Boiler Overflow Pipe Dripping Outside: What It Means

First, which pipe is actually dripping?

Most houses and flats have several pipes poking out of the wall near the boiler, and only one of them is the boiler overflow; water running from the flue is a different problem again. Stand outside and match what you see to this list before you worry.

• Thin copper pipe, about pencil width (15 mm), usually bent back down towards the wall: the boiler pressure relief valve discharge. This is what people call the boiler overflow
• White or grey plastic pipe, 21.5 mm to 32 mm, often running down into a drain or gully: the condensate pipe. Some dripping while the boiler is running is normal
• Plastic pipe high up on the wall, often from the loft: the overflow from a cold water storage tank or a heating feed and expansion tank, found on older open-vented systems
• Copper pipe, often 22 mm, running from a tundish (a funnel with a visible air gap) beside a hot water cylinder with no loft tank: the discharge from an unvented cylinder, not the boiler
• Plastic flue terminal with water running from it: condensation in the flue, which usually comes out as steam or light dripping in cold weather

If it is the copper pipe: why a combi overflow drips

The copper pipe only carries water when the pressure relief valve inside the boiler has opened. That valve is set to about 3 bar, so a dripping copper pipe means one of two things. Either the heating pressure is going that high, or the valve opened at some point and now fails to close fully.

Look at the gauge on the front of the boiler. With the heating cold it should read around 1 to 1.5 bar. If it is at 2.5 bar or more, the drip is the valve doing its job. The usual causes are an expansion vessel that has lost its air charge, a filling loop tap left slightly open or letting by, or, on a combi, a plate heat exchanger letting mains water into the heating side. Our guide to a boiler pressure relief valve leaking shows how to tell those three apart with a day of gauge readings.

If the gauge is normal and the pipe still drips, the valve itself is at fault. Once it has lifted, grit or scale can hold it open slightly. A drip every few seconds on a normal pressure reading usually means a new valve.

Never block, cap or shorten the copper pipe to stop the drip. It is the outlet for a safety valve. Water from it can also be hot, so keep children and pets away if it is running.

If it is the plastic pipe: condensate, and when it is a problem

Every condensing boiler makes acidic water from its exhaust gases, often 10 litres or more a day in winter. It drains through a plastic pipe to the outside, and seeing it drip while the boiler is firing is expected. What is not expected is water spilling from a joint, the pipe dripping when the boiler has been off for hours, or the boiler locking out with a fault code on a cold morning.

In freezing weather the condensate can freeze in the pipe outside, back up into the boiler and trip it out. Pouring warm water, not boiling, over the outside section usually clears it. Current guidance in BS 6798 calls for external runs with an internal diameter of at least 30 mm, which in practice means 32 mm pipe and freezes far less readily than the thinner pipe on many older installs. More in our guide to a boiler condensate pipe leaking.

If it is higher up on the wall: an old tank overflow

Homes with a regular boiler and a hot water cylinder often have two tanks in the loft: a large cold water storage tank and a small feed and expansion tank for the heating. Each has an overflow pipe to the outside, placed where you will notice it. When one of them drips, the float valve in that tank is not shutting off. A drip from the small tank can also mean the heating is pumping over, pushing water up the vent pipe.

That is not a boiler fault as such, and it rarely needs an emergency visit. See a header tank overflowing and loft tank ballcock problems for the checks, which usually start with the float arm.

How urgent is a dripping boiler overflow?

A slow drip from the copper pipe with the pressure between 1 and 2.5 bar is not an emergency. The boiler can run, but book a repair in the next day or two, because a valve that keeps weeping will drain the system bit by bit and a loop letting by will keep pushing the pressure back up.

It is more urgent if the pipe is running rather than dripping, if the gauge keeps hitting 3 bar, or if the pressure has dropped so far the boiler has locked out and you have no heating or hot water. Switch the boiler off at its fused spur in those cases. A running discharge onto a path in winter also turns to ice, which is a slip risk worth fixing quickly.

Do not keep topping the pressure up. Each top-up brings fresh water and oxygen into the system and adds to the drip.

What the engineer does, and what it usually costs

The engineer confirms which pipe is discharging, checks the pressure cold and hot, then drains the heating side to test the expansion vessel charge. They check the filling loop for a let-by and, on a combi, whether mains water is crossing into the heating circuit through the plate heat exchanger. The relief valve is replaced if it will not reseal.

Typical UK market prices, parts and labour: a relief valve £100 to £200, an expansion vessel recharge often done within the same visit, and a new vessel £150 to £450. A filling loop valve is usually a small part. You can see how a visit is priced on our pricing page.

Our boiler repair visit has the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers.