Design and Construction

Guidance

The ACOP says that plant or water systems should be designed and constructed to be safe and without risks to health when used at work. The following section on design and construction offers guidance on how to do this in hot and cold water systems.
The overall choice of system depends on the size and configuration of the building and the needs of the occupants.A key issue is whether cold water storage is required and how much. Some activities (health care, catering, etc.) rely on the continuous availability of hot and cold water but others would not be severely disadvantaged by a short-term loss of supply. Hot and cold water storage systems in commercial buildings are often over-sized in relation to the actual usage because of uncertainties in occupation at the design stage. This leads to excessive safety margins. If the design needs to allow for future growth in demand, this should be organised in a modern fashion. This enables additional plant to be added at a later stage, if required.

Water service systems have to comply with the Water Supply (Water Fittings) Regulations 1999. This includes the prevention of backflow, the use of approved materials for pipework, water fittings and jointing materials. General issues of design, sizing, layout, construction and commissioning are discussed in BS6700:1998. Certain aspects of the system will also have to comply with the appropriate buildings regulations.

Hot and cold water systems should be designed to aid safe operation by preventing or controlling conditions which permit the growth of legionella and to allow easy cleaning and disinfection. In particular, the following points should be considered:
* Materials such as natural rubber, hemp, linseed oil-based jointing compounds and fibre washers should not be used in domestic water systems. Materials and fittings acceptable for use in water systems are listed in the directory published by the Water Research Centre.
* Low corrosion materials (copper, plastic, stainless steel etc) should be used where possible.
* Water storage tanks should be fitted with covers which comply with the water regulations and insect screens fitted to any pipework open to the atmosphere, e.g. the overflow pipe and vent.
* Multiple-linked storage tanks should be avoided because of operational difficulties due to possible unequal flow rates and possible stagnation.
* Accumulator vessels on pressure-boosted hot and cold water services should be fitted with diaphragms which are accessible for cleaning.
* The use of point-of-use hot water generators, with minimal or no storage for remote low use outlets should be considered.
* Showers (excluding safety showers) should not be fitted where they are likely to be used less than once a week.
* Thermostatic mixing valves (TMVs) should be sited as close as possible to the point of use. Ideally, a single TMV should not serve multiple tap outlets but, if they are used, the mixed water pipework should be kept as short as possible. Where a single TMV serves multiple shower heads, it is important to ensure that these showers are flushed frequently.
TMV's should not be used with low-volume spray taps in buildings with susceptible populations.

Hot water systems

The storage capacity and recovery rate of the calorifier should be selected to meet the normal daily fluctuations in hot water use without any drop in the supply temperature. The vent pipe from the calorifier which allows for the increase in volume of the water should be large enough and suitably sited on the water circuit to prevent hot water being discharged. However, if discharged, the water should go to a tun-dish.
When more than one calorifier is used, they should be connected in parallel and, if temperature is used as a means of control, each should deliver water at a temperature of at least 60C. All calorifiers should have a drain valve so that accumulated sludge can be drained easily and the vessel emptied in a reasonable time. A separate drain should be provided for the hot water system vent (particularly if the feed to the calorifier incorporates a non-return valve).
If temperature is used as the means of controlling legionella, the hot water circulating loop should be designed to give a return temperature to the calorifier of 50C or above. The pipe branches to the individual hot taps should be of sufficient size to enable the water in each of the hot taps to reach 50C within 1 minute of turning on the tap. Thermometer/ immersion pockets should be fitted on the flow and return to the calorifier and in the base of the calorifier, in addition to those required for control.
In larger calorifiers, the fitting of time-controlled shunt pumps should be considered to overcome temperatures stratification of stored water.
Hot water distribution pipes should be insulated.
If temperature is used as a means of controlling legionella, trace-heating should be provided on non-recirculating hot water distribution pipework where the discharge temperature would not otherwise reach 50C in 1 minute.

Cold water Systems

Low-use outlets should be installed upstream of higher use outlets to maintain frequent flow; e.g. a safety shower can be installed upstream of a WC. Access ports should be provided on cold-water tanks for inlet valve maintenance, inspection and cleaning (more than one hatch may be needed on large tanks).
The volume of cold water stored should be minimised; it should not normally be greater than one day's water use. Multiple cold water storage tanks require care in the connecting piping to ensure that the water flows through each of the tanks, so avoiding stagnation in any one tank.
The cold-water storage tank should be sited in a cool place and protected from extremes of temperature by thermal insulation. Piping should be insulated and kept away from hot ducting and other hot piping to prevent excessive temperature rises in the cold water supply; typically not more than 2C increase should be allowed. The pipework should be easy to inspect so that the thermal insulation can be checked to see that it is in position and has remained undisturbed.

< Back - Next >