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water_damageUnvented Cylinder Fault

Why Is Water Dripping From My Tundish?

A dripping or running tundish means a safety valve on your unvented cylinder is passing. Here is what it actually means, what causes it, and what it costs to put right. G3 qualified, across Orpington, Bromley and Kent.

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helpWhat It Means

What a Dripping Tundish Is Telling You

Where the tundish sits
Where the tundish sits in an unvented cylinder discharge arrangementA safety valve on the cylinder discharges through a short pipe, D1, into a tundish with a visible air gap. From there the discharge pipe, D2, drops straight down at least 300 millimetres, then falls continuously to a safe place outside.UnventedcylinderT&P reliefvalveD1: 600 mm max.Tundishvisible air gapD2300 mm min.straight dropfalls 1 in 200 min.Outside wallOutsideSafe placesuch as a gully
Never block, cap or pipe the tundish into anything that could back up: the visible air gap is part of the safety arrangement.
Where the tundish sits. Water appearing here means a safety device has operated — switch off the heat source safely and get it looked at promptly. An explanatory schematic, not an installation drawing; the pipe rules shown are from Approved Document G.
Read this diagram as text
An unvented cylinder has a temperature and pressure relief valve on its side. If that valve operates, water passes along a short pipe, called D1, into a tundish, which is an open funnel with a visible air gap so that any discharge can be seen. D1 should be no more than 600 mm long. From the tundish a larger discharge pipe, D2, drops straight down for at least 300 mm before any bend, then falls continuously, at least 1 in 200, to a safe place outside such as a gully. The tundish and its discharge pipe must never be blocked, capped or piped into anything that could back up, because the visible air gap is a required part of the safety arrangement. This is an explanatory schematic rather than an installation drawing.

The tundish is the small open funnel in the pipework above your cylinder. It exists to give a visible air break — so that if a safety valve ever operates, you can see it happening rather than water quietly discharging somewhere hidden.

So water in the tundish is not a leak. It is a safety device doing its job and telling you something upstream is wrong.

Expansion vessel has lost its charge

The most common cause by far. Water expands as it heats, and the expansion vessel is what absorbs that. Lose the air charge and there is nowhere for the expansion to go — so it goes out through the relief valve instead, every single heating cycle.

Vessel set to the wrong pressure

I have found vessels charged 0.5 bar above what the incoming mains could actually deliver, which means the vessel could accept no expansion at all. I have also found vessels still at their original factory precharge from the year they were made, never commissioned to the property.

Failed temperature and pressure relief valve

T&P valves can stick, weep, or fail to reseat properly after operating once. Replacing the valve is straightforward, but a good engineer works out why it operated rather than just fitting a new one.

Failed expansion relief valve

Similar symptom, different valve. Debris on the seat is a common cause, particularly in hard water areas like Orpington, Bromley and North West Kent.

Incoming mains pressure too high

If the pressure reducing valve in the inlet control group has failed, mains pressure can exceed what the system is designed for and push the relief valve open. Measuring the incoming pressure is the first thing I do.

Layers of protection on an unvented cylinder
The layers of protection on an unvented hot water cylinderTwo scales. Pressure: the reducing valve holds the cylinder at a working pressure of about 3 bar; the expansion relief valve opens at about 6 bar and the temperature and pressure relief valve at about 7 bar. Temperature: the thermostat stores water at around 60 degrees; a thermal cut-out switches the heat source off at around 80 to 85 degrees; the temperature and pressure relief valve opens at 90 degrees.Pressurebar02468Working pressure, typically 3 bar7 bar T&P valve6 bar expansionrelief valve3 bar workingpressure, held bythe reducing valveTemperature°C020406080100Stored water, usually around 60 degrees90°C T&P valve80–85°C cut-outswitches the heat off60°C normalstorage, set bythe thermostat
  • Normal working pressure
  • Normal stored temperature
  • Safety device steps in
Typical settings for a domestic unvented cylinder; the data label on yours shows its own. Each device is a separate layer, and the T&P valve is the last one. If the expansion vessel loses its charge, pressure climbs to the relief valve and water appears at the tundish, which is why every device is tested at a service rather than just looked at.
Read this diagram as text
Pressure: the pressure reducing valve holds the cylinder at a working pressure of about 3 bar. As the water heats it expands, and the expansion vessel takes up the extra volume. If the vessel has lost its charge, pressure rises until the expansion relief valve opens at about 6 bar. The temperature and pressure relief valve opens at about 7 bar.

Temperature: the thermostat stores water at around 60°C. If it fails, a separate thermal cut-out switches the heat source off at around 80 to 85°C. If that fails too, the temperature and pressure relief valve opens at 90°C and discharges through the tundish. These are typical values; the cylinder’s data label gives its own.
helpFAQ

Tundish & Discharge Questions

Treat it as something to act on now rather than later. The tundish is the visible air break that shows a safety device has operated. If hot water is discharging from your cylinder's discharge pipe, switch off the heat source using the normal controls if you can do so safely, and contact a competent unvented-cylinder engineer promptly. Keep clear of hot discharge. Do not block the discharge pipe, adjust safety valves or attempt a repair yourself. Follow the safety instructions for your model.
Switch off the heat source using the normal controls if you can do so safely, and call me. If the discharge is heavy and the cold mains isolating valve is accessible, turn that off too. Do not block the discharge pipe.
In most cases the expansion vessel has lost its charge or was set to the wrong pressure, so there is no room for the water to expand as it heats. Second most common is a failed temperature and pressure relief valve or expansion relief valve.
Diagnosis is included in a £140 G3 service visit. Most repairs are a combination valve, an expansion relief valve or re-priming the vessel — all quoted at a fixed price agreed before I start.
Usually not. A dripping tundish is nearly always a valve or expansion problem, not a failed cylinder. Be wary of anyone quoting a replacement before they have measured your incoming pressure and checked the vessel charge.
Only to catch water while you arrange a visit, and only if you can do it without obstructing anything. The tundish and its discharge pipe must never be blocked, capped or piped into something that could back up — it is a required visible air break.
Almost certainly. The underlying causes — a discharged expansion vessel, a failing valve, or excessive incoming pressure — do not resolve themselves.
I cannot assess that remotely, and nor should anyone else. What I can tell you is that a safety device has operated and the cause needs establishing by someone competent. Switch off the heat source if you safely can, keep clear of the discharge, and get it looked at promptly.
Depends on the day, but I am local to Orpington and Bromley and I will tell you honestly when I can be there rather than promising a slot I cannot make.

Get Your Tundish Sorted

£140 G3 service with full diagnosis, no VAT, across Orpington, Bromley, Sevenoaks and Kent.

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What To Do Now

What To Do Right Now

Before anything else, work out which of these you have — it changes how urgent this is.

Water pouring or running continuously

Switch off the heat source using the normal controls if you can do so safely. If the cold mains isolating valve on the inlet control group is accessible, turn that off too, or use your main internal stopcock. Then call me. Continuous discharge means a safety device is operating repeatedly, and the cause needs investigating by someone competent.

An occasional drip after the cylinder heats up

Not something to live with. It means a safety device is operating, and the cause needs investigating. Book it in rather than watching it.

A one-off you have never seen before

Get it looked at. A safety device has operated and the reason needs establishing. Safety valves that operate once and reseat properly sometimes do not reseat the second time.

What not to do

Do not cap, plug or block the tundish or the discharge pipe. It exists specifically so that discharge is visible and can escape safely. Blocking it removes the protection the whole system depends on.

photo_cameraThe Valves

The Devices That Discharge

When water reaches your tundish, one of these has operated.

Expansion relief valve and temperature and pressure relief valve on a cylinder
Reliance 8 bar expansion relief and TPR15 temperature & pressure relief valve
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