Appendix 2 - types of hot and cold water systems
There are three main types of hot and cold water systems:
1. Gravity system with recirculation
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 (electrically or by boiler / heat exchanger). 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.
2. Gravity system without recirculation
This system is found in most houses and small buildings and is similar to the system described above but does not have a hot water loop and re-circulating pump. The cold water may be fed by gravity direct from the cold storage tank to the point of use or may be supplied direct from the mains.
3. Mains pressure (closed) systems
Cold water is fed directly from the mains, is heated and fed directly to the point of use, without the need for water storage tanks. 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 (combi) boilers.
With the increase in green technologies, heat bank systems are becoming more popular. These store a volume of liquid which is heated through solar, ground source, wind or other methods, and forms a 'heat bank'. Mains water is fed through this and is instantly heated on demand to supply the outlets. The liquid in the heat bank may be used for central heating purposes.
In mains pressure systems, the risk from Legionella is reduced because there is no storage of water to which people are regularly exposed.
Some of the features of gravity hot water systems which influence the risk of exposure to Legionella can be eliminated by moving to mains pressure systems. However this requires confidence in the reliability and continuity of the mains supply and may not be appropriate in all cases. Problems such as maintaining the hot water temperature throughout the distribution system and changes in demand are sometimes overcome by point of use water heaters which have minimal or no storage.