What is a tundish? Unvented cylinder discharge pipe (D1/D2)

What the tundish is for

The tundish sits in the safety discharge route from an unvented hot-water system. Its job is not to make the fault disappear; it makes discharge visible and provides the air-gap break in the route. A dripping tundish is therefore a fault clue, not a feature to cap, hide or route into ordinary waste pipework without checks.

Tundish, drain, overflow or valve? Clearing up the terms

The tundish itself is not a valve and not a plain overflow — it is an open funnel that breaks the discharge route so the flow is visible. The valves sit upstream: the expansion (pressure) relief valve and the combined temperature and pressure relief valve on the cylinder. What people call the “tundish drain” is really the D2 discharge pipe carrying flow away from the tundish. Nor is it a routine overflow like a cistern warning pipe — anything showing at the tundish has come from a safety device operating, so it points to a fault to investigate, not normal running.

D1 and D2 in plain English

D1 is the short pipework from the safety valve outlet down to the tundish. D2 is everything downstream of the tundish, from the funnel to the final discharge point. The split matters because the two lengths are judged by different rules: D1 is about keeping the valve close to a visible break, D2 is about carrying a hot discharge safely all the way to somewhere it cannot hurt anyone.

The learner mistake is treating D2 like ordinary overflow or waste pipe. It may have to carry water near boiling point, and steam, from a safety device that has just operated. Size, material, route, support and termination all matter — and D2 is never smaller than D1.

• D1 — valve outlet to tundish: keep it as short as it can be, with no more than 600 mm of pipe between the valve outlet and the tundish.
• The tundish sits vertically, below the valve, somewhere it can actually be seen.
• D2 — tundish to termination: at least one pipe size larger than D1.
• D2 drops at least 300 mm vertically below the tundish before any elbow or bend.
• After that vertical leg, D2 runs with a continuous fall — 1 in 200 is the minimum guide value.

Sizing D2: why the length limit moves

D2 is not sized by diameter alone. Approved Document G sets a maximum resistance-allowed length for each combination of valve outlet size and D2 diameter, and every bend eats into that allowance — roughly 0.8 m of the permitted length per elbow. Add enough bends to a long run and a pipe that looked compliant on the drawing no longer is.

For the common ½ inch (G½) temperature relief valve with 15 mm D1, the pattern to remember is that going up a size buys you a lot of length: about 9 m in 22 mm, about 18 m in 28 mm and about 27 m in 35 mm, each figure before deducting for bends. Larger valves shift the same ladder up a size. Treat these as the guide values to work from and confirm against the current Approved Document G table and the cylinder manufacturer’s instructions for the job in front of you.

The revision point is the method, not the memorised number: identify the valve outlet size, pick the D2 diameter, read the permitted length, then subtract for every bend on the actual route. If the route will not fit inside the allowance, go up a pipe size or shorten the run.

Approved Document G points to remember

• Discharge should be visible at the tundish.
• D2 needs at least 300 mm of vertical pipe below the tundish before bends are introduced.
• After that initial vertical section, D2 should run with a continuous fall of at least 1 in 200.
• D2 material must be suitable for the temperature of the discharge.
• D2 sizing is not just a nominal pipe size; bends and effective length reduce capacity.
• A common discharge pipe serving more than one system needs extra sizing care.
• Termination must be in a safe place where people are not put at risk by hot discharge.

Termination examples

Approved Document G gives example discharge arrangements such as a suitable trapped gully arrangement, guarded low-level discharge, or high-level discharge into metal rainwater goods or onto a suitable roof area. The exam point is the logic: visible, safe, heat-resistant and not routed where discharge can injure people or damage unsuitable materials.

Soil-stack trap

Connecting into a soil discharge stack is not a casual shortcut. The stack and branch need to be shown suitable for discharge temperatures, foul air must not ventilate back through the tundish, and sanitary appliances should not be connected into the same branch. For revision, treat soil-stack routes as a controlled design decision, not as a default.

Common exam mistakes

• Saying the tundish is just an overflow.
• Hiding the tundish where discharge cannot be noticed.
• Running D2 uphill or through a route that traps water.
• Using ordinary plastic waste pipe without proving temperature suitability.
• Forgetting bends when thinking about D2 sizing.
• Treating repeated tundish discharge as normal operation.