Internal services

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8.1.5Hot water service

Hot water service shall be provided in accordance with statutory requirements and be adequate for the demand and consumption.

Hot water services should be designed in accordance with Tables 3, 4 and 5, and:

  • the minimum flow rate should be in accordance with the statutory requirements and generally be available; it may be less where the pressure and flow rate of the incoming supply falls below 1.5 bar
  • have the design flow rate available at each outlet when the total demand does not exceed 0.3L/s (where simultaneous discharge occurs, the flow rate at individual outlets should not be less than the minimum rate).

Table 3: Flow rate and temperature requirements


1 The design flow rate should be used to establish the hot and cold pipe sizes to provide the flow rate quoted at each outlet when that outlet is used on its own.
2 The minimum flow rate should be available at each fitting when that fitting is used simultaneously with one or more other fitting(s) as shown in Table 4.
3 The supply temperature is the temperature at the outlet. In accordance with BS 8558 the water temperature at an outlet or thermostatic mixing valve should be at least 50°C within 1 minute of running the water.

Table 4: Hot water demand and simultaneous use


1 Shower may be over the bath or in a separate enclosure within the bathroom.
2 Demand based on ‘Design’ flow rate of bath.
3 Demand based on minimum acceptable boiler output.
4 Demand based on use of the shower in preference to the bath.
5 The hot water system should supply at least the hot water demand stated and take account of distribution heat losses through the pipework. The suitability of
instantaneous systems (combination boilers) will be limited by their performance as quoted by the boiler manufacturer.

Hot water storage should comply with the minimum capacity in Table 5 (based on a draw-off temperature of 60°C), and where appliances require greater volumes, the capacity should be increased accordingly.

Table 5: Minimum storage requirements

Shower onlyBath onlyBath and showers(s)(1)Two baths


1 Maximum of two showers (excludes instantaneous electric showers).

Where systems are heated by off-peak electricity, the storage capacity should be in accordance with the recommendations of the electricity supplier.

Where homes have one bathroom or shower room, the system should be able to provide adequate hot water:

  • immediately after the bath has been filled, for tasks such as washing
  • for a second bath after 20 minutes.

Where homes have two or more bathrooms, the system should be able to provide adequate hot water immediately after each of the baths have been filled, for tasks such as washing.

Where a shower is installed, adequate provision should be made to ensure that the outlet temperature of the water is not significantly affected by the use of other hot or cold outlets in the home. This may be achieved by the provision of a thermostatic shower mixing valve, the appropriate design of pipe sizes or dedicated supplies.

Instantaneous systems (using combination boilers) produce hot water on demand (generally at lower flow rates than storage systems), and should only be used where:

  • simultaneous demand for hot water is limited. Where there are three or more outlets, the design for simultaneous discharge can omit the outlet at the kitchen sink
  • storage combination boilers have the capacity as required in Table 5. Where boilers can control and prioritise hot water outputs, the storage capacities can be less than the figures in Table 4 subject to manufacturer’s recommendations on meeting the demand.

Storage systems provide higher flow rates than instantaneous systems, and:

  • require a suitable space for the siting of the storage vessel
  • where vented, should be provided with an expansion pipe.

Unvented hot water storage systems should be:

  • assessed in accordance with Technical Requirement R3, or meet the requirements of BS EN 12897 and be the subject of third-party certification, e.g. Kitemarking (applies to both the assembled system and components)
  • installed by competent installers.

Hot water cylinders should be:

  • supported in accordance with manufacturer’s recommendations
  • installed vertically, unless designed otherwise
  • accessible for maintenance
  • insulated as specified in the design.

Where an immersion heater is fitted, it should be:

  • appropriate for the type of water supplied to the home
  • controlled by a thermostat
  • located to facilitate replacement
  • fitted with an on/off switch.