Water trickling from a pipe on an outside wall almost always traces back to your heating system. Here's how to work out which "overflow" is leaking, what's safe to check yourself, and when it's an engineer's job.
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People say "overflow pipe", but on a heating system there are usually two very different pipes it could be — and they point to completely different faults. Before you do anything, have a look at where the water is coming from and what the pipe is made of. Getting this right tells you whether it's a quick homeowner check or a call to an engineer.
The two suspects: a small copper or plastic discharge pipe near the boiler (the pressure-relief outlet on a modern sealed system), or a larger plastic overflow from a tank in the loft (an older open-vent system). Identify which one and the rest of the diagnosis falls into place.
If you have a combi or sealed system boiler, the pipe in question is the pressure-relief discharge pipe — sometimes loosely called the overflow. It's a short length of copper or plastic that runs from the boiler to a nearby external wall and ends in a downward bend, usually low down and close to where the boiler sits inside.
This pipe is fed by the pressure relief valve (PRV), a safety device that opens when system pressure climbs too high — typically around 3 bar. When it opens, it deliberately dumps water outside to protect the boiler and pipework. So a one-off discharge after a fault is the valve doing exactly what it should. The problem is when it keeps dripping or running.
A continually dripping discharge pipe almost always comes down to one of three things:
We cover the diagnosis in more depth in our guide to pressure relief valve problems, which explains why the valve is usually a symptom rather than the root cause.
The only homeowner-safe step here is to look at whether high pressure is driving the discharge. None of this involves the gas supply, the flue, the casing or the sealed parts of the boiler.
Do not touch the PRV or the discharge pipe. Never loosen, "test", cap or block them. The valve is a sealed safety component, and the pipe is a deliberate safety outlet — blocking it removes the boiler's protection against over-pressure. Replacing a PRV means draining the system and working inside the boiler, which is strictly a job for a Gas Safe registered engineer.
If your home has an older open-vent (heat-only) system rather than a sealed one, the leaking pipe may instead be the overflow from the feed-and-expansion tank in the loft. This is a smaller version of the cold-water storage tank, often called the "F&E" or "header" tank, and it's what tops the heating system up automatically.
Like a toilet cistern, the tank has a float-operated ball valve that's supposed to shut off the incoming mains water at the right level, with an overflow pipe as a backstop. If the ball valve sticks open or the float fails, the tank overfills and water runs out of the overflow pipe — typically a larger plastic pipe discharging from high up under the eaves, not down by the boiler.
Telltale signs it's the F&E tank rather than the PRV:
A sticking ball valve is plumbing rather than gas work, so a competent plumber (or a confident DIYer) can deal with the valve and float. But getting up into a loft, handling mains water and the fact that an overfilling tank can also point to a deeper problem — such as the system pushing water up the vent pipe — mean it's often worth having a heating engineer look at it rather than guessing.
| What you see | Likely source | What it usually means |
|---|---|---|
| Small copper/plastic pipe, low down by the boiler | Pressure-relief discharge pipe | High pressure, faulty expansion vessel or worn PRV |
| Larger plastic pipe high up, under the eaves | Feed-and-expansion tank overflow | Stuck ball valve / failed float in the loft tank |
| Combi or sealed system with a pressure gauge | Pressure-relief discharge pipe | Check the gauge first — see above |
| Hot-water cylinder and tanks in the loft, no gauge | Feed-and-expansion tank overflow | Ball valve almost always the culprit |
For a sealed-system discharge pipe, try the safe pressure check once. Call a professional if:
Any work inside the boiler — the PRV, the expansion vessel, the sealed circuit — must be done by an engineer on the Gas Safe Register. (You may still hear older tradespeople say "CORGI"; that scheme was replaced by Gas Safe in 2009, so check for a Gas Safe ID card.) If you ever smell gas or suspect a leak, leave the property and call the National Gas Emergency line on 0800 111 999.
A failed expansion vessel, a worn PRV or a faulty ball valve are exactly the kind of repairs a boiler cover policy is built for — the call-out, labour and parts may be included rather than landing as an unexpected bill, subject to your plan's terms, excess and any exclusion period. Cover won't reimburse you for checking the gauge or bleeding a radiator yourself, but for the engineer-only fixes it can pay for itself in a single visit. It's worth weighing up whether a policy makes sense for your boiler's age and condition before a fault appears.
A dripping discharge pipe usually ends in an engineer's visit. See how policies from our selected panel of providers compare on price, call-out limits and what's included.
Compare boiler coverUsually not an immediate emergency, but it shouldn't be ignored. A steadily dripping pressure-relief pipe means the sealed system isn't holding pressure as it should, and an overflowing loft tank wastes water and can cause damp. Get the cause diagnosed rather than leaving it. If you ever smell gas, that is an emergency — call 0800 111 999.
That's a strong sign the expansion vessel has lost its air charge. With nowhere to absorb the expansion of heated water, the pressure spikes as the boiler fires up and pushes past the safety threshold, so the PRV releases. Re-charging or replacing the vessel is an engineer's job.
No. The pressure-relief discharge pipe is a deliberate safety outlet. Capping or blocking it removes your boiler's protection against dangerous over-pressure. Leave the pipe alone and have the underlying fault — high pressure, a failed expansion vessel or a worn valve — fixed properly.
Look at where the water comes from. A pressure-relief pipe is small (copper or plastic) and exits low down near the boiler; a feed-and-expansion overflow is a larger plastic pipe high up under the eaves. If your boiler has a pressure gauge it's a sealed system, so the discharge pipe is the likely culprit.
Many plans cover faults like a failed expansion vessel, a worn pressure relief valve or a stuck ball valve — the call-out, labour and parts may be included, subject to the policy's terms, excess and any exclusion period. Check your documents before paying out of pocket, and compare cover levels if you don't yet have a plan.
This article is general information, not advice, and reflects typical UK boilers in 2026. Any work involving the gas supply, the flue or the inside of your boiler must be carried out by a Gas Safe registered engineer. We compare a selected panel of providers, not the whole market, and may earn a commission.