UK trade guideCylinder Spares 4 min readUpdated 9 July 2026

How to Replace a Tundish (UK)

Fit a new 15/22 mm tundish on an unvented discharge pipework. Cure drips and building regs compliance.

RelianceAltecnic

Discharge sizing

D1 = tundish inlet from valves (typically 15 mm). D2 = tundish to termination (22 mm minimum). Length: max 9 m for 22 mm; 18 m for 28 mm.

Fit

Undo compression at inlet and outlet. Support the D2 vertical drop before removing. Fit new tundish with 300 mm vertical drop below it (visible air break).

Frequently asked questions

Why is the air gap visible?
Building Regs G3 — must be observable to prove the discharge pipe is not blocked back to pressure.
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