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SECTION 8 Schedule 2: Paragraphs 17, 18, 19, 20, 21, 22, 23 and 24: Hot water services (1) Every unvented water heater, not being an instantaneous water heater with a capacity not greater than 15 litres, and every secondary coil contained in a primary system shall- be fitted with a temperature control device, and either a temperature relief valve or a combined pressure and temperature relief valve; or be capable of accommodating expansion within the secondary hot water system. (2) An expansion valve shall be fitted with provision to ensure that water is discharged in a correct manner in the event of a malfunction of the expansion vessel or system. Appropriate vent pipes, temperature control devices and combined temperature pressure and relief valves shall be provided to prevent the temperature of the water within a secondary hot water system from exceeding 100°C. Discharges from temperature relief valves, combined temperature pressure and relief valves and expansion valves shall be made in a safe and conspicuous manner. (1) No vent pipe from a primary circuit shall terminate over a storage cistern containing wholesome water for domestic supply or for supplying water to a secondary system. (2) No vent pipe from a secondary circuit shall terminate over any combined feed and expansion cistern connected to a primary circuit. Every expansion cistern or expansion vessel, and every cold water combined feed and expansion cistern connected to a primary circuit, shall be such as to accommodate any expansion water from that circuit during normal operation. (1) Every expansion valve, temperature relief valve or combined temperature and pressure relief valve connected to any fitting or appliance shall close automatically after a discharge of water. (2) Every expansion valve shall- be fitted on the supply pipe close to the hot water vessel and without any intervening valves; and only discharge water when subjected to a water pressure of not less than 0.5 bar (50 kPa) above the pressure to which the hot water vessel is, or is likely to be, subjected in normal operation. (1) A temperature relief valve or combined temperature and pressure relief valve shall be provided on every unvented hot water storage vessel with a capacity greater than 15 litres. (2) The valve shall- be located directly on the vessel in an appropriate location, and have a sufficient discharge capacity, to ensure that the temperature of the stored water does not exceed 100°C; and only discharge water at below its operating temperature when subjected to a pressure of not less than 0.5 bar (50 kPa) in excess of the greater of the following- the maximum working pressure in the vessel in which it is fitted, or the operating pressure of the expansion valve. (3) In this paragraph "unvented hot water storage vessel" means a hot water storage vessel that does not have a vent pipe to the atmosphere. No supply pipe or secondary circuit shall be permanently connected to a closed circuit for filling a heating system unless it incorporates a backflow prevention device in accordance with a specification approved by the regulator for the purposes of this Schedule.

Guidance

Unvented hot water systems:

G17.1   Every unvented water heater or storage vessel, and every secondary coil contained in a heater and not being an instantaneous water heater or a thermal storage unit of 15 litres or less capacity, should be fitted with:

1. a temperature control device; and
2. either a temperature relief valve or combined temperature and pressure relief valve; and
3. an expansion valve; and
4. unless the expanded water is returned to the supply pipe in accordance with Regulation 15(2)(a), either;
   1. an expansion vessel; or
   2. contain an integral expansion system, such that the expansion water is contained within the secondary system to prevent waste of water.

G17.2   An expansion valve should be fitted to all unvented hot water storage systems, with a capacity in excess of 15 litres, to ensure that expansion water is discharged in a correct manner in the event of a malfunction of the expansion vessel or system. See Figure 8.1.

G17.3   Where expansion water is accommodated separately the expansion vessel should preferably be of an approved 'flow through type' and should comply with the requirements of BS 6144 and BS 6920.

Temperature of hot water within a storage system

G18.1   Irrespective of the type of fuel used for heating, the temperature of the water at any point within a hot water storage system should not exceed 100^oC and appropriate vent pipes, temperature control devices and other safety devices should be provided to prevent this occurring.

Hot water distribution temperatures

G18.2   Hot water should be stored at a temperature of not less than 60^oC and distributed at a temperature of not less than 55^oC. This water distribution temperature may not be achievable where hot water is provided by instantaneous or combination boilers.

G18.3   The maintenance of acceptable water temperatures may be achieved by efficient routing of pipes, reducing the lengths of pipes serving individual appliances and the application of good insulation practices to minimise freezing of cold water pipes and to promote energy conservation for hot water pipes. For references, see Comments and Recommendations of Clause 2.6.4 of BS 6700.

Temperature of hot water supplies at terminal fittings and on surfaces of hot water pipes

G18.4   Where practicable the hot water distribution system should be designed and installed to provide the required flow of water at terminal fittings to sanitary and other appliances at a water temperature of not less than 50^o C and within 30 seconds after fully opening the tap. This criteria may not be achievable where hot water is provided by instantaneous or combination boilers.

G18.5   Terminal fittings or communal showers in schools or public buildings, and in other facilities used by the public, should be supplied with water through thermostatic mixing valves so that the temperature of the water discharged at the outlets does not exceed 43^oC.

G18.6   The temperature of water discharged from terminal fittings and the surface temperature of any fittings in health care premises should not exceed the temperatures recommended in HS(G)104 - Safe hot water and surface temperatures.

Energy conservation

G18.7   All water fittings forming part of a primary or secondary hot water circulation system and all pipes carrying hot water to a tap that are longer than the maximum length given in Table 8.1 should be thermally insulated in accordance with BS 5422.

Table 8.1   Maximum recommended lengths of uninsulated hot water pipes
Outside diameter of pipe in millimetres	Maximum length in metres
12 Over 12 and up to and including 22 Over 22 and up to and including 28 Over 28	20 12 8 3

Discharge pipes from safety devices

G19.1   Discharge pipes from expansion valves, temperature relief valves and combined temperature and pressure relief valves should be installed in accordance with the guidance given in this document and should also comply with the requirements of Building Regulation G3.

G19.2   Where discharge pipes pass through environments outside the thermal envelope of the building they should be thermally insulated against the effects of frost.

G19.3   The discharge pipe from a temperature relief valve or combined temperature and pressure relief valve should:

1. be through a readily visible air gap discharging over a tundish located in the same room or internal space and vertically as near as possible and in any case within 500 mm of the point of outlet of the valve; and,

2. be of non-ferrous material, such as copper or stainless steel, capable of withstanding any temperatures arising from a malfunction of the system; and,

3. have a vertical drop of 300 mm below the tundish outlet, and thereafter be laid to a self draining gradient; and,

4. be at least one size larger than the nominal outlet size of the valve, unless its total equivalent hydraulic resistance exceeds that of a straight pipe 9 metres long. Where the total length of the pipe exceeds 9 metres equivalent resistance, the pipe shall be increased in size by one nominal diameter for each additional, or part of, equivalent 9 metres resistance length. The flow resistance of bends in the pipe should be taken into consideration when determining the equivalent length of pipe; and,
   [ Note: Alternatively, the size of the discharge pipe may be determined in accordance with Annex D of BS 6700.]

5. terminate in a safe place where there is no risk to persons in the vicinity of the point of discharge. See Building Regulation G3.

Discharge pipes from expansion valves

G19.4   The discharge pipe from an expansion valve may discharge into the tundish used for the discharge from a temperature relief valve or from a combined temperature and pressure relief valve as described in G19.1; or:

1. discharge through a readily visible air gap over a tundish located in the same room or internal space and vertically as near as possible and in any case within 500 mm of the point of outlet of the valve; and,

2. be of non-ferrous material, such as copper or stainless steel; and,

3. discharge from the tundish through a vertical drop outlet and thereafter be laid to a self draining gradient; and,

4. not be less than the nominal outlet size of the expansion valve and discharge external to the building at a safe and visible location.

Vent pipes

G20.1   Vent pipes from primary water systems should be of adequate size but not less than 19 mm internal diameter. They may terminate over their respective cold water feed and expansion cisterns, or elsewhere providing there is a physical air gap, at least equivalent to the size of the vent pipe, above the top of the warning pipe, or overflow if there is one, at the point of termination.

G20.2   Vent pipes from hot water secondary storage systems should be of adequate size but not less than 19 mm internal diameter and be insulated against freezing.

G20.3   Where vent pipes, from either a primary or secondary system, terminate over their respective cold water feed cisterns, they should rise to a height above the top water level in the cistern sufficient to prevent any discharge occurring under normal operating conditions. See Figure 8.3 for determination of minimum height required to prevent discharge.

Hot water systems supplied with water from storage cisterns

G20.4   In any cistern-fed vented or unvented hot water storage system the storage vessel should:

1. be capable of accommodating any expansion water; or
2. be connected to a separate expansion cistern or vessel; or
3. be so arranged that expansion water can pass back through a feed pipe to the cold water storage cistern from which the apparatus or cylinder is supplied with water. See Figure 8.2.

G20.5   Where the cold water storage cistern supplying water to the hot water storage vessel is also used to supply wholesome water to sanitary or other appliances, any expansion water entering the cistern through the feed pipe should preferably not raise the temperature of the wholesome water in the cistern to more than 20⁰ C.

Vented systems requiring dedicated storage cisterns or mechanical safety devices

G20.6   Every vented and directly heated hot water storage vessel, single feed indirectly heated hot water storage vessel, or any directly or indirectly heated storage vessel where an electrical immersion heater is installed, should be supplied with water from a dedicated storage cistern unless:

1. where the energy source is gas, oil or electricity, a non-self-setting thermal energy cut-out device is provided in addition to the normal temperature-operated automatic-reset cut-out; or,
2. where the energy source is solid fuel, a temperature relief valve complying with BS 6283: Part 2, or a combined temperature and pressure relief valve complying with BS 6283: Part 3, is provided complete with a readily visible air-break to drain device and discharge pipe as described in G19.3.

G20.7   Every double feed indirectly heated hot water storage system which is heated by a sealed (unvented) primary circuit, or the primary circuit heating medium is steam or high temperature hot water, or where an electric immersion heater is installed, should:

1. be supplied with water for the secondary circuit from a dedicated cold water storage cistern; or,
2. be provided with a non-self-setting thermal energy cut-out device to control the primary circuit, and any electric immersion heaters, in addition to any temperature-operated automatic-reset cut-out.

G20.8   No water in the primary circuit of a double feed indirect hot water storage vessel should connect hydraulically to any part of a hot water secondary storage system.

G20.9   Vent pipes from primary circuits should not terminate over cold water storage cisterns containing wholesome water for supply to sanitary appliances or secondary hot water systems.

G20.10   Vent pipes from secondary hot water systems should not terminate over feed and expansion cisterns supplying water to primary circuits.

G20.11   No water in the primary circuit of a single feed indirect hot water storage vessel, under normal operating conditions, should mix with water in the secondary circuit. Single feed indirect hot water storage vessels should be installed with a permanent vent to the atmosphere.

Primary feed and expansion cisterns

G21.1   Every expansion cistern, and every cold water combined feed and expansion cistern connected to a primary or heating circuit should be capable of accommodating any expansion water from the circuit and installed so that the water level is not less than 25 mm below the overflowing level of the warning pipe when the primary or heating circuit is in use. See Fig. 8.4.

Expansion and safety devices

G22.1   Expansion valves, temperature relief valves or combined temperature and pressure relief valves connected to any fitting or appliance should close automatically after an operational discharge of water and be watertight when closed.

G22.2   Expansion valves should comply with BS 6283: Part 1. They should be fitted on the supply pipe close to the hot water vessel and without any intervening valves, and only discharge water when subjected to a water pressure of not less than 0.5 bar (50 kPa) above the pressure to which the hot water vessel is, or is likely to be, subjected to in normal operation.

Temperature and combined temperature relief valves

G23.1   Except for unvented hot water storage vessels of a capacity of 15 litres or less, a temperature relief valve complying with BS 6283: Part 2, or a combined temperature and pressure relief valve complying with BS 6283: Part 3, should be provided on every unvented hot water storage vessel. The valve should:

1. be located directly on the storage vessel, such that the temperature of the stored water does not exceed 100°C; and,
2. only discharge water at below its operating temperature when subjected to a pressure not less than 0.5 bar (50 kPa) greater than the maximum working pressure in the vessel to which it is fitted, or 0.5 bar (50 kPa) greater than the operating pressure of the expansion valve, whichever is the greater.

Non-mechanical safety devices

G23.2   If a non-mechanical safety device such as a fusible plug is fitted to any hot water storage vessel, that vessel requires a temperature relief valve or combined temperature and pressure relief valve designed to operate at a temperature not less than 5°C below that at which the non-mechanical device operates or is designed to operate.

Filling of closed circuits

G24.1   No primary or other closed circuit should be directly and permanently connected to a supply pipe unless it incorporates an approved backflow prevention arrangement.

G24.2   A connection may be made to a supply pipe for filling or replenishing a closed circuit by providing a servicing valve and an appropriate backflow prevention device, the type of which will depend on the degree of risk arising from the category of fluid contained within the closed circuit, providing that the connection between the backflow prevention device and the closed circuit is made by:

1. a temporary connecting pipe which must be completely disconnected from the outlet of the backflow prevention device and the connection to the primary circuit after completion of the filling or replenishing procedure (Figure 8.1b shows an acceptable method, of filling a closed circuit, providing that the fluid in the closed circuit is not a greater hazard than fluid category 3) ; or

2. a device which in addition to the backflow prevention device incorporates an air gap or break in the pipeline which cannot be physically closed while the primary circuit is functioning; or

3. an approved backflow prevention arrangement.

Figure 8.1: Diagrams of unvented hot water storage systems

Figure 8.2: Direct and indirect hot water storage systems

Figure 8.3: Determination of minimum height of top of vent pipe

Figure 8.4: Details of feed and expansion cisterns

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Published 22 December 1999

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