Tundish Dripping on Your Unvented Cylinder? What It Means and What to Do

The tundish is the small funnel with a gap in it, usually on the pipework near your cylinder. It's there so you can see when one of the cylinder's safety valves is letting water out. A dry tundish is what you want. If it's dripping or running, the cylinder is telling you something.

First, if hot water is pouring through the tundish

If hot water is running through it continuously, or it’s steaming:

1. switch off the heat source for the cylinder: the immersion heater and the boiler or heat pump
2. don't turn off the cold water supply to the cylinder, and don't try to adjust or block any of the valves
3. call a G3-qualified engineer straight away

Hot water discharging can mean the cylinder has overheated, and that needs looking at the same day.

The common causes of a dripping tundish

1. The expansion vessel has lost its charge

On most modern unvented cylinders, a separate expansion vessel absorbs the water's expansion as it heats. Over time, the air charge inside it drops. Once it's low, the expansion relief valve has to release water every time the cylinder heats up, so you get a drip at the tundish, usually during or after a heat-up. This is the most common cause, and recharging the vessel at a service usually fixes it.

2. The air gap has gone (older Megaflo models)

Older Megaflos use an internal air gap instead of a separate vessel. When it’s lost, the result is the same: expansion water comes out through the tundish. See my post on Megaflo pipes buzzing.

3. The pressure reducing valve is letting by

If the valve that controls the incoming mains pressure is worn, the pressure creeps up and the relief valve opens to release it.

4. A relief valve isn't sealing

Bits of debris or scale can stop the expansion relief valve, or the temperature and pressure relief valve, from closing fully, which leaves a constant slow drip.

5. Overheating

If the cylinder's thermostat or the heat source controls fail, the water can get too hot, and the temperature and pressure relief valve opens. That's the hot-water case above, and it's the serious one.

Why you shouldn't ignore it

A dripping tundish is the safety system working, but it means something upstream needs attention. Left alone, small drips can turn into constant discharge, wasted water, and in some cases water ending up where it shouldn't if the discharge pipework isn't right.

What I do

As a G3-qualified engineer, I'll check the expansion vessel or air gap, the pressure reducing valve, both relief valves and the cylinder controls, find which one is causing it, and put it right. Most of the time it's sorted as part of a £140 cylinder service, with no VAT to add. Parts are extra if a valve needs replacing.