Water under your boiler: condensate drain, condensate trap or a real leak?

Short answer: condensate under the boiler almost always comes from a blocked or cracked condensate drain, not from the heating water. In winter a condensing boiler produces a few litres of condensate a day and discharges it via a condensate trap (siphon) into the drain. If that trap is clogged with soot and limescale or the plastic has cracked, the water runs out. You can usually clean it yourself; a new condensate trap costs €10 to €39, depending on brand and type. If the system pressure drops as well, it is not condensate but a leak.

First determine: condensate or heating water?

That distinction decides everything. Condensate is mildly acidic, colourless and comes from the plastic drain at the bottom of the boiler. Heating water is often darker, sometimes black with sludge, and you can see its loss on the pressure gauge: the pressure drops.

If the water pressure stays at 1.5 bar while there is water under the boiler, it is almost certainly condensate. If the pressure does drop, read our guide on water pressure that keeps dropping.

Why a condensing boiler produces water

A condensing boiler extracts so much heat from the flue gases that the water vapour in them condenses. That is exactly where its efficiency comes from. An average home therefore produces a few litres of condensate a day in winter. That water is mildly acidic, which is why the drain is made of plastic and not metal.

The condensate trap underneath holds a water seal, so that no flue gas can enter the room via the drain pipe. That water seal must therefore always be full. If a boiler is idle for a long time, for example a whole summer, the water seal can evaporate; some appliances, such as Bosch models, therefore run a trap-filling programme on start-up.

Where it goes wrong

By far the most common cause: the condensate trap is blocked. Soot and limescale are carried along in the condensate and settle in the bend. The water can then no longer get through and overflows. Manufacturers flag this too: Intergas shows code 5 for a blocked condensate drain, Remeha reports E 425 and Vaillant and AWB use F.77 for a problem with the condensate pump or flue gas flap.

In addition, plastic cracks after years of use, especially where the condensate trap clicks into the boiler. And with boilers in a loft or basement, where the drain has to go upwards, there is a small condensate pump in between that can also fail.

Cleaning or replacing the condensate trap

On most appliances you can clean it without opening the boiler: the condensate trap is at the bottom and can usually be removed with a twist. Rinse it with lukewarm water, then refill it with clean water and click it back. Refitting an empty trap is not a good idea, because you then briefly have no water seal.

If the plastic is cracked or the trap no longer fits tightly, replace it. Examples from our catalogue:

Part	Manufacturer number	Price
Intergas condensate trap L=235 for HRE	844787	€21.95
Nefit condensate drain pipe	7101531	€26.95
Vaillant condensate trap VHR3/C including hose	180984	€35.95
Nefit condensate trap for Economy HR	75969	€38.95

All variants can be found under condensate traps and condensate drains.

Check the drain pipe itself too

The pipe from the condensate trap to the drain must slope downwards and must not sag. A hose that forms a loop somewhere holds water and silts up. Also, never connect the pipe directly and airtight to the drain; there should be an open connection or an extra trap in between, otherwise the drain sucks the water seal empty and you will smell it straight away.

If the drain discharges into a sink or washbasin, check that trap as well: the acidic condensate speeds up deposits there.

DIY or call an engineer?

Removing, cleaning and refitting the condensate trap is a job you can do yourself; you do not touch the gas section. Switch the boiler off, have a bucket and a cloth ready and take your time. If you are not sure how the trap is fixed, or if the boiler casing has to come off to reach it, have a professional do it.

Cleaning the condensate trap is a standard part of annual servicing. If the boiler overflows when it has just been serviced, report this to your installer.

Frequently asked questions

Where does condensate under the boiler come from?

A condensing boiler cools the flue gases so far that the water vapour in them condenses. That condensate runs via the condensate trap to the drain. If it collects under the boiler, the trap or the condensate drain is blocked, cracked or loose. If the water pressure on the gauge stays the same, it is almost certainly condensate and not heating water.

How do you clean the boiler's condensate drain?

Switch the boiler off, hold a bucket under the condensate trap and remove it with a twist. Rinse it with lukewarm water, with a soft brush if needed, and check the drain pipe for a loop or sag. Fill the trap with clean water before refitting it; without a water seal, flue gas can enter the room.

How much condensate is normal?

A few litres a day in winter for an average home. You do not see it, because it drains away immediately. Only when the drain is not working do you notice how much it actually is.

Can I discharge condensate onto the lawn or into the garden?

No, do not do that. Condensate is mildly acidic and should be discharged via the drain, exactly as the installation instructions require.

Can I unblock the condensate trap with drain unblocker?

Do not use aggressive drain unblockers: they attack the plastic and the seals. Rinsing with lukewarm water and a soft brush if needed is enough.

I can smell flue gas near the boiler. What now?

Switch the appliance off, ventilate the room and call a certified gas engineer. An empty or leaking water seal can let flue gas into the room, and that is a carbon monoxide risk. Background information is available from Milieu Centraal.