A tundish is an open fitting that receives appliance discharge across a visible air break, then carries the water into a downstream drain.
It lets you see when water is flowing and keeps the upstream discharge pipe from becoming a closed direct connection. It does not explain why the water appeared.
Which parts make up the tundish arrangement?
The visible cup is only one part of the complete discharge path.
- Upstream discharge pipe: Carries water from a relief valve, condensate outlet or another approved source.
- Open air break: Separates that pipe from the receiving fitting and keeps the flow visible.
- Tundish body: Catches the discharge without closing the open separation.
- Downstream drain: Carries the expected flow to a lawful destination using the required capacity, material and fall.
The opening must remain clear and observable. A cap, hose or direct connection can defeat that function and may obstruct a safety discharge path.
Is a tundish a trap or backflow device?
A tundish is not the same thing as a trap. A trap holds a water seal against drainage gases, while a tundish receives visible discharge.
A trap may form another part of the downstream connection. Its need and position depend on the complete drainage design.
The open separation can contribute to backflow protection, but a tundish is not a universal backflow device. Hazard level and the whole installation determine the required protection.
Our guide to drinking-water backflow prevention explains why one visible air break cannot be applied to every risk.
Where are tundishes used?
Water heaters can use a tundish within a relief-valve drain arrangement. It makes discharge visible while the downstream pipe carries water to the approved point.
Some air-conditioning and mechanical equipment uses a product-specific tundish for condensate. That cool condensate arrangement is not interchangeable with a hot relief-water drain.
Other building services may also discharge through an observable air break. The source, water temperature, expected flow and contamination risk control the fitting and drain design.
A tundish does not make an unsuitable destination compliant. Do not combine discharge lines or redirect them to sanitary drainage, stormwater or ground without design and authority evidence.
Should water be running through it?
Visible water can be expected during some appliance cycles. For example, some storage-water-heater relief discharge can occur while the water is heating and expanding.
Continuous, heavy, newly changed or overflowing flow needs assessment. It can point to a valve, pressure, temperature, appliance or downstream drainage problem.
Check which appliance feeds the tundish and when the flow occurs. Photograph the fitting and data plate from a safe position without opening covers or touching the discharge.
For a water heater, compare the symptoms with how a relief valve normally discharges. The tundish receives the water; it is not the valve creating it.
Warning signs around a tundish
- Hot or forceful discharge: Keep people clear because relief water can cause burns.
- Overflow at the cup: The downstream pipe may be restricted, undersized, poorly graded or unable to accept the flow.
- Water near electrical equipment: Do not touch wet equipment or enter standing water. Arrange urgent electrical and plumbing assistance.
- Cracks or leaks: Water escaping from the body or drain needs assessment before it damages nearby materials.
- Unknown discharge: Avoid contact when the source, temperature or possible contamination is unclear.
Do not put a finger, wire or tool into the opening. A concealed drain restriction can remain even when the tundish itself looks clear.
Can you cap or alter a tundish?
No. Never cap, plug, clamp, valve off or bridge the visible air break. Do not connect the upstream pipe directly into the downstream drain.
A water-heater relief line must stay open and unrestricted. Blocking it can interfere with a safety function rather than stopping the underlying fault.
Do not operate a relief-valve lever unless the current instructions for that exact appliance direct you to do so. Remain clear of hot discharge at all times.
The QBCC treats water-heater drain lines and tundishes as part of a compliant installation. Its water-heater drain-line guidance also points back to the applicable plumbing requirements.
Who should inspect it in Queensland?
A resident can observe the flow, identify the connected appliance and record when it happens. Fixed relief, water, drainage, gas or electrical work belongs with the appropriately licensed trade.
The correct contractor depends on the source. Hot-water discharge, air-conditioning condensate and another building-service outlet can require different technical and licensing scopes.
If a water-heater tundish is flowing continuously, heavily or unsafely, arrange a hot-water repair assessment. Keep the discharge path open and share the model and photographs before attendance.