Why is my unvented cylinder discharging through the tundish?

Why is water coming out of my tundish?

Because something upstream is relieving pressure, and in four cases out of five that something is a flat expansion vessel. Heating 200 litres of water from 10 °C to 60 °C expands it by three to four litres. An unvented cylinder is a sealed system, so that volume has to go somewhere: into the expansion vessel, or out through a relief valve. When the vessel loses its air charge, there is no cushion left, pressure rises sharply during every heating cycle, and the expansion relief valve opens.

The classic signature is a drip that starts when the cylinder is heating and stops once it has cooled. That pattern is diagnostic — it is the pressure cycling, not a failing valve.

Which device is actually discharging?

A typical unvented installation has three protective devices in series, each with a different trigger. Match the symptom to the device before touching anything:

How do I recharge or replace the expansion vessel?

The charge is checked with the water side depressurised — checking a pressurised vessel gives a meaningless reading. The sequence is: isolate the cold feed, open a hot tap to drop the cylinder pressure to zero, then read the Schrader valve on the vessel with a tyre gauge. The pre-charge should match the incoming pressure set by the pressure reducing valve, normally 3.0–3.5 bar. Top up with a foot pump, close up, and repressurise.

If the vessel will not hold that charge for a few days, the diaphragm has ruptured and the vessel must be replaced. For a 200 litre cylinder that means a 12–18 litre vessel. Some cylinders use an internal air gap or floating baffle instead of an external vessel; those are recharged by draining the cylinder to re-establish the air pocket, which is a routine annual service task rather than a fault.

In England and Wales, unvented hot water systems fall under Building Regulations Part G3, and work on their safety devices must be carried out by a competent person. Elsewhere in Europe the construction and safety requirements sit in EN 12897, which covers indirectly heated unvented cylinders for supply pressures of 0.5 to 10 bar and storage up to 95 °C — including how the expansion vessel and relief devices must be arranged.

When is tundish discharge a safety issue rather than a nuisance?

When the water discharging is very hot or flashing to steam. That means the temperature and pressure relief valve has opened — the last line of defence — and something has failed in the temperature control chain: a stuck immersion thermostat, a defeated thermal cut-out, or a boiler control fault. Shut off the heat source and call an engineer the same day. A tundish quietly dripping cool water during heating cycles is a repair job; a tundish discharging scalding water is a live safety fault.

The tundish itself exists precisely so this is visible. UK Approved Document G3 practice puts the tundish within 500 mm of the relief valve, with the discharge terminating where it can be seen and where nobody can be scalded. Never plumb a tundish out of sight or cap it — that removes the only warning the system gives you.

How do I stop it happening again?

• Check the expansion vessel pre-charge annually. It is a two-minute job and it prevents the most common failure outright.
• Verify the pressure reducing valve setting, especially if incoming mains pressure is high or variable.
• Descale in hard water areas. Scale on the valve seat and on the immersion element causes both nuisance dripping and overheating faults.
• Keep the storage temperature sensible. Cycling to 70 °C every day expands more water, stresses the vessel harder and accelerates scaling; 55–60 °C is enough for legionella control with a thermostatic mixing valve at the outlets.
• Service the magnesium anode on enamel-lined tanks at the same visit — a corroded tank is a much larger problem than a wet tundish. Solimpeks builds its enamel-coated tanks to DIN 4753 with accessible anodes for exactly this reason.