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. Size D2 against the safety valve outlet and the resistance of the discharge route.

Keep the measurements separate: 600 mm is the maximum D1 pipe length, 300 mm is the minimum straight vertical D2 drop before a bend, and 1 in 200 describes the fall afterwards — 5 mm per metre. These figures use England’s guidance.

• 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: size from the safety valve outlet, allowing for the route and bends.
• 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

The bend allowance changes with the copper pipe diameter. In Table 3.1 of England’s Approved Document G, each 22 mm bend adds 0.8 m of equivalent length, each 28 mm bend adds 1.0 m, and each 35 mm bend adds 1.4 m. Do not carry the 0.8 m allowance across every pipe size.

For a G½ temperature relief valve with 15 mm D1, the straight-equivalent limits are 9 m for 22 mm copper D2, 18 m for 28 mm and 27 m for 35 mm. Add the actual pipe length and the bend allowances, then compare with the limit for that valve and candidate diameter. Plastic systems need their own bore and resistance data.

Worked D2 sizing check

Take a G½ valve, 15 mm D1 and a proposed copper D2 route measuring 6 m with five elbows. At 22 mm, the equivalent length is 6 + (5 x 0.8) = 10 m, exceeding the 9 m limit.

Recheck at 28 mm using its own bend allowance: 6 + (5 x 1.0) = 11 m, within the 18 m limit. That passes this sizing check; the fall, initial vertical drop, materials and termination still need checking against the applicable guidance and cylinder instructions.

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 connection and mechanical seal

A soil-stack connection needs a stack suitable for the discharge temperature, a mechanical seal that keeps foul air out without using a water trap, and a separate branch with no sanitary appliances. The branch also needs suitable materials and warning markings. This is a controlled arrangement, not an ordinary waste connection.

What a discharge at the tundish tells you

Water at the tundish means a relief valve has opened. Note three things before touching anything: which valve it is coming from, whether it appears only while the cylinder heats or all the time, and whether it is cold or very hot. The pattern narrows the search; it does not name the failed part. Settings and the order of checks differ between cylinders, so the fault-finding chart in the installed cylinder’s instructions decides, and work on these controls needs unvented competence.

• Only while the water heats, from the expansion relief valve: the expanding water has nowhere to go. Check the expansion provision by the manufacturer’s method — an external vessel’s pre-charge with its water side depressurised, or the recharge procedure for an internal air gap.
• All the time, from the expansion relief valve, even with no heat input: compare the pressure reducing valve’s outlet pressure with the cylinder’s stated setting, then look for debris or damage on the relief valve seat.
• Very hot water or steam from the temperature and pressure relief valve: treat it as a thermal control failure. Switch off every heat source to the cylinder — immersion heaters and boiler — and keep clear of the discharge. Some manufacturers also say to leave the water supply on until it stops, so read the instructions before isolating anything else.
• Nothing discharging today, but staining or scale below the tundish: signs of past discharge are one of the things an annual service looks for. Record them and check the same three points. The G3 revision notes cover expansion vessels and the service checks in full.

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.