Why Is My Boiler Leaking Water? Common Causes & Fixes

A boiler leaking water can be caused by high boiler pressure, loose joints and gaskets, corroded pipes, and poor installation.

The most common causes for a boiler leaking water are the boiler pressure being too high, leaking plumbing under the boiler or damage to the central heating pump seals.

Other reasons for a boiler to visibly leak include faulty components such as the blowdown valve, faulty heat exchanger or auto-air vent.

Homeowners should never ignore a leaking boiler, as excess moisture can lead to potential electrical hazards, making the leaking boiler dangerous over time.

Phone a local Gas Safe plumber if you have a visible leak under your boiler.

Understanding the reasons behind a boiler leak and the appropriate fixes is crucial for maintaining a safe and efficient home heating system. This guide delves into the common causes of a leaking boiler, offering clear solutions and preventive measures.

Water Leak under boiler from corrosion

Boiler Is Leaking Water From The Bottom

Water will leak from underneath a boiler when either there is too much pressure in the system or pump seals are damaged.

Boiler leaks are a concerning problem for homeowners and must be promptly addressed. A leaking boiler negatively affects your central heating system and results in reduced efficiency, pressure loss and severe water damage to your home – and wallet!

Let’s look at a few common reasons why your boiler is leaking water from the bottom:

Pressure issues

  • Cause: A Boiler will lose pressure when there is a boiler leak.
  • Solution:
    Check the boiler pressure gauge to see if it’s within the optimum range (between 1.5 and 2.5 bars). The pressure gauge is found on the front of your boiler unit; usually a dial on an old boiler or a digital display screen for modern boilers.
  • If the boiler pressure is too low, you need to repressurise the boiler via the filling loop. The filling loop is found underneath the boiler or close to a sink. Top the boiler up with water until it reaches the desired pressure on the pressure gauge.
  • If the boiler pressure is too high, you need to reduce the pressure via the pressure relief valve.
    The pressure release valve (PRV) is a brass fitting found inside the boiler unit, attached to the discharge pipe. Check the boiler manual for its precise location. The PRV automatically regulates boiler pressure. If it does not, the PRV is faulty.
  • Bleeding radiators are another solution to your leaky boiler problem as this method releases trapped air and debris that causes increased pressure.
    Locate the bleed valve on the side of your radiator and turn the bleed key (or flathead screwdriver) anti-clockwise to open the valve. Use a bucket to catch excess water and sludge that is released from the heating system. Once the excreted water runs clear, close the valve by turning the bleed key clockwise.

Loose joints

  • Cause: Loose plumbing joints, connections and fittings are common boiler leak issues.
  • What to look for:
  1. Damp patches or water dripping around pipe fittings, valves, and other fixtures on the boiler unit occur when a boiler leaks.
  2. Hissing, whistling, or gurgling sounds occur due to escaping water.
  3. Visible cracks or signs of wear and tear are common boiler leak issues.
  • Solution:
  1. Turn off your boiler to prevent further water damage and potential safety hazards.
  2. If the loose joint(s) are minor and are easily accessible, tighten the connections with a wrench or screwdriver. Apply a boiler leak sealant to close up any gaps or cracks.
  3. If there is a significant amount of water leaking from the boiler, call a Gas Safe registered engineer to attend to the boiler problem.

Damage to pump seals

  • Cause: Damage to the seals of the central heating pump is another reason why boilers leak. The boiler pump is responsible for circulating hot water through the heating circuit/heat exchanger which is fed to radiators and pipes to heat our homes.
  • What to look for:
  • Water dripping from the pump housing unit is a telltale sign of a faulty seal.
  • Unusual sounds, like gurgling or grinding, are often heard when pump seals are worn out.
  • Reduced boiler pressure is noticed when seals leak as the boiler’s heating efficiency is affected.
  • Solution:
  • Stop using your boiler as soon as you notice any of the above signs as continuous use will damage internal components.
    Pump seal repairs are not a DIY fix and require a heating engineer to attend to the boiler problem.
  • Early detection and prompt action are crucial to minimise further damage to the boiler, particularly when you notice leaking water.

Temperature too high

  • Cause: When the water temperature is too high inside the boiler it causes the boiler to leak. Overheating leads to excessive pressure as water expands and contracts too quickly, resulting in water leaks.
  • Solution:
  • Regularly check the temperature gauge on the boiler. The temperature gauge can be found on the front display panel of modern boilers or as a dial on older models.
  • Adjust the temperature valve controls to ensure boiler temperature is within the optimal range – between 60°C and 70°C.
  • If the boiler is within range and continues to leak water, contact a professional Gas Safe engineer to assess your boiler problem.

While the bottom of the boiler is a common leak location, various other components can be culprits, which we’ll touch on next.

Other Reasons For A Boiler Leaking

Other less common reasons for a boiler to leak water include:

Boiler blowdown valve

A faulty boiler blowdown valve can cause a leaking boiler. Understanding its function and potential issues is crucial when troubleshooting leaky boilers.

The boiler blowdown valve removes unwanted impurities and sediment from the water supply. If the sediment accumulates, it causes blockages that lead to boiler leaks and other internal malfunctions.

There are two main types of blowdown valves in boiler systems:

  1. Surface blowdown – located near the top of the boiler unit and removes impurities from the steam drum.
  2. Bottom blowdown – located on the lowest part of the boiler and removes sludge and sediment that settles to the bottom of boilers.

Why blowdown valves leak:

  • Worn-out internal parts
  • Corrosion
  • Too much debris buildup (restricting proper closure of the valve)
  • Frequent or overused blowdown cycles can put stress on the valve

Heat exchanger

A faulty heat exchanger can cause boilers to leak. Heat exchangers transfer heat from the combustion chamber to the water that warms our home. You will need a Gas Safe engineer to repair or replace a faulty heat exchanger as this is not a DIY fix.

Why blowdown valves leak:

  • Limescale buildup
  • Corrosion
  • Thermal shock – rapid changes in boiler temperature
  • Manufacturing defects

Leaky auto-air vent

A faulty leaky auto-air vent causes water leaks by allowing air into the central heating system. These airlocks wreak havoc on boiler systems often causing them to leak. Auto-air vents play a critical role in keeping our heating system running smoothly.

Why auto-air vents leak:

  • Worn out seals
  • Debris buildup
  • Corrosion
  • Faulty internal mechanisms (e.g. float device, internal spring)

Sediment trapped in pressure valve

Sediment trapped inside the pressure valve causes the boiler to have too much pressure – resulting in boiler leaks. This interferes with the expansion and contraction vessels that cannot open and close sufficiently to regulate the pressure in the system.

Over time our radiator systems become clogged with rust, limescale and debris. This debris is called sludge and forms a sediment layer or coating on internal boiler parts. Debris affects boiler functionality and its heating ability, causing water leaks when the buildup is too much.

The sediment acts as an insulator that prevents water from flowing properly within the system. This results in many boiler problems, including boiler leaks.

Tip: Use a central heating inhibitor (cleaner) or magnetic boiler filter to catch debris particles before they clog up integral boiler components.

What Do I Do If My Boiler Is Leaking Water?

If your boiler is visibly leaking water, you need to call a Gas Safe registered local plumbing and heating engineer, especially if the above DIY fixes have proven unsuccessful.

ADI Leak Detection is the UK’s water leak detection specialist that provides a reliable nationwide service. We help homeowners, landlords, tenants, and businesses to identify and repair leaks, with proven cost-effective solutions.

Our team of qualified plumbing and central heating experts use non-destructive leak detection methods to find hidden leaks in your home, such as tracer gas, acoustic listening devices, and borescope cameras. ADI only finds hidden leaks, if a boiler is dripping water you need to phone a local plumber.

Adrian Morgan

Adrian Morgan CIPHE RP RHP is the founder & Managing Director of ADI Pro Leak Ltd. Adrian is also the Head Plumbing, Heating & Leak Detection engineer at ADI Leak Detection.

Adrian CIPHE RP RHP is proud to be registered with the Chartered Institute of Plumbing & Heating Engineering, a Registered Plumber and a Registered Heating Professional.