A boiler overflow pipe that is dripping or running usually means the pressure relief valve has opened because the heating system pressure went too high, or that the valve itself is worn and letting water by. It is not dangerous in itself, because the valve is doing its safety job, but it will not fix itself: check the pressure gauge, make sure the filling loop is closed, and call a Gas Safe registered engineer if it keeps happening.

First, make sure it is the right pipe

Several pipes leave a house near the boiler, and only one of them should never drip. Identifying it saves a lot of worry.

Pipe	What it looks like	Is dripping normal?	What it means if it runs
Pressure relief valve discharge pipe	Copper, about 15 mm, from the bottom of the boiler through the wall, usually turned back to the wall	No	The heating pressure is too high or the valve is letting by
Condensate pipe	Plastic, usually wider, running to a drain or soakaway	Yes, when the boiler is running	Normal. A condensing boiler makes acidic water that has to drain away
Unvented cylinder discharge pipe	Copper, from a tundish near a hot water cylinder, often to a gully	No	A hot water cylinder safety valve has opened. See our unvented cylinders service
Loft tank overflow	Plastic, from high on the wall or under the eaves	No	A float valve in a cold water or feed tank is not shutting off

If your dripping pipe is the plastic condensate pipe and the boiler is running, that is normal. If the boiler has stopped in freezing weather and that pipe is frozen, our guide to a frozen condensate pipe shows how to thaw it safely. The rest of this guide is about the copper pressure relief pipe.

Why the pressure relief valve opens

A combi or system boiler runs on a sealed heating circuit. When cold, it is normally filled to around 1 to 1.5 bar, and it rises a little as the water heats up and expands. To stop that expansion pushing the pressure too high, the boiler has an expansion vessel: a steel tank with a rubber diaphragm and a cushion of air or nitrogen behind it. The heated water squeezes into the vessel and the pressure stays in a safe range.

If the pressure still climbs too high, the pressure relief valve opens and lets water out through the discharge pipe to the outside. Ideal’s installation manual for the Logic Max combi, for example, gives a relief valve setting of 3 bar and a 15 mm copper discharge pipe. Other makes use a similar setting, and your boiler’s manual gives the exact figure.

So when you see water from that pipe, one of two things is happening: the pressure really did reach the limit, or the valve is no longer sealing properly and is letting water through below it. Our boiler repair engineers find out which on the first visit.

Check these first

These checks are safe for any homeowner.

1. Read the pressure gauge. Look at it with the heating off and the system cold, then again with the heating running. Cold, it should sit roughly between 1 and 1.5 bar. If it is close to 3 bar when hot, the relief valve is opening because the pressure is too high.
2. Check the filling loop is closed. The filling loop is the flexible hose or built-in lever under the boiler used to top up the pressure. If a valve on it has been left open, or is letting water through, mains water keeps feeding in and the pressure creeps up. Close the valves as your user guide shows.
3. Watch what happens after it drips. If the pressure shoots up every time the heating comes on and then falls well below 1 bar when it cools, the expansion vessel has very likely lost its charge.
4. Bleed a radiator to lower a high pressure. With the heating off and the radiators cool, open a radiator bleed valve a little with a bleed key and let out some water, catching it with a cloth, until the gauge drops to around 1 to 1.5 bar. Our guide on how to bleed a radiator covers it step by step.
5. Note any low pressure afterwards. Each discharge loses water from the system, so the pressure may now be low. If you top it up, stop at 1 to 1.5 bar and close the filling loop fully.
6. Look for signs of long-term dripping. White or green staining on the wall below the pipe, or a wet patch on the ground, means the valve has been opening regularly.

Common causes of a leaking boiler overflow pipe

Cause	What you might notice	Can you fix it yourself?	What the engineer does
System overfilled when topping up	Gauge near 3 bar soon after you used the filling loop	Yes, bleed a radiator to bring it down	Nothing needed unless it recurs
Filling loop left open or letting by	Pressure climbs even with the heating off	You can close the valves. If it still climbs, no	Replaces the filling loop valve or its internal parts
Expansion vessel has lost its charge	Pressure jumps when hot, drops low when cold, drips during heating	No	Checks and recharges the vessel to the maker’s setting, or replaces it if the diaphragm has failed
Worn or dirty relief valve	Steady drip even at normal pressure	No	Replaces the pressure relief valve
Internal leak between mains water and heating water (combis)	Pressure rises when hot water is used	No	Tests the plate heat exchanger and related parts, then replaces the faulty one
Debris or scale on the valve seat	Drip started after a pressure spike or system work	No	Cleans the system and replaces the valve

Vaillant notes that sludge, limescale and overfilling through the filling loop can all stop a relief valve sealing. The expansion vessel is the cause we find most often on boilers a few years old, because the charge slowly leaks away over time. It is also one of the main reasons behind a boiler that keeps losing pressure.

Is a dripping overflow pipe an emergency?

Usually not. The relief valve is a safety device, and while it is dripping the boiler is protecting itself. The risks are indirect: the system slowly loses water, the pressure falls and the boiler eventually stops on a low pressure fault, and fresh water keeps getting drawn into the system, which brings in oxygen and speeds up corrosion inside radiators.

Treat it as urgent if the pipe is pouring water continuously, if the gauge is in the red, or if you also have water coming from inside the boiler casing. In those cases, turn the boiler off at the switch and call an engineer. We answer the phone 24 hours a day and offer 24/7 emergency cover.

What an engineer will check

• The pressure cold and hot, and how fast it rises when the heating starts.
• The expansion vessel charge, measured with the system pressure released. It should match the figure in the boiler’s installation manual.
• The filling loop for passing water, including internal filling loops built into many combis.
• The relief valve for signs of weeping or debris, and whether it reseats after operating.
• On combis, whether mains pressure is crossing into the heating side through an internal fault.
• The condition of the system water, because sludge is often behind a relief valve that will not seal.

After the repair, the system should be refilled to the right pressure and the discharge pipe checked for a correct, safe termination.

What it costs with Vantage

In normal working hours there is no call-out fee. Gas faults start with a fixed-price Gas Safe diagnostic visit, and if we can fix it on the same visit, part of that diagnostic charge comes off the repair. After the first hour, time is charged in half-hour steps, and parts are quoted before we fit them. Evening, weekend and overnight visits are charged at the out-of-hours rates. Every rate is published on our prices page, so you know what you will pay before we arrive.

Recharging an expansion vessel usually costs less than replacing one, and a relief valve is a small part, but the labour depends on how easy the boiler is to work on.

Frequently asked questions

Why is water coming out of my boiler overflow pipe?

The pressure relief valve has opened. Either the heating pressure reached its limit, often because the expansion vessel has lost its charge or the filling loop is letting water in, or the valve itself is worn and letting water past at normal pressure.

Can I fix a dripping boiler overflow pipe myself?

You can check the pressure, close the filling loop and bleed a radiator to lower a high pressure. If that stops the drip and it does not come back, no further work is needed. Anything inside the boiler, including the expansion vessel and the relief valve, needs a Gas Safe registered engineer.

Is it safe to use my boiler while the overflow pipe drips?

Usually yes, as long as the pressure gauge is not in the red and there is no water leaking inside the casing. The drip means the safety valve is working. Get it looked at soon, though, because the system will keep losing water and pressure.

Why does my boiler overflow pipe only drip when the heating is on?

That pattern points strongly to the expansion vessel. When the water heats up it expands, and with no working cushion to absorb it the pressure rises until the relief valve opens.

Should the condensate pipe drip?

Yes. The plastic condensate pipe carries away the water a condensing boiler makes while it runs, so dripping from it is normal. It is only a problem if it is blocked or frozen.

Help with a leaking boiler overflow pipe

If the checks above have not stopped the drip, the next step is a proper diagnosis by an engineer. Our own Gas Safe registered engineers (registration 956889) cover Hertfordshire, Bedfordshire and Cambridgeshire from our Stevenage base with fully stocked vans, and we are rated 4.9 on Google. Book a boiler repair or call 01462 506374, any time of day or night.