will be temperature stratification providing favourable conditions for microbial growth including legionella at the base of the vessel. However, in times where there is little heat gain from the panels there may be a larger volume at a reduced temperature than in conventional systems. These systems should be designed so that the hot water temperature is not compromised during times when there is little heat gain from the solar panels. If the solar coil does not generate temperatures that bring about thermal inactivation of legionella bacteria; and the residence time for water in contact with the boiler coil at 60 °C is less than that required to effect thermal inactivation, a further level of control should be provided. For example, consideration should be given to programming the boiler coil to heat the entire contents of the solar hot water cylinder once daily, preferably during a period when there is little demand for hot water. A shunt pump may also be used to move hot water from the top of the calorifier to the base, however, it should not be used continuously except for about one hour daily and in all cases the pump should be controlled by a time clock. Where temperature control is not achieved, other measures such as using appropriate biocides should be considered.
The risk from legionella growing in peripheral parts of the domestic water system, such as dead legs off the recirculating hot water system, 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 risk of legionella proliferation in the system. Once started, this procedure has to be sustained and logged, as lapses can result in a critical increase in legionella at the outlet. Where there are high-risk populations, eg healthcare and care homes, more frequent flushing may be required as indicated by the risk assessment