A tundish looks like a small fitting, but in G3 revision it represents a serious safety chain. If a temperature or pressure relief device discharges, the system needs a visible warning point and a discharge route that can handle hot water and steam without creating a new hazard.
The tundish makes safety discharge visible; the D2 pipe then has to carry hot water and steam safely to a visible, safe termination.
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.
D1 and D2 in plain English
D1 is the short pipework from the relief valve to the tundish. D2 is the discharge pipework downstream of the tundish. The learner mistake is treating D2 like normal overflow or waste pipe. It may need to carry high-temperature water and steam from a safety device, so size, material, route, support and termination matter.
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.
Quick answers
What does a tundish do on an unvented cylinder?
It makes safety discharge visible and forms part of the discharge route from relief devices, so faults are noticed rather than hidden.
What are D1 and D2 discharge pipes?
D1 is the pipe from the relief valve to the tundish. D2 is the pipe from the tundish to the final discharge point.
Can unvented discharge go into plastic waste pipe?
Only if the route and material are suitable for the discharge temperature and the current guidance/manufacturer instructions are satisfied. Do not assume ordinary waste pipe is acceptable.
Is a dripping tundish normal?
No. Visible discharge is a fault clue. The cause should be investigated rather than capped or ignored.
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