Tundish and discharge pipe basics (D1 and D2)

The safety discharge is part of an unvented cylinder’s protection. D1 runs from the safety valve to the tundish; D2 continues after the tundish to a suitable termination. The cylinder’s cold-water inlet is a separate supply, not either discharge section.

D1 and the visible air break

Under Approved Document G for England, D1 is metal and at least the nominal size of the safety-valve outlet. Position the tundish vertically, in the same space as the cylinder, below and close to the valve, with no more than 600 mm of pipe between them. Discharge at the tundish must be visible; consider an additional suitable warning where an occupant cannot see it.

D2 carries the discharge by gravity

Provide at least 300 mm of straight vertical pipe below the tundish before a bend or elbow. The onward route needs a continuous fall of at least 1:200. D2 may be metal or another material with demonstrated suitability for the discharge conditions and permanent product/performance identification. Do not assume ordinary plastic waste pipe is suitable, or extend the D2 material options to D1.

Include the bends when sizing

Table 3.1 sizes copper discharge pipe by the safety-valve outlet and the route’s equivalent resistance. For a G½ outlet, the minimum D1 is 15 mm and 22 mm copper D2 has a 9 m equivalent-length allowance. Each bend or elbow in that 22 mm route uses 0.8 m of the allowance.

For example, 6 m of pipe with two such bends uses 6 + (2 × 0.8) = 7.6 m. A measured 9 m run with those bends would exceed the 22 mm allowance. Recalculate using the table’s allowances for the proposed larger diameter; different materials need their own supported sizing data. Increasing D1 alone does not change the valve outlet used for sizing.

Finish safely

Choose a permitted termination suited to the site. A guarded external outlet, trapped gully and specified soil-stack connection each have different conditions. Visibility at the tundish does not require every length of D2 to remain exposed. Never leave an open end discharging into a concealed wall void, and never fit a shut-off valve in the safety route. Confirm the complete arrangement against the applicable guidance and manufacturer’s instructions.