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G3 REQUIREMENTS AND GUIDANCE - UK ONLY

Discharge pipe from safety valves

The Building Regulation G3 requires that any discharge from an unvented system is conveyed to where

it is visible, but will not cause danger to persons in or about the building.

The tundish and discharge pipes should be  tted in accordance with the requirements and guidance

notes of Building Regulation G3. The G3 Requirements and Guidance sections 3.50 - 3.63 are detailed

below.

For discharge pipe arrangements not covered by G3 Guidance advice should be sought from your

local Building Control Oﬃ cer.

Main characteristics :

• Any discharge pipe connected to the pressure relief devices (Expansion Valve and Temperature/

Pressure Relief Valve) must be installed in a continuously downward direction and in a frost free

environment.

• Water may drip from the discharge pipe of the pressure relief device.

• This pipe must be left open to the atmosphere.

• The pressure relief device is to be operated regularly to remove lime deposits and to verify that it

is not blocked.

A typical discharge pipe arrangement is shown on next page.

General remarks

• Discharge pipe-work D2 can now be a plastic pipe but only pipes that have been

tested to a minimum 110°C must be used.

• Discharge pipe D2 can now be plumbed into the soil stack but only soil stacks that can

handle temperatures of 99°C or greater should be used.

Extract from “The Building Regulation G3” :

Discharge pipe D1

3.50 Safety devices such as temperature relief valves or combined temperature and pressure and

pressure relief valves (see paragraphs 3.13 or 3.18) should discharge either directly or by way of a

manifold via a short length of metal pipe (D1) to a tundish.

3.51 The diameter of discharge pipe (D1) should be not less than the nominal outlet size of the tem-

perature relief valve.

3.52 Where a manifold is used it should be sized to accept and discharge the total discharge from the

discharge pipes connected to it.

3.53 Where valves other than the temperature and pressure relief valve from a single unvented hot

water system discharge by way of the same manifold that is used by the safety devices, the man-

ifold should be factory fitted as part of the hot water storage system unit or package.

Tundish

3.54 The tundish should be vertical, located in the same space as the unvented hot water storage sys-

tem and be fitted as close as possible to, and lower than, the valve, with no more than 600mm of

pipe between the valve outlet and the tundish.

Note: To comply with the Water Supply (Water Fittings) Regulations, the tundish should incorporate a

suitable air gap.

UK 3.55 Any discharge should be visible at the tundish. In addition, where discharges from

safety devices may not be apparent, e.g. in dwellings occupied by people with im-

paired vision or mobility, consideration should be given to the installation of a suitable safety

device to warn when discharge takes place, e.g. electronically operated.

Discharge pipe D2

3.56 The discharge pipe (D2) from the tundish should:

(a) have a vertical section of pipe at least 300mm long below the tundish before any elbows or

bends in the pipework; and

(b) be installed with a continuous fall thereafter of at least 1 in 200.

3.57 The discharge pipe (D2) should be made of:

(a) metal; or

(b) other material that has been demonstrated to be capable of safely withstanding tempera-

tures of the water discharged and is clearly and permanently marked to identify the product and

performance standard (e.g. as specified in the relevant part of BS 7291)

3.58 The discharge pipe (D2) should be at least one pipe size larger than the nominal outlet size of the

safety device unless its total equivalent hydraulic resistance exceeds that of a straight pipe 9m

long, i.e. for discharge pipes between 9m and 18m the equivalent resistance length should be at

least two sizes larger than the nominal outlet size of the safety device; between 18 and 27m at

least 3 sizes larger, and so on; bends must be taken into account in calculating the flow resist-

ance. See figure, table and the worked example.

3.59 Where a single common discharge pipe serves more than one system, it should be at least one

pipe size larger than the largest individual discharge pipe (D2) to be connected.

3.60 The discharge pipe should not be connected to a soil discharge stack unless it can be demon-

strated that that the soil discharge stack is capable of safely withstanding temperatures of the

water discharged, in which case, it should:

(a) contain a mechanical seal, not incorporating a water trap, which allows water into the

branch pipe without allowing foul air from the drain to be ventilated through the tundish;

(b) be a separate branch pipe with no sanitary appliances connected to it;

be either polybutalene (PB) to Class S of BS 7291-2:2006 or cross linked polyethylene (PE-X) to

Class S of BS 7291-3:2006; and (d) be continuously marked with a warning that no sanitary ap-

pliances should be connected to the pipe.

Note:

1. Plastic pipes should be joined and assembled with fittings appropriate to the circumstances in

which they are used as set out in BS EN ISO 1043-1.

2. Where pipes cannot be connected to the stack it may be possible to route a dedicated pipe along-

side or in close proximity to the discharge stack.

Termination of discharge pipe

3.61 The discharge pipe (D2) from the tundish should terminate in a safe place where there is no risk

to persons in the vicinity of the discharge.

3.62 Examples of acceptable discharge arrangements are:

(b) to a trapped gully with the end of the pipe below a fixed grating and above the water seal;

parks, hard standings, grassed areas etc. are acceptable providing that a wire cage or similar

guard is positioned to prevent contact, whilst maintaining visibility; and

(d) discharges at high level: e.g. into a metal hopper and metal downpipe with the end of the dis-

charge pipe clearly visible or onto a roof capable of withstanding high temperature discharges

of water and 3m from any plastic guttering system that would collect such discharges.

WaterMaster 25 - 35 - 45 - 70 - 85 - 120 : A1006482 - 664Y7300 • D

TECHNICAL CHARACTERISTICS

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