A dripping boiler overflow pipe usually means your boiler pressure is too high, the expansion vessel has lost its charge, or the pressure relief valve is worn and no longer sealing. On combi and system boilers, the “overflow” is actually the pressure relief valve discharge pipe, a copper pipe that runs from the boiler to the outside wall. It’s a safety device, and water coming out of it is a sign something needs attention.
We’re Clever Energy Boilers, Gas Safe registered (no. 554138) and accredited Worcester Bosch installers based in Bingley. We fit boilers across Yorkshire, the North West and the Midlands, and we’re often asked what that dripping pipe outside actually is. Here’s how to tell, and what to do about it.
What is a boiler overflow pipe?
On a combi or system boiler, the “overflow pipe” is the discharge pipe from the pressure relief valve. It’s a copper pipe, usually 15mm, that leaves the bottom of the boiler and runs through the wall to the outside, often turned back towards the brickwork so any water discharges safely. Its only job is to let water out if the system pressure gets dangerously high. Regular boilers with a loft tank may also have a separate tank overflow.
Which pipe is dripping?
There are usually two or three pipes coming out of your wall near the boiler, and they do very different jobs. Work out which one is dripping before you do anything else.
| Pipe | What it looks like | Is dripping normal? |
|---|---|---|
| Pressure relief valve (PRV) pipe | Copper, about 15mm, often turned back towards the wall | No. It should be dry |
| Condensate pipe | White or grey plastic, wider, running to a drain | Yes, when the boiler is running |
| Cold water tank overflow | Plastic, usually high up from the loft | No. Usually a float valve problem in the loft tank |
If it’s the plastic condensate pipe, that’s normal. Our guide to the boiler condensate pipe explains why it drips. The rest of this guide is about the copper PRV pipe.
Is a leaking boiler overflow pipe dangerous?
Not usually, but don’t ignore it. The PRV is doing its job by letting out water when pressure gets too high. The water can be hot, so keep children and pets away from it. The bigger concern is that your boiler is losing water and pressure, which will eventually stop it working.
Why is my boiler overflow pipe dripping?
Is the boiler pressure too high?
The PRV opens at around 3 bar. If your gauge is creeping up towards that, the valve will release water to protect the system. Our guide to boiler pressure that’s too high explains how to bring it down, and our post on what boiler pressure should be has the normal readings.
Has the filling loop been left open?
If the filling loop isn’t fully closed, mains water keeps trickling into the system, pressure rises and the PRV releases it. Check both valves on the loop are turned off firmly.
Does the expansion vessel need re-charging?
The expansion vessel absorbs the extra volume when water heats up. If it loses its air charge, pressure spikes every time the heating comes on and the PRV dumps the excess. You’ll often see pressure swing high when hot and low when cold. Re-charging it is an engineer’s job.
Is the pressure relief valve worn?
Once a PRV has opened a few times, grit or scale can stop it closing properly. Then it drips even when pressure is normal. It needs replacing by a Gas Safe engineer.
What can I check if my boiler overflow pipe is dripping?
- Read the pressure gauge with the heating off and again with it on.
- Check the filling loop valves are fully closed.
- If pressure is over 2.5 bar, bleed a radiator to bring it down to about 1.5 bar.
- Keep an eye on the pipe for a day or two.
If the dripping stops once the pressure is normal, it was simply over-pressurised. If it carries on, the PRV or expansion vessel needs looking at.
Can I block or cap the overflow pipe?
No, never. The PRV pipe is a safety device that stops your system building up dangerous pressure. Blocking it could cause serious damage. Fix the reason it’s dripping instead.
When should I call a Gas Safe engineer?
Call one if the pipe keeps dripping after the pressure is back to normal, if pressure keeps rising with the filling loop closed, or if the boiler keeps losing pressure. Our guide on a boiler losing pressure explains the connection.
Is a dripping PRV a sign I need a new boiler?
On a boiler over 8 years old, a worn PRV or a flat expansion vessel is often a sign that other seals and parts are near the end too. Fixing them one at a time adds up, and older boilers are less efficient anyway.
We offer boiler replacement with fixed prices and Gas Safe engineers, and we can arrange long, market-leading warranty cover for your new boiler through Clever Energy. As accredited Worcester Bosch installers, we can also talk you through the latest Greenstar models.
Boiler overflow pipe FAQs
Should water come out of my boiler overflow pipe?
No. The copper PRV pipe should stay dry. Water from it means pressure has been too high or the valve isn’t sealing.
Why does my overflow pipe only drip when the heating is on?
Because pressure rises as the water heats up. If it tips over 3 bar, the PRV opens. That usually points to a flat expansion vessel or too much water in the system.
Why is my combi boiler overflow pipe leaking outside?
Same reasons: high pressure, a filling loop left open, a flat expansion vessel or a worn PRV.
Is it the boiler overflow or the condensate pipe?
Copper means PRV, and dripping isn’t normal. Plastic running to a drain means condensate, and dripping is expected.
