Why Is My Overflow Pipe Leaking? (And How to Fix It)

A leaking overflow pipe might seem minor — but it’s usually a sign of something going wrong inside your plumbing or heating system. Whether it’s dripping from your toilet cistern, boiler, or loft tank, ignoring it can lead to water damage, higher bills, or system failure.

At Pleasant Plumbers, we help homeowners and landlords across London diagnose and fix overflow issues quickly and safely. Here's how to figure out what's causing the leak, and what to do next.

An overflow pipe is a safety feature. It carries excess water away from tanks or cisterns inside your home — directing it outside to prevent leaks or flooding.

They’re most commonly found on:

• Toilet cisterns
• Cold water storage tanks
• Feed and expansion tanks (for central heating)
• Combination boilers
• Copper cylinders
• Sinks and basins

If your overflow pipe is dripping or running continuously, it means the water level is rising higher than it should — and the system isn’t shutting off properly.

1. Faulty Float Valve

Most overflow issues are caused by a stuck or worn-out float valve. These are designed to shut off water once it reaches a set level. If the valve gets stuck open, water keeps filling the tank until it flows out through the overflow.

2. Punctured Ball Float

If the plastic float ball is full of water, it can’t rise properly to shut off the valve. This allows the tank to keep filling and spill over.

3. High Water Pressure

Excessive mains water pressure can overwhelm cisterns and valves — especially in older homes without pressure regulators.

4. Washer Deterioration

Over time, the internal washer in a float valve can perish or split, leading to a slow, constant drip.

Step 1: Identify the Source

• Toilet overflow? Check under the cistern or look for a pipe exiting the wall just behind the toilet.
• Loft overflow? Look for a pipe dripping from under the eaves — likely from a cold water or central heating tank.
• Boiler overflow? If your combi boiler is dripping, it’s likely the pressure relief valve, not a traditional overflow.

Step 2: Check the Float Valve

Lift the float gently. If the water stops, the float valve may be stuck.
Check that the ball isn't full of water and moves freely. If it's waterlogged, it needs replacing.

Step 3: Turn Off the Water Supply

Use the isolation valve (for toilets) or the main stopcock to turn off the water. Drain down the tank or cistern before attempting any repairs.

Step 4: Replace Parts if Needed

• New washer: Inexpensive and easy to fit
• New float valve: If damaged or not responding
• New ball float: If it’s waterlogged or misshapen

• The float valve isn’t closing
• The float is set too high
• The washer has worn out
• The water pressure is too high

Adjust the float level or change the washer. If the valve is old, replacing the entire unit is often quicker and more reliable.

This usually comes from:

• A stuck float valve in the cold water storage tank
• Water backfeeding from the taps (caused by a faulty mixer tap or shower)
• A perforated coil in the hot water cylinder, which causes systems to overfill and push water out the overflow

If the cause isn’t obvious, call a professional to diagnose the issue — especially where multiple tanks are involved.

Combi boilers don’t have traditional overflow pipes — but they do have a pressure relief valve (PRV). If your boiler pressure is too high (above 3 bar), the PRV will release water to protect the system.

Causes include:

• A failed expansion vessel
• A faulty PRV
• The system being over-pressurised during a top-up

This is a job for a Gas Safe registered engineer.

Sink overflows are built into the basin or bath. If you notice water pooling or dripping underneath:

• Clean out the overflow — it could be blocked with soap or hair
• Check the overflow pipe connection under the basin
• Tighten or reseal the joint if it’s leaking

If water is dripping down an external wall from a small pipe:

• Toilet cistern – faulty float valve
• Loft tank – stuck valve or excess pressure
• Boiler – pressure relief due to system fault

It’s not a coincidence — it means something inside is failing. The longer it leaks, the higher the risk of damage, freezing, or a more costly repair.

At Pleasant Plumbers, we diagnose and fix overflow problems quickly, whether it’s a toilet, tank, cylinder or boiler issue. We also offer free video call advice — if we think you can fix it yourself, we’ll guide you through it.

📲 Call or WhatsApp 0800 046 1000
📧 Email [email protected]
🌐 Visit pleasantplumbers.com

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