Water Discharging From The Cylinder Pipe Outside

What's happening?

An unvented cylinder stores hot water at mains pressure, which is why it gives a strong shower without a loft tank. Because it is sealed and pressurised, it carries layered safety devices: an expansion vessel to absorb the water expanding as it heats, an expansion relief valve, a temperature and pressure relief valve, and a thermal cut-out. If any of those operate, water goes to a tundish — the small open funnel in the pipework — and out through a discharge pipe to the outside, where you see it. Discharge is the system working as designed, but it is never something to leave: it means one of the controls has failed or the vessel has lost its charge, and the whole point of the layered protection is that you fix the first layer before you rely on the last one. Installing or servicing an unvented cylinder requires specific competence under Building Regulations Part G3.

What usually causes it?

• An expansion vessel that has lost its air charge, so heated water has nowhere to expand into
• A failed or debris-fouled expansion relief valve passing water continuously
• A failed cylinder thermostat letting the water overheat until the temperature relief valve lifts
• Mains pressure at the property exceeding the cylinder's rating, with no pressure-reducing valve or a failed one
• Limescale on the immersion or the coil causing localised overheating in a hard-water area
• A relief valve seat scaled or damaged so it never re-seats after operating
• Debris in the mains supply lodging under a valve seat after work on the street

When should you call a heating engineer?

Call a heating engineer with unvented hot water qualification the same day, and turn the heat source off in the meantime. Treat it as urgent if the water discharging is hot, if discharge is continuous, or if the cylinder is making noise. Unvented cylinders must be worked on by someone competent under Part G3; ask for that qualification when you book, because not every heating engineer holds it.

What will the heating engineer actually do?

• Identify which safety device has operated by testing each in turn rather than replacing blindly
• Check and recharge or replace the expansion vessel, and confirm the charge against the incoming pressure
• Test and replace the expansion relief valve or the temperature and pressure relief valve as needed
• Check incoming mains pressure and fit or reset a pressure-reducing valve where it is too high
• Test the cylinder thermostat and thermal cut-out and replace failed controls
• Recommission the cylinder, confirm no discharge over a full heat cycle, and record the service

Reading the discharge

• Cold water, only while the cylinder heats — usually the expansion vessel has lost its charge.
• Cold water, continuously — usually the expansion relief valve is passing or fouled with debris.
• Hot water — the temperature and pressure relief valve has lifted, which means the cylinder overheated. Turn the heat source off and treat it as urgent.
• Discharge that started after work on the water main — often debris under a valve seat.