Boiler Overflow Pipe Leaking

Why Does an Overflow Pipe Leak?

A leaking boiler overflow pipe typically occurs when the system is trying to protect itself.

The overflow pipe is a safety feature designed to release water when the system's pressure becomes too high.

Here are the most common causes:

1. High Boiler Pressure

2. Faulty Pressure Relief Valve

3. Stuck Filling Loop

4. Faulty Expansion Vessel

5. Issues with Loft Tank in Open Vented Systems

Let’s break down these causes and what you can do next.

1. High Boiler Pressure

High boiler pressure is one of the most frequent causes of an overflow pipe leak.

When the pressure in your system rises above normal levels, the excess water is pushed out through the overflow pipe to maintain balance.

Signs of this problem

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

• Pressure rises quickly when the heating starts.

What you can check yourself

• When the system is cold, check the pressure gauge. It should be between 1 and 1.5 bar.

• If it’s too high, you can bleed a radiator to reduce the pressure.

When to call an engineer
If the pressure continues to rise despite your checks, there may be an underlying issue requiring professional attention. Call a Gas Safe engineer.

2. Faulty Pressure Relief Valve

The pressure relief valve (PRV) releases water when the system’s pressure gets too high. If the valve is faulty or stuck, it may allow water to leak from the overflow pipe even when the pressure appears normal.

Signs of this problem

• A steady drip from the overflow pipe, even when the pressure seems normal.

• The PRV discharge pipe may feel warm.

What to do
A faulty PRV needs to be replaced by a Gas Safe engineer. This is not something you can fix yourself.

3. Stuck Filling Loop

The filling loop is used to top up your boiler with water. If the taps or check valves are left open, water may continue entering the system, causing the pressure to rise and triggering the overflow pipe to leak.

Signs of this problem

• The pressure continually rises even after you top up the boiler.

• You hear water trickling into the system when the heating is off.

What you can check yourself
Ensure both filling loop taps are fully closed.

When to call an engineer
If the pressure continues to climb despite closing the taps, the filling loop valves might need replacing.

4. Faulty Expansion Vessel

The expansion vessel absorbs pressure changes as the system heats up. If it becomes waterlogged or damaged, the pressure can rise too quickly, leading to an overflow pipe leak.

Signs of this problem

• Pressure rises rapidly when the system is on.

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

What to do
A faulty expansion vessel requires professional repair. The vessel may need to be recharged or replaced.

5. Fault with Loft Tank (Open Vented Systems)

Older systems with a loft tank can experience overflow issues due to a stuck float valve or a malfunctioning ball valve.

Signs of this problem

• Water dripping from the high-level overflow pipe outside.

• The loft tank appears higher than usual.

What to do
Do not adjust the float or ball valve yourself.

Call an engineer to inspect the tank, replace the faulty valve, and check the rest of the system.

Is a Leaking Overflow Pipe Dangerous?

A small drip from your boiler’s overflow pipe is usually not an emergency.

However, if you notice rapidly rising pressure, hissing sounds, or steam coming from the boiler, it could be a sign of a more serious problem.

What you should do

• Turn off your heating.

• If you notice hissing or steam, immediately call a Gas Safe engineer.

What You Can Safely Check at Home

If you feel comfortable performing some checks, here are the safe steps you can take:

1. Check the Boiler Pressure
   The pressure should be around 1 to 1.5 bar when the system is cold.

2. Make Sure the Filling Loop is Closed
   Ensure both taps are fully closed to prevent water from entering the system.

3. Bleed a Radiator
   If the pressure is slightly high, try bleeding a radiator to release some water.

4. Look for Steady Dripping Outside
   Check if the overflow pipe outside is dripping steadily, indicating a constant leak.

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

Cost to Fix a Leaking Overflow Pipe

Repairs to a leaking overflow pipe typically cost between £60 and £180, depending on the cause of the problem and the parts needed.

Preventing Future Overflow Leaks

To avoid future overflow leaks, here are a few maintenance tips:

• Check boiler pressure every few weeks.

• Don’t overfill the system when topping up the pressure.

• Bleed radiators before winter to release any trapped air.

• Have the boiler serviced annually to maintain efficiency and prevent issues.

• Replace faulty valves promptly to avoid bigger problems later.

Final Thoughts and iHeat Support

A leaking overflow pipe is often a sign that your boiler is relieving pressure or that a valve needs attention.

Simple checks can help identify the cause, but lasting issues require a professional fix.

If your boiler is struggling to hold pressure or the issue is related to an older system, iHeat installs modern, efficient boilers from trusted brands.