Leaking Overflow Pipe - Causes and Fixes

Why an Overflow Pipe Leaks

An overflow pipe leaking outside the house is a sign that your heating system is trying to protect itself. Most leaks come from:

• High boiler pressure

• A faulty pressure relief valve

• A stuck filling loop letting water in

• A faulty expansion vessel

• A fault with the loft tank in an open vented system

Below is a clear look at each cause and what you can do next.

1. High Boiler Pressure

If the pressure gauge rises too high, the system pushes excess water out through the overflow pipe.

Signs of this problem

• The pressure gauge reads over 2.5 bar when the heating is on.

• Pressure rises quickly every time the heating starts.

Safe checks you can do

Look at your boiler’s pressure gauge when the system is cold. It should sit around 1 to 1.5 bar.

If it is higher, you can release a small amount of water from a radiator using a bleed key.

When to call an engineer

If pressure rises again within hours or days, there is an underlying fault that needs professional repair.

2. Faulty Pressure Relief Valve

The pressure relief valve (PRV) releases water when pressure becomes too high.

If the valve is worn or stuck slightly open, water can leak constantly from the overflow pipe.

Signs of this problem

• A steady drip from the overflow even when pressure is normal.

• The PRV discharge pipe outside is warm.

Engineer only fix

A faulty PRV needs repairing or replacing by a Gas Safe engineer. This should not be attempted yourself.

3. Filling Loop Letting Water In

The filling loop is used to top up the system. If the taps or check valves are stuck open, water may enter the system all the time, pushing pressure too high.

Signs of this problem

• Pressure keeps rising after you top up the boiler.

• You hear water trickling into the system even when not in use.

Safe check you can do

Make sure both filling loop taps are fully closed.

When to get help

If pressure still climbs, the filling loop valves may need replacing.

4. Faulty Expansion Vessel

The expansion vessel absorbs changes in water pressure when the system heats up.

If it is flat or waterlogged, pressure will rise sharply and push water out of the overflow pipe.

Signs of this problem

• Pressure rises quickly during heating cycles.

• Pressure drops back to zero or very low when the system cools.

Engineer only fix

An expansion vessel needs recharging or replacing by a qualified engineer.

5. Fault with Loft Tank (Open Vented Systems)

Older heating systems have a tank in the loft that can spill water through the overflow pipe.

This is often caused by a stuck float or a faulty ball valve.

Signs of this problem

• Water dripping from the high level overflow pipe outside.

• Loft tank visibly higher than normal.

Safety note

Only observe the tank. Do not adjust the float or ball valve yourself.

Engineer fix

A professional can replace the valve or float and check the heating system for related issues.

Is a Leaking Overflow Pipe Dangerous

A small drip is usually not an emergency.
However, constant leaking, rapidly rising pressure or any hissing or steam from the boiler can indicate a safety issue.

Turn the heating off and call a Gas Safe engineer if you notice any of these warning signs.

What You Can Safely Check at Home

These checks are simple and safe for most homeowners:

1. Check the boiler pressure

Cold pressure should be around 1 to 1.5 bar.

2. Make sure the filling loop is closed

Both taps must be fully turned off.

3. Bleed a radiator if pressure is slightly high

Do this only if you are comfortable using a radiator key.

4. Look for steady dripping outside

This helps identify whether the leak is continuous or linked to heating cycles.

If pressure continues to rise after these checks, stop troubleshooting and get an engineer involved.

Fast Troubleshooting Table

Cost to Fix a Leaking Overflow Pipe

Repairs usually cost between sixty and one hundred eighty pounds, depending on the cause, parts needed and the type of system you have.

Preventing Future Overflow Leaks

• Check boiler pressure every few weeks

• Do not overfill the system

• Bleed radiators before winter

• Have the boiler serviced annually

• Replace faulty valves promptly

Regular servicing helps reduce sudden pressure problems and keeps the heating system working efficiently.

Final Thoughts and iHeat Support

A leaking overflow pipe usually means your boiler is relieving pressure or a valve needs attention.

Simple checks can help, but lasting issues need professional repair to keep the system safe.

If your boiler is struggling to hold pressure or the overflow problem is linked to an older heating system, iHeat installs modern and efficient boilers from trusted brands.

You can get a free online quote along with helpful guidance tailored to your home.

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