D1 runs from the valve to the tundish, D2 from the tundish to a safe end: the air break, positions, fall, material and sizing with bends.
The safety discharge has two named sections: D1 from the relief valve to the tundish, and D2 from the tundish to a safe termination.
The tundish
The tundish provides an open, visible break between the safety-valve outlet and the onward discharge pipe. This separates the outlet from the drain and shows when water passes. Some suitable products combine this air break with a mechanical odour seal for an internal connection; the seal is a separate drainage function and does not replace the relief valve.
Under Approved Document G for England, fit it vertically, in the same space as the cylinder, as close as possible to and lower than the valve, with no more than 600 mm of pipe between them. D1 is metal and at least the nominal size of the valve outlet. Keep the tundish visible and unobstructed, even inside a cupboard.
D2 runs by gravity
Below the tundish, D2 runs straight down for at least 300 mm before any bend, so the discharge falls away without backing up and spilling at the tundish. After that it falls continuously at not less than 1 in 200 (5 mm per metre). The air break means D2 is not a sealed extension of the cylinder. Size and route it for gravity discharge with the required fall; do not rely on backing-up water or cylinder pressure to carry it over a joist.
D2 must withstand near-boiling water: metal, or another material whose suitability has been demonstrated and which carries permanent marking. Ordinary plastic waste pipe is not suitable just because it fits.
Sizing with bends
For the basic copper sizing approach, start at least one pipe size larger than the valve outlet and include every bend in the equivalent length. For a G½ valve, D1 is 15 mm, and 22 mm copper D2 allows 9 m with each bend counting 0.8 m: 6 m with two bends uses 6 + (2 × 0.8) = 7.6 m, which fits.
For a G¾ valve, 28 mm allows 9 m (1.0 m per elbow) and 35 mm allows 18 m (1.4 m per elbow). A 6.0 m run with four elbows is 10.0 m at 28 mm, too long, but 11.6 m at 35 mm, so 35 mm is the smallest compliant size. Size from the valve outlet: an oversized D1 does not change it.
A concealed section or specified soil-stack final connection can be acceptable while the tundish stays visible. Never fit an isolating or shut-off valve that could block relief. A suitable mechanical odour seal without a water trap may form part of the specified stack arrangement and must pass the required discharge.