Water Running Into the Tundish: Emergency or Not?

What is the tundish and why does water in it matter?

The tundish is the small funnel, often with a clear or slotted body, on a pipe near your hot water cylinder. You will mostly find it on unvented cylinders, the kind fed straight from the mains with no tank in the loft, though a sealed heating system can share it. Its job is to give an open, visible air gap between the cylinder’s safety valves and the drain pipe, so you can see when a valve is discharging and so dirty water can never be drawn back towards the supply.

Two valves discharge into it. The expansion relief valve on the cold inlet opens on pressure alone. The temperature and pressure relief valve near the top of the cylinder opens if the water overheats or the pressure climbs too high. In normal use neither should let any water through. So any water in the tundish drain, even a drip, is a fault. The only question is how fast it needs attention.

Is it an emergency? Read the water

Stand at the tundish for a few minutes and note three things: how hot the water is (judge it by steam and by holding your hand near, not on, the pipe below the funnel), how fast it runs, and whether it follows the heating cycle.

• Hot, steady flow, sometimes with hissing, knocking or steam: the temperature and pressure relief valve has opened because the cylinder has overheated. Emergency.
• Hot water overflowing the tundish itself onto the cupboard floor: the drain pipe after the funnel is blocked or too small to carry the flow. Emergency, because scalding water is now escaping indoors.
• Cool or lukewarm drips that start when the cylinder is heating and stop when it is up to temperature: the expansion relief valve is lifting on pressure. Not an emergency tonight, but book it.
• A constant cold trickle, day and night, even when no one has used hot water: the expansion relief valve is letting by, often because the pressure reducing valve before it has failed or grit is sitting on its seat. Book it within a few days.
• A pulse of water only after someone runs a tap and closes it quickly: pressure spikes, again pointing at the expansion vessel or reducing valve.

For a hot discharge: switch off the boiler and the immersion heater at their switches. Leave the cold water supply to the cylinder on until the discharge runs cold, because that fresh cold water is what brings the temperature down. Keep children and pets away from wherever the drain pipe comes out outside.

What usually causes a dripping tundish

In most cases of a dripping tundish the cylinder is not overheating at all. The pattern is pressure. When water heats it expands, and on a sealed unvented system that extra volume has to go somewhere. The expansion vessel, a small white or blue tank by the cylinder (some cylinders use an air bubble inside instead), holds a cushion of air behind a rubber membrane to take it. Over time that air leaks away. With no cushion, every heat-up pushes pressure up until the expansion relief valve opens and the surplus drips into the tundish.

Other causes to rule out:

• A split membrane in the expansion vessel. The vessel fills with water, cannot compress, and the same drip appears. Recharging it will not last; it needs replacing.
• A failed pressure reducing valve on the incoming main. If it no longer cuts mains pressure down to the cylinder’s working pressure, often 3 bar, the relief valve sees too much pressure. More in pressure reducing valve problems.
• Grit or scale on the relief valve seat, often after the valve has lifted once or after work on the mains. The valve never fully closes again.
• An overheating cylinder: a stuck motorised valve on the boiler coil, a failed cylinder thermostat, or an immersion thermostat welded on. This is the cause behind hot discharges, and why the temperature and pressure relief valve is fitted.

Where the tundish drain should go, and signs it is wrong

Building Regulations Part G, section G3, sets out how the pipework must run. The funnel should sit upright in the same space as the cylinder, close to and lower than the valve it serves with only a short run of pipe between them, and where people living there can see it. The pipe after the tundish, known as D2, has to drop vertically for at least 300mm before any bend, be at least one pipe size bigger than the safety valve outlet, and run with a continuous fall to a safe, visible outlet. A common arrangement is to finish below the grating of an outside gully but above the water level in it.

Some problems you can spot without touching anything. A tundish hidden behind panels or boxed into a cupboard nobody opens means discharges go unnoticed for months. A plastic waste pipe used for D2 can soften with a hot discharge. A drain pipe that runs uphill, or disappears into a sink waste, can back up and overflow at the funnel. If the outside end is covered by a patio, bin store or extension, the water has nowhere to go. Any of these is worth pointing out to the engineer when they come for the drip.

What you can safely do, and what to leave alone

Everything on the cylinder’s safety side should be handled by an engineer holding the unvented (G3) qualification. That still leaves some useful things you can do.

• Write down when it drips: during heating, overnight, after baths. That timing points to the cause faster than anything else.
• Put a container under the outside outlet if it is splashing a path, and keep the outlet clear.
• Check the cylinder’s thermostat dial if it is external and easy to see. Around 60°C is normal. A dial set to maximum makes pressure build up faster.
• Know how to isolate the cylinder in case the drip turns into a flow. Our guide on how to turn off the hot water supply from a cylinder shows where the valves usually are.
• Do not twist or lift the relief valve caps to try to reseat them, pump air into the expansion vessel, or adjust the reducing valve. A valve that will not close again turns a drip into a constant flow.
• Do not cap, block or kink the tundish drain to stop the noise or the wet patch outside. That removes the only safe route for scalding water.

What the repair involves

The engineer will first check the stored water temperature to rule out overheating, then read the incoming pressure after the reducing valve and test the air charge in the expansion vessel with a gauge on its valve. If water comes out of the vessel’s air valve, the membrane has gone.

Most dripping tundish jobs end with one of three fixes: re-pressurising the expansion vessel to the maker’s figure, replacing the vessel or the relief valve cartridge, or replacing the pressure reducing valve cartridge. None usually needs a new cylinder. Where the cylinder did overheat, the failed thermostat, cut-out or motorised valve is replaced as well, and the relief valve checked to make sure it has seated. The repair price is agreed with you on site before any work starts, and the cost of the call-out is set out on one page. Book an engineer for the cylinder.