Tundish Dripping: What the Water Is Telling You

What a tundish actually is and why there is a gap in the pipe

An unvented cylinder is a sealed metal vessel fed straight off the incoming mains, with no cold tank in the loft above it. That is what gives you mains pressure hot water at every tap. It also means the stored water sits under real pressure, so if anything goes wrong with the controls there has to be a route for that pressure to escape safely rather than through the cylinder wall. Every unvented cylinder carries safety valves, and every valve needs somewhere to discharge.

The tundish is the deliberate break in that discharge route. A short pipe runs from the valve down into a small plastic or brass funnel with a visible air gap across it, and another pipe leaves the bottom and runs to a safe outside point or a drain. The gap does two jobs. It stops anything at the far end of the discharge pipe siphoning or backing up into the safety valve, so the valve always sees clean atmosphere below it, and it makes any discharge visible, because a safety valve venting into a buried pipe tells nobody anything.

That is the whole design intent. The tundish is a warning light made of plastic. It is normally bone dry, mounted a short distance from the valve at eye level or just above, usually in the airing cupboard beside the cylinder casing. The pipework around it falls under building regulations and its sizing and routing are the installing engineer and building control matter, not something to modify afterwards. Work on a sealed cylinder and the safety pipework around it is the province of a G3 qualified plumber, which is a separate certification from general plumbing and a perfectly fair thing to ask about before anyone opens a cupboard.

Most unvented units have more than one valve discharging into the same tundish. A pressure relief valve forms part of the cold water inlet group where the mains enters, and opens on pressure alone. A temperature and pressure relief valve is screwed into the cylinder body near the top, and opens if the stored water gets dangerously hot as well as if pressure climbs. Knowing that two valves share one funnel is the key to reading the water, because they report completely different failures. If you already know it is the pressure side, our page on a leaking pressure relief valve goes through that valve on its own. A pressure relief valve leaking with the temperature valve above it bone dry is a narrower and calmer problem than a wet tundish of unknown origin.

• Tundish: an open funnel with a visible air gap, normally dry
• Fed by the pressure relief valve on the cold inlet group, which responds to pressure
• Also fed by the temperature and pressure relief valve on the cylinder, which responds to heat
• Discharges onward to a safe visible point outside or to a drain
• Found next to or just above the cylinder, commonly in an airing cupboard

How to tell whether a dripping tundish is serious

Before touching anything, watch it. Five minutes of observation narrows this down further than any amount of guessing, and it is exactly what an engineer will ask for. Note four things: hot or cold, drip or flow, whether it coincides with the cylinder heating, and whether it stops on its own.

A cold drip appearing towards the end of a heating cycle and stopping shortly afterwards is the classic pattern for lost expansion. Nothing is dangerous at that moment, but the system is dumping heated water down the drain several times a day and wearing the valve every cycle.

A cold drip that never stops, carrying on overnight with the heating and the immersion off, is usually a valve that has stopped sealing: after a few dozen lifts, one fragment of scale on the seat will hold it a fraction open forever. It can also mean incoming mains pressure has risen above what the system is set to sit at, in which case the valve is doing its job and the fault is further upstream.

Hot water, steam, gurgling, or a flow rather than a drip is the one that changes the day. That is the temperature and pressure relief valve doing its job as the last line of defence, which means the control thermostat and the overheat cut-out above it have both already failed to stop the heat going in. Treat that as urgent rather than annoying.

• Cold drip at the end of a heating cycle, then stops: expansion has been lost
• Cold drip continuously, day and night: valve seat not sealing, or mains pressure too high
• Drip that started after works in the street or a new stopcock: incoming pressure change
• Hot water or steam: the temperature relief valve has opened, treat as urgent
• Water only when a tap is used: unusual, and worth reporting as it points elsewhere

Do not put your hand under a discharging tundish to find out how hot it is. Water leaving a temperature relief valve can be close to boiling and will scald instantly. Look at it instead: steam, or a hissing flow rather than individual drops, tells you what you need to know from a safe distance.

Tundish dripping cold water after the cylinder heats

This is the common one, and the mechanism is simple physics. Water expands as it gets hot, by roughly four per cent between cold mains and stored hot water temperature. In a vented system that extra volume pushes back up into the loft tank. In a sealed unvented system it has nowhere to go unless space was deliberately provided for it, so every unvented cylinder has expansion provision: either an external vessel with a rubber diaphragm and a cushion of air behind it, or an air bubble held at the top of the cylinder itself. That single difference, unvented vs vented cylinder, is why only one of the two arrangements has a funnel in the cupboard at all.

When that provision stops working, the heating water has nothing to push against and pressure rises sharply every cycle until it reaches the setting of the pressure relief valve, which lifts, spits and closes again. The drip is therefore not random. It follows the heating schedule, and a household that notices the tundish wet at the same times each day has effectively already diagnosed it.

The loss happens two ways. An external vessel loses its air charge over the years, or the diaphragm perforates so the vessel fills with water and does nothing. An internal air bubble gets absorbed into the water over time, which is normal ageing rather than a defect and is one of the things restored during a proper service. Either way, checking and correcting it belongs to an engineer qualified on unvented systems, because it means isolating and depressurising a sealed pressure vessel in the right order.

The third cause sits further upstream. The mains supply into a London property can be well above what an unvented cylinder is designed to sit at, so the inlet group contains a pressure reducing valve that holds the cylinder at a set working pressure. Those valves creep as they age, and when one stops holding, the cylinder sees full mains pressure, which on some London streets is considerably higher than the relief valve setting. The symptom is a drip that is worse at night, because mains pressure across the network rises when nobody is drawing water. That detail alone often separates a failed reducing valve from a failed expansion vessel before anyone opens anything.

A tundish dripping cold water is not an emergency, but it is not something to live with either. Every drip is heated mains water going to waste, the valve seat is being eroded each time it lifts, and the failure mode at the end of that road is a valve that will no longer close at all.

Tundish overflowing with hot water or steam

If hot water is pouring through the tundish, the temperature and pressure relief valve has opened, and the reason it opened is that the water in the cylinder got dangerously hot. Understand what that means in sequence. The control thermostat should have switched the heat source off at around sixty degrees. When it fails, a separate overheat cut-out should have cut the supply at around eighty five. When that fails too, or has been reset repeatedly without the underlying cause being found, the temperature relief valve is the only thing left between you and a cylinder full of water above boiling point held under pressure. It opens at around ninety to ninety five degrees and it dumps that water deliberately.

So a hot discharge is not a plumbing nuisance. It is the system announcing that two layers of protection have been used up and the third is now working. The correct response is to remove the heat, not to stop the water.

Turn off the immersion heater at its wall switch, and turn the boiler off if the cylinder is heated through a coil. Keep children and pets away from the cupboard, because the discharge and the pipework around it will be hot enough to scald. Do not close any valve on the discharge pipe, do not try to hold the relief valve shut, and do not cap anything. That pipe is the only controlled route the energy has.

The underlying cause is usually a failed thermostat, a thermostat pushed out of contact with the cylinder, a wiring fault that leaves an immersion element permanently live regardless of what the controls are asking for, or a cut-out that has been reset without anyone finding out what tripped it. All of those are engineer work, and on a sealed unvented system they are specifically work for an engineer qualified on unvented hot water. The valve itself will very often need replacing afterwards as well, because a relief valve that has passed a hard discharge frequently will not reseat cleanly again. If the taps have gone scalding as well, read what to do about an overheating hot water cylinder, because the heat sources need isolating before anything else happens.

• Switch off the immersion heater at the wall switch
• Switch off the boiler if the cylinder is heated through a coil
• Do not run the hot taps to cool it down and do not close anything on the discharge pipe
• Keep people away from the cupboard while it is discharging
• Get an engineer out that day rather than waiting to see if it settles

A tundish overflowing with hot water means every automatic control above it has already failed. Treat it as urgent. Removing the heat source is the only thing that should be done before an engineer attends.

Which valve is discharging, and how to tell them apart

Because two valves can feed the same funnel, identifying the source is the first practical step and it decides what gets tested. The pressure relief valve sits in the inlet group where the mains arrives, usually a cluster of brass fittings low down on the cold side. The temperature and pressure relief valve is screwed directly into the cylinder body higher up, with a probe reaching into the stored water. Trace the short pipe entering the top of the tundish back to whichever it comes from, and most installations make that obvious in about thirty seconds.

Where both discharge pipes join before the tundish, temperature is the giveaway. Cold is the pressure side, hot is the temperature side. The pipes themselves also tell on you: a valve passing for months leaves a chalky white scale trail and often a green stain on the brass below it.

From there the tests are straightforward for someone with a gauge. Incoming mains pressure is measured at a tap, including when the street is quiet if the drip is worse at night. The pressure the cylinder is actually sitting at is read at the inlet group. The expansion provision is tested or recharged to suit the design of the unit. The relief valve setting stamped on the valve body is compared with what the system is doing, and where the discharge has been hot the thermostat, the cut-out and the heat source wiring are tested rather than replaced on a hunch.

This is where guessing costs money. Fitting a new relief valve without finding out why it was asked to open gets you a clean valve that drips again a few weeks later. The valve is the messenger almost every time.

• Trace the pipe entering the tundish back to the valve it came from
• Cold means the pressure relief valve on the inlet group
• Hot means the temperature and pressure relief valve in the cylinder body
• White scale and green staining show which valve has been passing longest
• Mains pressure, held pressure and expansion all get measured before any part is changed

Why a dripping tundish is worth fixing even when it seems minor

It is tempting to leave a drip alone, particularly when it only appears twice a day and the cupboard floor stays dry. There are three reasons not to.

The first is wear. A relief valve is designed to open rarely, in an emergency. Using it as a daily pressure regulator drags scale and grit across the sealing face every time it lifts, and in a hard water area like most of London those deposits build fast. The valve eventually reaches a point where it no longer closes, and the intermittent drip becomes a permanent trickle that only a new part will stop. The water going down it was metered and heated, so it is not free either.

The second is the discharge pipe. Repeated discharge of hot hard water deposits scale inside the pipe below the tundish, and a partially blocked discharge pipe is a genuine safety problem, because that pipe is the route the cylinder relies on in a real overheat. A pipe quietly furring up for two years is not something to discover during the one event it was installed for.

The third is diagnostic. A tundish that is already wet all the time cannot warn you about anything else. Once the household stops looking at it, the system has lost its only visible signal, and a hot discharge starting on a cylinder that has dripped for a year gets noticed days late rather than minutes late.

Can a dripping tundish be capped or plumbed out?

This needs saying plainly because it does get done. Capping the tundish, fitting an isolating valve on the discharge, piping the outlet into a hidden void or running it into a container somebody empties are all ways of removing the symptom while leaving a pressurised vessel with a compromised safety route. The air gap is not optional.

The gap matters physically. Without it the discharge pipe is directly connected to the safety valve, and anything in that pipe, a blockage, a frozen external section, water backing up from a drain, can act against the valve and interfere with its ability to open and clear. It matters just as much for visibility: discharge routed into a soil stack or a hidden drain works perfectly right up to the point where nobody notices a cylinder venting hot water for a fortnight. The funnel is at eye level in the cupboard precisely so somebody can see it.

The same applies to the small test lever on most relief valves. It is there for an engineer to operate during a service, in controlled conditions, knowing the valve may not reseat and being equipped for it if it does not. Lifting it out of curiosity on a cylinder that is already dripping is a reliable way to turn a drip into a flow that will not stop, in a cupboard full of bedding, on a weekend.

If anyone has fitted a valve, a cap, a bung or a container on the discharge from a tundish, have it put back to an open discharge. Anything that can stop or hold back a safety discharge defeats the only protection a sealed cylinder has.

What you can check yourself before calling anyone

Some of this you genuinely can do, and none of it involves opening the cylinder, touching a valve or removing a cover. Doing it first shortens the visit, because what you gather is what gets asked on the phone. If the discharge is hot, skip the list and go straight to switching the heat source off.

The honest limit is clear. Everything past observation, on a sealed unvented cylinder, is work for someone qualified on unvented systems. These units hold a large volume of water under mains pressure and above the temperature at which it would flash to steam if released suddenly, which is why the safety devices exist and why they are not householder maintenance. That is the work a G3 qualified engineer is certificated for, and it is worth asking whoever attends to confirm they hold it before they open anything.

• Note whether the water is hot or cold, without putting a hand under it
• Note whether it is a drip or a flow, and whether it stops on its own
• Note the time of day it happens and whether it matches the heating or immersion schedule
• Check whether the cylinder thermostat has recently been turned up by anyone in the house
• Look for white scale trails on the pipes above the tundish, which show it has been happening a while
• Check whether it started after mains works in the street or after any plumbing was done
• Photograph the data badge on the cylinder so the make, model and settings are to hand
• Leave the relief valve test lever alone, whatever a video suggests

What a visit for a dripping tundish involves

The visit starts with the same question you have been answering above: which valve, hot or cold, constant or cyclical. From there it is measurement rather than parts swapping, using the checks set out in the section above. Unvented cylinder repair is diagnostic work before it is parts work, and a relief valve changed without a pressure reading behind it usually ends up behaving exactly like the one it replaced.

What gets corrected depends on what those readings show. A perforated diaphragm in an external expansion vessel is a replacement. A unit relying on an internal air pocket has that pocket reinstated by the manufacturer procedure, which on some designs is expected periodic work rather than a defect. A pressure reducing valve that has stopped holding is a replacement, and it is the actual fix, because until the cylinder sits at its correct working pressure a new relief valve will do exactly the same thing.

Where the discharge has been hot, the work is on the control chain rather than the valves: the thermostat and its contact with the cylinder, the overheat cut-out and why it did not hold, the immersion wiring, and whichever heat source was running when it happened. A relief valve that has passed a hot discharge is usually replaced as well, because the seat rarely survives it cleanly.

Older cylinders reach a point where this stops being economic, and a good engineer will say so. A unit twenty years old, scaled inside from hard London water, on discontinued parts and venting daily is a replacement conversation rather than a repair one, and you are better off hearing that early. At that point the honest question stops being which valve to change and becomes whether a hot water cylinder replacement is the better answer over the years ahead. Where the unit is sound, this is ordinary unvented cylinder repair work, and booking the annual unvented cylinder service at the same time means the expansion side and both relief valves get checked properly rather than only the one that was dripping. Our rate is fixed before we attend and the total depends on how long the job takes.