Overflow Pipe Dripping on a Vented Cylinder System

Which pipe is actually dripping?

A vented system has a hot water cylinder that is open to the air through a vent pipe, with water supplied by gravity from tanks in the loft. Those tanks have overflow pipes that discharge outside through a wall or under the eaves, and each pipe has a single job: to show you that a tank is full and still receiving water. A drip from one of them is a symptom, and the tank behind it is the first thing to identify.

There are normally two tanks. The larger one is the cold water storage tank, which feeds the cylinder and often the cold taps upstairs. The smaller one is the feed and expansion tank, which tops up the heating circuit and lets it expand as it warms. Look in the loft with a torch and follow each overflow pipe back to its tank. Do not lean on the tanks or the ceiling, and use a proper loft ladder and board.

If the house has an unvented cylinder instead, the pipe you can see dripping is probably a tundish, an open funnel-shaped fitting near the cylinder, and the cause is a different one. Go to tundish dripping: causes and checks instead. If you are unsure which type you have, how to tell if a cylinder is vented or unvented shows the difference in a minute.

Is it a worn float valve?

This is the commonest cause and the cheapest to put right. The float valve, often called a ball valve, has an arm and a float that rise with the water level and close the inlet. When the washer or seat inside it wears, or the arm is set too high, the valve cannot shut properly and the tank keeps filling until it spills. A cold water tank carries an overflow or warning pipe, as the Water Supply (Water Fittings) Regulations 1999 expect, so that a failing inlet valve shows itself outside the house. A tank spilling while its valve is still open usually means a worn or wrongly set valve.

The tell-tale is timing. A float valve problem shows after the tank has been drawn down, for example after a bath or a flush, and then the tank refills and carries on dripping instead of stopping. Check the water level against the overflow with the tank still. If the level is just at the overflow and the inlet is still hissing, the valve is not closing. If the valve closes cleanly and the drip stops, a high setting or a surge was the cause.

Adjusting the float arm so the valve closes with a few centimetres of margin below the overflow is simple. Renewing the valve involves isolating the tank supply at its servicing valve, which should be fitted on the inlet pipe, and draining the line first. That is ordinary plumbing and not specific to the cylinder, so this page stops at the diagnosis.

Is the heating coil leaking between the circuits?

If the overflow is on the small feed and expansion tank, and the float valve is closing properly, look at the cylinder. On an indirect vented cylinder a copper coil carries heated water from the boiler through the stored water. That coil is a closed loop connected to the heating circuit and the feed and expansion tank. If the coil develops a pinhole, water passes across whichever way the pressure difference runs. If stored water is at the higher head, it can push into the heating circuit and raise the level in the small tank until it overflows.

The clues are an overflow that continues however the float valve is adjusted, a small tank that never seems to settle, and sometimes stored water that tastes or smells of inhibitor, or heating water that suddenly looks cleaner. The reverse can also happen: heating water seeps into the hot water side, which is why dark or inhibitor-smelling hot water belongs to the same investigation. See smelly or discoloured hot water and cylinder heating coil fault.

Confirming a coil leak is an engineer test, normally by isolating the coil connections and watching the pressure and levels. It is not a householder repair, and a confirmed leaking coil points toward the cylinder rather than the float valve. The usual question is then repair or replacement of the cylinder, which cylinder repair or replace takes through.

Could the cylinder be overheating?

The vent pipe from a vented cylinder rises from the top of the cylinder and turns over the cold water storage tank (Approved Document G, paragraph 3.14). If the stored water boils, expansion pushes hot water and steam up that pipe and into the tank, which warms the tank and drives water out of its overflow. The overflow pipe then drips hot water, and the loft often smells of steam.

Overheating comes from a heat source that is not stopping. On an electric cylinder that means an immersion thermostat that has failed in the closed position, and on an indirect cylinder it means a cylinder thermostat or a two-port valve that is stuck open so the boiler keeps pumping heat into the coil. A hot drip that is worse after the heating has run for a while points this way, and a drip that stops when the heating is off supports it.

Do not open the cylinder controls or immersion cover to look. Overheating is a safety fault and the response is to isolate and call. If the water at the tap is hotter than normal and the overflow is warm, treat it as overheating rather than a float valve problem. Two-port valve and no hot water explains how that valve behaves when it fails open or shut.

What does the engineer check, and what is the repair?

The check order follows the one above. The engineer identifies the tank, then tests the float valve and the level, because that is the quick elimination. Next they check the cylinder temperature and the thermostat, and watch whether the tank only overflows while heating. Finally they test the coil for a leak between circuits.

The repair follows the result. A float valve is renewed. A failed cylinder thermostat or a stuck valve is replaced. A leaking coil usually means the cylinder, because the coil is inside it and cannot be changed in place. The hot water cylinder diagnosis page covers how this order is applied on a visit.