That Little Copper Funnel Beside Your Cylinder: When a Drip Is a Warning, Not Condensation

If you have an unvented hot water cylinder in your Watford home, there is a good chance you have walked past a small copper funnel without giving it a second thought. It usually sits in an airing cupboard, near the cylinder, with a short pipe above it and a discharge pipe below. That funnel is called a tundish, and its job is to give you a visible gap in the discharge pipework — a place where you can see water if it ever needs to escape.

Most of the time, the tundish should be dry. Not damp, not streaked, not quietly dripping into the drain. Dry. So when you notice a trickle, a drip or a faint hiss coming from that little copper cup, it is worth paying attention. This is one of the few plumbing components that is deliberately designed to show you a problem before it becomes a bigger one.

What the tundish actually does

An unvented cylinder stores hot water under pressure. Because it is a sealed system, it needs safety devices to cope with expansion and with pressure that could otherwise build up inside the cylinder. The tundish sits on the discharge pipe from the pressure-relief valve. If that valve opens, water passes through the tundish and away to a safe discharge point, usually outside or to a drain.

The tundish is not a drain fitting and it is not there to catch condensation. It is an indicator. If water is passing through it, one of the safety paths has opened. That might be because the pressure-relief valve is passing slightly, because the expansion vessel has lost its charge, or because the system pressure is too high. Sometimes it is a combination of small issues. It is almost never a case of “they all do that” or “it is just the weather.”

Why a drip is not just condensation

Condensation around a hot water cylinder is common enough, especially in a cool airing cupboard. You might see damp patches on pipework or a bit of moisture on a cold feed pipe. But condensation does not usually appear as water flowing through the tundish. The tundish is a dry gap by design. If water is making its way across that gap, it has come from the cylinder side of the system, not from the air.

A continuous trickle is the clearest warning. An occasional drip that appears only when the cylinder is reheating is also worth noting, because it can point to expansion pressure rather than a constantly passing valve. Either way, the sensible approach is to treat it as a fault to be checked, not a quirk to be ignored.

A safe visual check you can do

You do not need to dismantle anything to gather useful information. Start by looking, not touching. If the pipework or tundish is hot, keep your hands clear. Hot water from a discharge pipe can cause scalds, and the cylinder store is hot enough to be dangerous.

Look at the tundish and the pipe below it. Is the drip constant, or does it appear only after the hot water has been running? Is there a steady trickle, or just an occasional bead of water? Check whether the discharge pipe outside is wet or has been staining the wall. If there is a pressure gauge on the cylinder or nearby, note what it reads. Do not adjust valves or try to tighten anything. Do not block the tundish or put a container under it as a permanent fix. The discharge pipe is a safety route, and restricting it is a bad idea.

A dry paper towel held near — not against — the tundish gap can help you see whether the drip is active, but keep it clear of hot surfaces. If the water is running steadily, or if you cannot safely get near the cylinder, stop there and call an engineer.

What to note before you call

A few details will help your engineer diagnose the issue faster. Note whether the drip is constant or intermittent. Does it happen after a shower, after the heating has been on, or overnight? Has the pressure gauge moved? Is there any water staining on the wall outside? Have you had any work done on the cylinder recently, or has the system been drained and refilled? If you are a landlord, note the date you first spotted it and whether tenants have reported any change in hot water performance.

It is also useful to say whether the cylinder has been serviced in the last year. Unvented cylinders need an annual service by someone competent to work on them, and a service can catch a weakening expansion vessel before it becomes a wet cupboard floor.

Why a G3-qualified engineer matters

Unvented hot water cylinders are not a DIY repair. In the UK, work on unvented systems above a certain size should be carried out by an engineer with the appropriate G3 qualification. That is not a box-ticking exercise. These systems combine stored hot water, pressure and safety valves, and getting the discharge pipework or relief valve wrong can create a genuine risk.

A G3-qualified engineer will understand how the expansion vessel, pressure-reducing valve and relief valve work together. They can test the relief valve safely, check the discharge pipe route and confirm whether the tundish is doing what it should. They will also know whether the fault is urgent or whether it can be booked in as a routine repair.

Recharging versus replacing the expansion vessel

One of the most common causes of a dripping tundish is a failed or flat expansion vessel. The vessel contains a trapped air charge that absorbs expansion as the water heats. Over time, that charge can be lost. When it is, the system pressure rises further than it should, and the relief valve opens to let water escape. That water appears at the tundish.

Sometimes the vessel can be recharged. An engineer will isolate the vessel, check its charge pressure and top it up if the vessel itself is still sound. In other cases, the vessel has failed internally, or the bladder has perished, and replacement is the better answer. Recharging is not a universal fix, and it is not something to attempt without the right tools and knowledge. A relief valve that has been passing may also need attention, and the engineer will check whether it has reseated properly or whether it needs replacing.

Urgent or routine?

A steady trickle through the tundish, water staining outside, or a pressure gauge that keeps climbing are reasons to call promptly. If the cylinder is leaking from the body itself, or if there is water near electrics, treat it as urgent. A slow, occasional drip that appears only during reheating is still worth booking in, but it is less likely to be an emergency. The key is not to let it drift for months. Small faults in unvented systems have a habit of becoming wet-cupboard problems at the least convenient moment.

In a Watford home, where airing cupboards can be tight and cylinders are often tucked away, it is easy to forget the tundish exists. A quick glance every few weeks is enough. Dry is good. Dripping is a message. And like most messages from your plumbing, it is cheaper to read it early.