Noticed a mysterious drip-drip-drip from a copper pipe sticking out of your wall? That’s your boiler’s overflow pipe - also known as the pressure relief pipe - and it’s trying to tell you something’s up. Think of it as your boiler’s way of throwing a hissy fit.
A leaking overflow pipe usually means your boiler’s pressure system isn’t functioning properly. It’s not always an emergency, but it’s definitely not something to ignore.
In this guide, we’ll walk you through what the overflow pipe does, why it might be leaking, and what you can do about it - whether you’re handy with a spanner or just want to impress the engineer with your newfound boiler wisdom.
Let’s get into it…
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Key Takeaways:
A dripping overflow pipe is your boiler’s cry for help, often due to high pressure or a dodgy pressure relief valve.
Overpressurisation, a knackered expansion vessel, or limescale gunk are usually the culprits.
You might fix simple issues yourself, but anything tricky needs a Gas Safe engineer - don’t be a hero.
Ignore it, and you’re risking water damage, a sulky boiler, or a repair bill that’ll make your eyes water.
What is The Overflow Pipe?
Think of your boiler as a moody chef under pressure, and the overflow pipe as its safety valve - there to blow off steam (well, water) when things get a bit too heated.
This small copper (or sometimes plastic) pipe runs from your boiler to the outside of your home, often poking out near a drain or discreet corner - ideally not right above your favourite flowerbed.
It’s connected to the pressure relief valve (PRV), which acts like the boiler’s personal bouncer, chucking out water if the internal pressure climbs above a safe level (usually over 2.5–3 bar).
Most of the time, the overflow pipe just sits there quietly, doing nothing - a proper wallflower. So if it starts dripping like a leaky tap, it’s your boiler’s way of waving a little white flag.
You might hear it called the discharge pipe or safety valve pipe in older systems, but they’re all doing the same job: stopping your boiler from having a high-pressure meltdown.
Let’s unpack why it’s started leaking - and what you can do about it…
Why’s My Overflow Pipe Leaking Like a Sieve?
If your overflow pipe’s dripping, it usually means one of two things: the pressure relief valve (PRV) is doing what it’s meant to - releasing excess pressure - or it’s gone a bit rogue and started leaking when it shouldn’t.
The PRV is designed to open if your boiler pressure gets too high, but if the valve is faulty or something else in the system is off-kilter, it might start letting water out for no good reason.
Common culprits include sky-high pressure, a tired or faulty PRV, a dodgy expansion vessel, or a system clogged with limescale and sludge (nice).
Before you spiral into Googling “cost of a new boiler,” let’s break down how to figure out what’s actually going on.
Checking Your Boiler Pressure (No PhD Required)
First things first - let’s play boiler detective.
Find the pressure gauge on your boiler; it’ll usually be on the front or underneath, looking a bit like a tiny speedometer, with a needle and numbers in bars.
When your boiler’s cold (no heating or hot water running), the pressure should be sitting comfortably between 1 and 2 bar, with 1.5 bar being the Goldilocks zone for most systems.
If the needle’s crept past 2.5 bar into the red zone, your boiler’s overpressurised, and the overflow pipe is likely releasing water to keep things safe.
If it’s below 1 bar, you’ve got a low-pressure issue - but that’s probably not the cause of the leak.
How to Check:
Switch off the boiler and let it cool fully.
Take a look at the pressure gauge and jot down the reading.
If it’s high, try bleeding your radiators - this releases a bit of water and air, which should lower the pressure (think: letting air out of an overpumped tyre).
Need a step-by-step? Our boiler pressure guide has you covered - minus the risk of turning your lounge into a splash zone.
Once bled, check the gauge again. If the pressure’s dropped and the leak stops, congrats - you’re officially a boiler whisperer.
If not? Keep reading…
What’s Causing the Leak (and What It Means for Your Boiler)
A leaking overflow pipe is your boiler waving a red flag - usually about pressure. But a few different culprits could be behind the mess.
Overpressurisation: Too Much Pressure, Too Little Chill
If your boiler’s pressure is higher than your mate’s blood pressure during the football, the pressure relief valve opens to release the excess, straight through the overflow pipe.
This might be because:
You (or someone) topped up the system too much.
The filling loop (that flexible braided hose under your boiler) was accidentally left open.
Fix:
Check that both filling loop valves are fully closed - the levers should sit perpendicular to the pipe, like a “no entry” sign.
Still high? Bleed the radiators and monitor the pressure for a day or two. If it keeps creeping up, time to call in the pros.
🎥 Watch: How to Bleed a Radiator (Without Flooding Your Floor):
Faulty Pressure Relief Valve: The Bouncer’s Lost the Plot
The PRV is meant to open only when the pressure’s too high.
But if it’s faulty, old, or crusted with limescale, it might start leaking even when the pressure is normal.
Fix:
This isn’t a DIY job. The PRV is part of the boiler’s safety system, and replacing it involves draining the system and (potentially) handling gas components.
Leave this one to a Gas Safe engineer - trying to tackle it yourself is like doing surgery with a butter knife. Hard pass.
Expansion Vessel Failure: The Shock Absorber’s Shot
Your boiler’s expansion vessel absorbs water expansion as it heats.
Inside is a rubber diaphragm separating air and water - if the diaphragm’s split or air pressure is too low, the boiler can’t stabilise pressure, leading to overflow pipe leaks.
Fix:
An engineer can check the vessel pressure (imagine a fancy bike pump setup). If it’s just low, they’ll repressurise it.
If the diaphragm’s torn, the vessel needs replacing - a bigger job involving full system drainage.
Bottom line: don’t try to DIY this one - your boiler isn’t a party balloon.
Limescale or Sludge Buildup: The Boiler’s Blocked Arteries
In hard water areas (hello, London and the South East), limescale builds up inside pipes and boiler components.
Add some sludge - a charming mix of rust and grime - and you’ve got pressure spikes or damaged valves.
Fix:
A Gas Safe engineer can do a power flush or chemical flush to blast out the gunk. To prevent future issues, consider:
A scale reducer
A magnetic filter
Regular boiler servicing
Think of it as giving your boiler a health check - and saving yourself from surprise leaks later.
Can I Fix This Myself - or Am I Out of My Depth?
Good news: some overflow pipe leaks are totally fixable without calling in the cavalry, as long as you’re reasonably confident and own a radiator key.
Start with the filling loop. Make sure both valves are fully closed - they’re sneaky little things and can easily be left open just enough to let water slowly creep in and raise the pressure.
Next up: bleed your radiators. This releases trapped air and a bit of water, which helps bring the pressure back to the 1–2 bar sweet spot.
It’s simple stuff, and our boiler pressure guide walks you through it step by step - no faff, no floods.
Don’t Forget the Pipe Itself:
In cold weather, the overflow pipe can freeze if it’s not insulated properly. A blocked pipe means water backs up and leaks elsewhere - sometimes inside.
To clear it:
Gently pour warm (not boiling) water over the pipe to melt any ice.
Clear out any visible debris - leaves, dirt, the occasional spider web (yes, really).
Avoid using force - you don’t want to crack the pipe and turn a quick fix into a full-blown repair.
If none of that works, or the pressure just won’t behave, it’s time to step back and call a Gas Safe engineer. Some things are best left to the pros - especially when gas and pressurised systems are involved.
DIY Checklist Before Calling an Engineer
Find the pressure gauge and check if it’s in the 1–2 bar range.
Make sure the filling loop is fully closed (both valves off).
Bleed radiators if the pressure’s above 2.5 bar, then recheck the gauge.
Inspect the overflow pipe for ice or blockages and clear gently if needed.
Keep an eye on the leak for 24 hours after your tweaks.
If the leak’s still going strong or the pressure keeps climbing, it’s time to wave the white flag and call a Gas Safe engineer.
Fiddling with valves or expansion vessels yourself is a recipe for disaster- think flooded floors or a boiler that refuses to play ball.
When to Call a Gas Safe Engineer
Some issues are just too tricky for a screwdriver and a YouTube tutorial. You’ll need a Gas Safe registered engineer if:
The pressure relief valve is faulty and needs replacing.
The expansion vessel is kaput or needs repressurising.
Limescale or sludge is clogging up the system.
The leak persists after your DIY attempts.
Boilers involve high-pressure systems and gas, so don’t be tempted to crack open the casing yourself - nobody wants to star in a DIY disaster story.
Is a Leaking Overflow Pipe Dangerous, or Just a Nuisance?
A little drip might seem like no big deal, but don’t be fooled- it’s your boiler waving a red flag.
Left unchecked, a leaking overflow pipe can cause water damage to your walls, floors, or garden (goodbye, prize-winning roses).
It can also lead to low boiler pressure, which might make your boiler throw in the towel and refuse to heat your home.
In rare cases, persistent high pressure could strain the system, increasing the risk of a breakdown or, worse, a safety issue.
If the leak is more like a steady stream or you hear weird noises (gurgling, banging, or hissing), don’t mess about- call an engineer pronto.
The sooner you act, the less likely you are to face a repair bill that’ll make you choke on your tea.
How Much Will It Cost to Fix?
The cost of fixing a leaking overflow pipe depends on what’s gone wrong.
If it’s just a case of high pressure, a quick DIY fix or a short engineer visit might set you back £50–£100 (or nothing if you nail it yourself).
Replacing a faulty pressure relief valve typically costs £100–£200, including parts and labour.
If the expansion vessel needs repressurising, expect to pay £150–£250; a full replacement could run £300–£400.
For limescale or sludge issues, a power flush might cost £300–£600, depending on the size of your system.
For a breakdown of typical costs, check out our guide on engineer visit costs.
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