Boiler Condensate Pipe Leaking? Causes and Fixes

Updated July 2026 · Emergency Plumber Chelsea

Start by making sure it really is the condensate pipe. Every modern condensing boiler has one: a plastic pipe, usually white or grey, coming out of the underside of the boiler and running to a drain, a sink waste, a soil stack or an outside gully. It is different from the pressure relief discharge pipe, which is normally a thinner copper pipe that runs through an outside wall and should be dry. Water from the plastic pipe or from the base of the boiler around where it connects points to the condensate. Water from the copper pipe points to the pressure relief valve, which is covered in our guide to a boiler pressure relief valve leaking. Water dripping from the heating pipes or valves under the boiler is heating water, and our guide on what to do if your boiler leaks covers that.

It helps to know what the condensate system does. A condensing boiler recovers extra heat from its flue gases, and as the gases cool, water vapour in them condenses into liquid. That water is mildly acidic, so it cannot simply run down the outside wall. Inside the boiler it collects in a condensate trap, sometimes called a siphon, which holds a small reservoir of water to stop flue gas escaping down the drain, and then discharges in bursts through the condensate pipe. A boiler running on a cold day can produce a surprising amount of it. Our guide to what a condensing boiler is explains the principle in more detail.

The most common cause of a leak is a blockage downstream. If the pipe drains into the kitchen sink waste, a sink that is slow to empty or completely blocked means the condensate has nowhere to go. It backs up the pipe, forces its way out of the weakest joint, and can spill from the base of the boiler. The same happens where the pipe runs to an outside gully choked with leaves, or to a soil stack with a partial blockage. Run the kitchen tap for a minute and watch the sink. If it drains slowly, clear the sink first with our guide on how to unblock a sink, then see whether the condensate leak stops. Many do.

In winter the blockage may be ice. When part of the pipe runs outside, the water in it can freeze in a cold snap, and the condensate then backs up into the boiler, which usually locks out with a fault code and may gurgle as it tries to push water past the ice. You may see water dripping from the boiler at the same time. That is a frozen pipe rather than a failed one, and our guide to a frozen condensate pipe shows how to thaw it safely with warm water and reset the boiler.

A joint that has come apart is the next suspect. Condensate pipework is usually plastic, joined with solvent-weld or push-fit fittings, and in many Chelsea flats it runs inside a kitchen unit, along the back of a cupboard or through a boxed-in void. A push-fit joint can creep apart if the pipe is knocked when someone stores things in the cupboard, and a solvent joint can fail where it was never properly made. The leak then shows as a drip or a damp patch along the run rather than at the boiler. Follow the pipe from the boiler with a torch and a dry tissue, feeling each joint, and you will usually find the wet one.

Check the fall of the pipe as you go. Condensate relies on gravity, so the pipe has to fall steadily from the boiler to the drain without dips, flat sections or sags between clips. Where a run has dropped off a clip, or was installed nearly level, water sits in the low point, can overflow a joint and, outside, freezes more easily. A pipe that runs uphill at any point is an installation fault. Re-clipping a sagging internal run to restore its fall is often all that is needed, and the manufacturer's installation instructions set out the minimum fall and pipe sizes for your boiler.

If the water is coming from inside the boiler casing rather than from the pipe, the condensate trap itself is the likely source. Traps can crack, their seals can perish, and after a service the trap has to be refitted and refilled correctly or it can leak or let the boiler lock out. A trap that has never been cleaned can also fill with sludge until condensate spills over. Do not take the boiler casing off to look. Inside it is a gas appliance, and removing the casing or disturbing the trap and its seals is work for a Gas Safe registered engineer, because a badly refitted trap can let flue gas escape into the room.

Flues can leak water too, and the cause is often similar. Condensation also forms inside the flue, and on a modern condensing boiler the horizontal flue is designed to slope slightly back towards the boiler so that water runs back and out through the condensate trap. If a flue has been fitted level or sloping the wrong way, water can drip from the terminal outside or collect at a joint. Water appearing where the flue meets the boiler, or running down inside from a vertical flue through the roof, can mean a failed seal, a joint that has separated, or rain getting past a damaged roof flashing or terminal. A small plume of vapour and some dripping from the terminal in cold weather can be normal, but water running down the inside of the flue or the boiler case is not.

Treat any flue leak as a Gas Safe job and do not try to seal or adjust a flue yourself. A flue joint that has separated enough to let water in may also let flue gases out, and those gases contain carbon monoxide. Make sure you have a working carbon monoxide alarm near the boiler, and if the alarm sounds, or anyone feels unwell with headaches, dizziness or nausea, turn the boiler off, open windows, leave and call the National Gas Emergency Service on 0800 111 999. Our guide to the signs of carbon monoxide from a boiler explains what to look out for.

Some things are safe to do yourself and some are not. You can switch the boiler off, put a tray or towel under the drip, clear a blocked sink waste, clear leaves from an outside gully, thaw a frozen external pipe with warm water, and push a clearly separated external push-fit joint back together if you can reach it from the ground. You should not open the boiler casing, remove or refill the condensate trap, cut into or re-route pipework connected to the boiler, or touch the flue. Condensate is mildly acidic, so rinse it off your skin, wipe it from metal and stone worktops promptly, and do not let it sit on a timber floor.

Chelsea's housing adds some particular causes. In mansion blocks and period conversions the boiler often sits in a kitchen cupboard with the condensate joined into the sink waste, so a slow sink is a very common trigger. Where the pipe has to run down a rear light well or across a flat roof, frost and blocked gullies are the usual culprits, and in conservation areas and listed buildings the better long-term answer is often an internal route into a soil stack rather than a visible external pipe. Our guide to boiler flues and pipes in conservation areas covers what that means for your frontage.

A leaking condensate pipe is not usually an emergency on its own, but it becomes urgent if water is reaching the boiler's electrics, soaking into a floor above a neighbour's ceiling, or the boiler has locked out in cold weather and you have no heating or hot water. Leaving a slow leak for weeks also tends to cost more in the end, because a damp cupboard base, swollen kitchen plinths and a stained ceiling below are dearer to put right than the pipe. Our guide to leaking boiler repair costs explains how the cause of a boiler leak affects what the repair involves.

Emergency Plumber Chelsea sends Gas Safe registered engineers to leaking condensate pipes, traps and flues across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We find the source, clear or re-route the pipe, replace a cracked trap or failed seal, correct a flue fall and check the boiler is running safely before we leave. We explain the options and the price before any work starts, and there is no call-out fee on jobs over 3 hours. See our boiler repair and emergency plumber pages, or call 020 3002 6667.

Quick tips

• Check the kitchen sink first. If the condensate joins the sink waste, a slow sink is the most common reason it leaks.
• Tell the plastic condensate pipe from the copper pressure relief pipe. They leak for completely different reasons.
• Follow the pipe with a torch and a dry tissue to find the wet joint, and look for sags where it has dropped off a clip.
• Never open the boiler casing or refit the condensate trap yourself. A badly seated trap can let flue gas into the room.
• Keep a working carbon monoxide alarm near the boiler, especially if the flue has ever leaked water.

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