Water under the boiler: find the source before you panic

First, make it safe

A little water under the boiler is rarely an emergency, but it needs dealing with. Vaillant’s advice for a heavier leak is to switch the boiler off for now, and close the gas tap if you switch it off completely. Water and the electrics inside a boiler don’t mix. Put a bowl or a towel under the drip, and never take the casing off: Vaillant says so in as many words.

Is it definitely the boiler? Water can run along a pipe from somewhere else, and the cold pipe feeding a combi can sweat in summer. My guide to a water leak at home covers the main stop tap, the test for condensation and your insurer. This page is about water from the boiler itself and the pipes under it.

One honest note: I work by appointment, 08:00-16:00, seven days a week, and I don’t run a 24-hour service. If water is pouring rather than dripping and you can’t stop it, you need an emergency plumber or your insurer’s helpline, not me.

The drain pipe under the boiler

Most of the leaks I’m asked about come from one thin pipe that runs from under the boiler to a drain. It carries more than one thing. The Intergas installation manual has the siphon’s condensate hose, the discharge from the safety valve (overstortventiel) and the discharge from the inlaatcombinatie on the hot-water side all going to the sewer through the same open connection. Vaillant describes the safety valve discharging through a drain pipe and an open siphon into the waste water. Flamco adds that the open connection is deliberate: a discharging safety valve is telling you something, and a funnel makes sure you see it.

So the first question is which of the three is dripping, and when.

Condensate: normal, until the drain blocks

A condensing boiler makes water. It collects in a tray, passes through a siphon and drains away, and a trickle from that hose while the heating runs is exactly what should happen. Trouble starts when the route is blocked, disconnected or broken; Vaillant lists those three as a cause of water under the boiler. In a cold garage or loft the pipe can freeze too, which is why Intergas requires the boiler to hang in a frost-free room.

A blocked condensate drain doesn’t just leak. Intergas lists it among the causes of fault codes 4 and 5, so a boiler that drips and then locks out often has one problem, not two. And one thing you must never do is pull the siphon off to stop the drip: Intergas warns that with the siphon empty, combustion gases can escape into the room. At a boiler service I rinse the tray and the siphon and refill it before it goes back.

The safety valve

The overstortventiel is a spring valve that opens if the pressure gets dangerously high; Intergas fits a 3 bar valve in the flow pipe within half a metre of the boiler. If its pipe drips only when the boiler is hot, look at the pressure gauge. High when hot and low when cold means the pressure is swinging, and the expansion vessel is the first suspect.

If it weeps all the time, at any pressure, Flamco’s fault list points at dirt, tape or hemp caught between the valve and its seat, often from when the system was first filled. Sometimes that flushes out; if not, the valve is replaced. What you don’t do is cap the pipe or tape it up, because then the valve can’t do its one job.

Fittings, the filling valve and the air vent

The joints where the pipes meet the boiler work loose over the years, and Vaillant names loose or worn connections as a common cause. Look for a drip on the nut itself, or a green or white crust. Flamco makes a useful point: at a slow leak the water often evaporates before it forms a drop, so rust or scale at a joint usually means a leak even when it looks dry.

Two other small ones: a filling valve (vulkraan) that isn’t fully closed can weep, and the automatic air vent on top of the boiler can start passing water instead of air. Vaillant lists a broken air vent among the causes too.

The heat exchanger and the expansion vessel

Now the two inside the casing. Vaillant names a damaged, rusted or broken heat exchanger and a worn or damaged expansion vessel as causes of water around the boiler. Neither is something to look at yourself.

A leaking expansion vessel is a part swap. A leaking heat exchanger on an older boiler is the one that changes the conversation, because the part is dear and the boiler has usually earned its retirement. That’s when I’d rather talk about what a new boiler costs than fit an expensive part to a tired one. Remeha adds a useful check: a leak often shows as wet patches around the boiler together with falling pressure, so note what the gauge is doing.

What you can do, and what to leave

Do:

• switch the boiler off if the water comes from inside the casing or doesn’t stop
• dry everything, lay kitchen paper under the pipes and see where the first drop lands
• note when it drips: heating on, hot tap running, or all the time
• read the pressure gauge, cold and hot
• take photos and a short video

Don’t:

• open the casing
• keep topping up a boiler that keeps losing water; my pressure guide explains why
• cap, tape or remove the drain pipe or the siphon

Getting it repaired

Send me a message on WhatsApp with the photos, the make and model, the pressure reading and your postcode, and tell me when it drips. My rate is €114.95 incl. BTW per hour, with no call-out charge, plus parts. A typical repair with a replacement part comes to around €300-€400 incl. BTW. Boiler repair explains how a visit works, and I agree the cost before I start.

If you smell gas, leave the property and call 0800 9009 from outside. If a CO alarm goes off or you suspect carbon monoxide, get everyone outside and call 112.