Fabdec EXCELSIOR, Installation Manual

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3.5

Discharge Pipework

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G3 Requirement 

‘...there shall be precautions...to ensure that the hot water discharged from safety devices is safely conveyed to where it is 

visible but will not cause danger to persons in or about the building.’ 

G3 Guidance Section 3.5 

The discharge pipe (D1) from the vessel up to and including tundish is generally supplied by the manufacturer of the hot 

water storage system. Where otherwise the installation should include the discharge pipe(s) (D1) from the safety 

device(s). In either case the tundish should be vertical, located in the same space as the unvented hot water storage 

system and be fitted as close as possible to, and lower than, the safety device, with no more than 600mm of pipe between 

the valve outlet and the tundish (See Figure 13). 

The discharge pipe (D2)  

The discharge pipe (D2) from the tundish should: 

Have a vertical section of pipe at least 300mm long below the tundish before any elbows or bends in the pipework (see 

Figure13).

Be installed with a continuous fall of at least 1 in 200 thereafter. 

The discharge pipe (D2) should be made of metal or other material that has been demonstrated to be capable of safely 

withstanding temperatures 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 7291-1:2006 Thermostatic pipes and fittings for hot and 

cold water for domestic purposes and heating installations in buildings, General Requirements).

Termination of discharge pipe

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. 

Examples of acceptable discharge arrangements are:

To a trapped gully with the end of the pipe below a fixed grating and above the water seal.

Downward discharges at low level; i.e. up to 100mm above external surfaces such as car parks, hard standings, grassed 

areas etc

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 are acceptable providing that a wire cage or similar guard is positioned to prevent contact, whilst maintaining 

visibility. 

Discharges at high level: e.g. into a metal hopper and metal downpipe with the end of the discharge 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. 

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The discharge would consist of high temperature water and steam. Asphalt, roofing felt and non-metallic rainwater goods 

may be damaged by such discharges 

Worked Example of Discharge Pipe Sizing

The example below is for a G

1/2

 temperature relief valve with a discharge pipe (D2) having 4 No elbows and length of 7m 

from the tundish to the point of discharge. From table 1: (overleaf) 

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Maximum resistance allowed for a straight length of 22mm copper discharge pipe (D2) from G1/2  temperature relief valve 

is 9m. Subtract the resistance for 4 No 22mm elbows at 0.8m each = 3.2m, therefore the permitted length equates to 

5.8m. This is less than the actual length of 7m therefore calculate the next largest size. 

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Maximum resistance allowed for a straight length of 28mm pipe (D2) from a G1/2 temperature relief valve equates to 18m 

Subtract the resistance of 4 No 28mm elbows at 1m each = 4m. 

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Therefore the maximum permitted length equates to 14m. As the actual length is 7m a 28mm (D2) copper pipe will be 

satisfactory. 

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