Boiler Leaking Water: What to Do and Who to Call in London

If your boiler is leaking water, switch it off at the fused spur on the wall. Catch the water in a bowl or towels and keep your hands off the casing. Don't top up the pressure. If it's more than a drip, turn off the stopcock. Then call a Gas Safe registered engineer to find and fix the fault.

A boiler leaking water is stressful, especially in winter when you need the heating most. Still, a few calm minutes now limit the damage to your home. They also give the engineer the clues they need to track the fault down.

Getting it wrong can cost you. In London flats and conversions, water can soak through to the home below before you've even spotted it. Topping up the pressure or tightening a nut can turn a slow drip into a steady flow. And a boiler left running with water near the electrics is a safety risk.

Boiler leaking water: what to do first

Switch the boiler off at the fused spur, catch the water and keep your hands off the casing. That's the safe first move, whatever's causing the leak.

Smell gas? Stop here. Don't use any switches, open the windows and leave if the smell is strong. Call the National Gas Emergency Service on 0800 111 999. It's free and open 24/7.

If there's no gas smell, work through these steps in order:

1. Turn off the power. The fused spur is the switched unit on the wall next to or below the boiler. It's often labelled "boiler". If it's wet or you can't find it, switch off the boiler circuit at your consumer unit (fuse box). Never touch a wet switch.
2. Catch the water. Put a bowl or towels underneath. Clear out the boiler cupboard and move anything that could get damaged.
3. Warn the neighbours if you're in a flat. Check whether water's getting into the floor. Let the people below, or your managing agent, know early.
4. Read the pressure gauge, but don't top it up. Make a note of the reading. Adding water to a leaking system just feeds the leak.
5. Turn off the stopcock if it's more than a drip. Turn it clockwise to shut it. It's usually under the kitchen sink or where the water supply enters your home.
6. Take photos or a short video. Get the leak, the gauge and any damage. They help the engineer, and your insurer if you need to claim.

Once the stopcock's off, keep an eye on the leak for a few minutes. What it does next tells you a lot.

Handy clue for the engineer: a leak on the mains or hot water side stops once the stopcock's off. A leak from the sealed heating circuit usually keeps going until the system pressure drops.

Jot down what you saw, with the time. It only takes a minute, and it means the engineer starts with facts instead of guesses.

Work out where the water's really coming from

A puddle under the boiler can come from several places, and some of them are harmless. A quick look, without opening anything, can rule a few out and give the engineer a head start.

Condensation is normal on cold days. Wipe the casing dry. A light film coming back is fine. Steady drips are a leak.

The condensate pipe is the white or grey plastic pipe, usually about 21.5mm, that carries waste water from the boiler to a drain. A drip at one of its joints can look like a boiler leak. In winter, the outside section can freeze, and that often comes with a lockout fault and gurgling noises. The water's mildly acidic, so wear gloves when you mop it up. You can thaw a frozen pipe by pouring warm water over it. Avoid boiling water.

The pressure relief pipe is copper and runs through an outside wall. If it's dripping and your gauge sits near 3 bar, the system's overpressurised. The valve is letting water out as designed, so the real fault lies elsewhere. There's more on that below.

Pipework and valves under the boiler are worth a careful look. Check the joints, the radiator pipes and the filling loop hoses for wetness. Look, but don't tighten anything. A nut turned the wrong way can turn a drip into a flood.

Water from above is common in London flats and conversions. A leak upstairs can track down walls and pipes. Check the ceiling and walls above the boiler for stains.

Two more clues help the engineer:

• Colour. Clear water could come from the mains or the heating. Black, brown or rusty water almost always comes from the heating circuit. It points to magnetite sludge or corrosion.
• Timing. Note whether it leaks only when the hot water runs, only when the heating's on, or all the time.

Why is my boiler leaking water?

Most boiler leaks come from a short list of parts. These are the pressure relief valve, the pump and diverter valve seals, the automatic air vent and the condensate trap. Loose or corroded joints are common too, and older boilers can develop a corroded heat exchanger.

Water runs down and collects at the lowest point. So a drip at the bottom of the boiler can start much higher up. That's one reason the casing has to come off to find it, and that's a job for an engineer.

New boilers can leak too. A poorly made joint can weep for weeks before anyone spots it.

Is your pressure relief valve leaking?

Industry guidance puts normal boiler pressure at about 1 to 1.5 bar when the system's cold. It rises a little once the heating's on, so check your boiler manual for its own range. Most pressure relief valves open at about 3 bar to let water out.

If yours is dripping, it's doing its job. Something is pushing the pressure up. The usual reasons are:

• A filling loop left open, or one that lets water through when it's closed.
• An expansion vessel that's lost its air charge. The sign is pressure jumping when the heating comes on.
• A worn relief valve that won't reseal properly after it's opened.

If the needle climbs quickly as the system heats up, mention it when you call. Don't keep bleeding radiators and topping up to manage it. That hides the fault and can make it worse.

Why is my combi boiler leaking from the bottom?

On a combi, a leak at the bottom usually traces back to one of five spots:

• Diverter valve seals. If it mostly leaks when you run a hot tap, this is a likely suspect.
• Pump seal. A worn seal can weep down inside the case.
• Automatic air vent. It can stick open and drip onto parts below.
• Condensate trap or sump. These can crack or block and overflow inside the boiler.
• Pipe connections under the boiler. These can work loose as the pipes heat up and cool down.

Your note of when it leaks helps narrow this down before the casing even comes off.

Corrosion, sludge and an ageing boiler

Over time, rust and debris from radiators and pipes build up as sludge. Sludge wears seals and blocks parts, which raises pressure in places it shouldn't be. Rust stains on the casing or pipes are an early warning. If sludge is behind the leak, a powerflush to clear sludge from your heating system can clean things out before new parts go in.

Industry guidance suggests most boilers last around 10 to 15 years, and corrosion leaks get more likely with age. A leaking heat exchanger on an older boiler is often where the repair or replace question comes up.

Do you need an emergency boiler repair today?

Call a Gas Safe engineer straight away if you see any of the warning signs below. They include water near the electrics, a steady leak, dark or rusty water, falling pressure, or someone vulnerable left without heat. A slow drip from an outside fitting can usually wait a day or two with the power off.

A gas smell is a different emergency. Call the National Gas Emergency Service on 0800 111 999 before anyone else.

On most modern boilers, the casing forms part of the sealed combustion chamber. So taking it off is gas work. Under the Gas Safety (Installation and Use) Regulations 1998, work on gas appliances has to be done by a Gas Safe registered engineer. Before anyone starts, ask to see their Gas Safe ID card. You can also check their registration on the Gas Safe Register website or by calling 0800 408 5500.

When you call, have these ready to speed up the diagnosis:

• Boiler make and model, and its rough age
• The pressure reading
• The water colour
• When it leaks (hot water, heating or all the time)
• Your photos or video

Our Gas Safe engineers offer 24/7 emergency support across Greater London. You can see what's involved on our boiler repair service page.

Signs you shouldn't wait

• Water dripping towards the control panel or wiring
• A constant flow that doesn't ease after you turn off the stopcock
• Black, brown or rusty water
• Pressure falling to zero, or the boiler locking out
• No heating or hot water for elderly people, babies or anyone unwell

Leaks that can usually wait a day or two

• A slow drip from an outside pipe joint once the boiler's off
• A pressure relief pipe dripping now and then while the gauge sits high
• A condensate pipe joint weeping outside the boiler

Book a visit anyway. Small leaks rarely fix themselves.

If you're a landlord or a tenant

Under section 11 of the Landlord and Tenant Act 1985, landlords have to keep the heating and hot water installations in repair. So if you rent, report the leak to your landlord in writing straight away. Don't try a repair yourself.

Landlords also need a gas safety check every 12 months under the 1998 regulations, and tenants must get a copy of the record within 28 days. If you're the landlord, arrange engineer access quickly. If the repair will take a while, think about temporary heaters for your tenants.

How much does a boiler leak repair cost in London?

The bill depends on what's failed. At InstaGasWorks, a breakdown call-out and diagnostic visit is £120. The repair itself, parts and labour, is priced on top once the engineer has found the fault.

When you book, confirm exactly what the £120 covers. Then ask for the repair price before any work goes ahead. A good engineer will talk you through the options and what each one involves.

These are the things that push the price up or down:

Water damage to floors or ceilings may be covered under escape of water on your home insurance. Check your policy and keep your photos.

About these figures: they're indicative, and every leak is different. For a price that fits your boiler, ask InstaGasWorks for a quote before any work starts.

Check your warranty and cover first

• Newer boilers often come with a manufacturer warranty, which usually depends on yearly servicing. Check it before you book anyone.
• If you've got boiler cover or a home emergency plan, call them first.
• If you rent, it's your landlord's job to arrange and pay for the repair.

When replacing makes more sense than repairing

A worn seal, valve or air vent is usually worth repairing. Replacement starts to make more sense when:

• The heat exchanger has corroded on a boiler that's 10 to 15 years old or more
• You've had repeat leaks or breakdowns in the same year
• Parts for your model are hard to find

Ask the engineer for a straight comparison of both options. If replacement comes out ahead, our guide to the best boiler brands for London homes is a good place to start.

How to stop your boiler leaking again

A yearly service, plus a quick monthly look at the pressure gauge and the pipes under the boiler, helps you catch leaks while they're still small.

Manufacturers generally recommend a service every 12 months. A yearly boiler service covers many of the leak points above. The engineer checks the seals, cleans the condensate trap, checks the expansion vessel charge and tests the pressure relief valve. It also keeps most warranties valid.

Between services, a few simple habits help:

• Keep the pressure in range. After topping up, close the filling loop fully. If you're topping up every few weeks, there's a leak somewhere, so get it checked.
• Fit a magnetic system filter. Along with the right level of inhibitor, it cuts sludge and corrosion.
• Think about hard water. Most of London is a hard water area. Limescale can build up on a combi's hot water side, so ask about a scale reducer. Our guide to hard water and your boiler in South West London explains more.
• Insulate the condensate pipe. Lag any outside section before winter to stop it freezing.
• Keep the boiler cupboard clear. You'll spot a drip, rust stain or damp patch much sooner.

Getting a Gas Safe engineer round in London

Power off, water caught, a few notes on what you saw, then a Gas Safe engineer: that's the fastest route back to heating and hot water. Our Gas Safe engineers handle boiler leaks and breakdowns across Greater London, and our breakdown call-out and diagnostic visit is £120. Get in touch if your boiler's leaking, and ask about a yearly service once it's fixed so the same leak doesn't come back.