Boiler Leaking Water? 6 Common Causes & What to Do

Boiler leaking water? Don’t ignore it. A leak may come from a joint or seal, excessive pressure, corrosion or an internal boiler component. The important thing is to find the actual source rather than repeatedly topping up the pressure or trying to repair it yourself.

If the water is coming from inside the boiler, don’t remove the casing. Note where the water appears, check the pressure display if it is safely visible, and look for any fault code before arranging professional diagnosis.

What Should I Do If My Boiler Is Leaking?

If you can do so safely, switch the boiler off using its normal controls and keep escaping water away from electrical equipment.

• Check the boiler pressure reading.
• Note any fault code on the display.
• Look for visible water around accessible pipes and valves.
• Take a photo of the leak if it is safe to do so.
• Do not remove the boiler casing.
• Arrange a repair if the leak continues or comes from inside the appliance.

Important: a water leak is different from a gas leak. If you smell gas or suspect a gas leak, follow HSE gas-safety guidance and call the National Gas Emergency Service on 0800 111 999.

Why Is My Boiler Leaking Water?

There isn’t one single cause. These are six of the faults an engineer may need to consider when investigating a leaking boiler.

1. Boiler Pressure Is Too High

If system pressure becomes excessive, the boiler’s safety controls may release water to reduce it. You may notice the pressure rising considerably when the heating gets hot.

Check the pressure against the recommended range in your boiler manual rather than relying on one pressure figure for every make and model.

If pressure is the main problem, see our guide to normal boiler pressure and pressure adjustment.

2. A Joint or Seal Is Leaking

Boilers and heating systems contain numerous connections and seals. Over time a joint can begin to weep or a seal may deteriorate.

This may start as a very small drip, so don’t assume a minor leak isn’t worth investigating. Water can eventually damage nearby components.

3. Expansion Vessel or Pressure-Control Problem

The expansion vessel helps accommodate changes in water volume as the heating system warms up and cools down.

If there is a problem with the expansion side of the system, pressure can behave abnormally and water may eventually be released through a safety component.

A useful clue is a large difference between the boiler pressure when the system is cold and when it becomes hot.

4. Corrosion

Corrosion can eventually weaken pipework, connections or boiler components and allow water to escape.

Visible staining or corrosion around accessible pipework can be useful information for an engineer, but corrosion inside the appliance cannot be properly assessed without professional inspection.

5. Heat Exchanger or Another Internal Component

A leak from an internal component can result in water appearing underneath the boiler even though the actual source is higher inside the appliance.

Engineer note: from hands-on boiler work, one thing worth knowing is that the place where the water finally drips from is not always where the leak started. Water can travel across components or pipework before becoming visible.

This is one reason we don’t recommend guessing the failed component from the drip location alone.

If you want to understand this particular component in more detail, see our guide to a faulty boiler heat exchanger.

6. Condensate Pipe or Connection

Modern condensing boilers produce condensate during normal operation. A problem with condensate pipework or one of its connections can sometimes result in visible water around the boiler.

An engineer can determine whether the water is coming from the condensate system or another part of the appliance.

Why Is My Boiler Leaking From Underneath?

If water is dripping from underneath the boiler, the source may not be visible without inspecting the appliance.

Before booking a repair, it helps to note:

• whether the leak is constant or only appears when the boiler runs;
• whether the pressure is rising or falling;
• whether heating and hot water still work;
• the boiler make and model;
• any fault code displayed.

Give this information to the engineer rather than removing the casing yourself.

Why Is My Boiler Leaking and Losing Pressure?

If water leaves a sealed heating system, boiler pressure can fall with it. A leak around the boiler, radiator valves, pipework or another heating-system component may therefore result in repeated pressure loss.

If you keep repressurising the boiler and the pressure drops again, read our guide to why boiler pressure keeps dropping.

Repeatedly adding water without identifying why the pressure is being lost is not a long-term fix.

Can I Fix a Leaking Boiler Myself?

You can make basic observations, but internal boiler repairs are not a DIY job.

Do not remove the casing or start dismantling boiler components. Instead, check the information that is safely available from outside the appliance and arrange professional boiler repair and fault diagnosis where necessary.

Does a Leaking Boiler Need Replacing?

Not automatically. Many boiler leaks can be repaired once the failed part or underlying pressure problem has been identified.

Replacement becomes worth comparing when the boiler is older, has suffered repeated faults, has significant corrosion or requires an expensive major component.

If you are weighing up repair against replacement, see our new boiler cost guide.

Boiler Leaking in Cardiff?

If you’re in Cardiff and your boiler is leaking, losing pressure or repeatedly locking out, see our dedicated Boiler Repair in Cardiff service page.

You can tell us the boiler make, model, pressure reading and any fault code when you book. You don’t need to diagnose the failed part yourself.

Quick Questions

Is a small boiler leak serious?

It should still be investigated. A small drip can worsen and water may eventually affect other boiler components.

Can high pressure make a boiler leak?

Yes. Excessive system pressure can cause water to be released through pressure-control components. The reason the pressure became excessive still needs to be identified.

Should I keep using a leaking boiler?

If there is an active leak, particularly if water is near electrical components or appears to come from inside the appliance, switch it off if this can be done safely and arrange professional advice.

Why does my boiler leak only when the heating is on?

A leak that changes as the system heats up can provide a useful diagnostic clue because system temperature and pressure also change. Tell the engineer when the leak appears.

Need Help With a Leaking Boiler?

Let’s Heat can investigate boiler leaks, pressure problems, lockouts, heating faults and hot-water problems.

You don’t need to know what component has failed before booking.

Book Boiler Repair →

Sources Checked

Technical information in this guide has been checked against guidance from
Vaillant
and
the Health and Safety Executive.