Guide · Boiler problems
A steady drip from the small pipe that exits your outside wall near the boiler is the system telling you something. Here's what that pipe is for, why it drips, and what to do about it.
The pipe — properly called the pressure relief discharge pipe — is a safety outlet. Inside the boiler is a pressure relief valve that opens if the pressure in your heating system climbs too high, letting water escape safely outside rather than stressing the boiler and pipework. If that pipe is dripping, either the valve has opened to do its job, or it has started to let water past when it shouldn't.
Water expands as it heats up, and in a sealed heating system that expansion has to go somewhere. The expansion vessel inside the boiler absorbs it in normal operation. If the vessel fails or the system is overfilled, pressure keeps rising — and at a set point the relief valve opens and dumps water through the discharge pipe. It's the last line of defence against over-pressure, so a dripping pipe should never be blocked off or ignored.
Look at the pressure gauge on the boiler. If it reads high — above roughly 2.5 to 3 bar on most systems — the valve has opened because the pressure genuinely is too high, and the drip is the symptom rather than the fault. Our boiler pressure too high guide explains why pressure climbs and what to check. If the gauge reads normal but the pipe still drips, the valve itself is usually worn or has debris stuck in it.
The most frequent story is this: the expansion vessel loses its air charge over the years, so each time the heating runs the pressure spikes, the relief valve opens, and a little water escapes. Each opening can let debris lodge in the valve seat, so it then weeps constantly, pressure falls, and you top the system up — which starts the cycle again.
Other causes include a filling loop left slightly open or passing, which overfills the system continuously, and simply a relief valve that has worn out. All of these are engineer jobs, but they're routine ones.
On its own, no — the system is doing what it's designed to do. But don't ignore it: a relief valve that's passing will slowly lose pressure, and the underlying cause (usually the expansion vessel) won't fix itself. If you also see water inside the boiler casing, or the pressure is climbing fast, turn the boiler off and call. Our boiler leaking water guide helps you tell an outside drip from an internal leak.
Older back boilers behind gas fires work differently — they're usually on open-vented systems with a feed and expansion tank in the loft. A drip from an overflow pipe on one of those systems often means the tank's float valve is sticking and overfilling. Either way, the principle is the same: an overflow that drips is reporting a fault elsewhere.
A dripping discharge pipe is a quick diagnosis for an engineer: check the vessel, check the valve, check the filling loop. See our boiler repairs page, or send the quote form with your boiler make and model and a note of what the pressure gauge reads.
Common questions
Clear answers before you arrange a visit.
Usually the pressure relief valve has opened because system pressure is too high — often due to a failed expansion vessel — or the valve itself is worn and letting water past.
A valve that has been opened once can trap debris and weep constantly, even at normal pressure. A filling loop left slightly open can also keep overfilling the system.
Never. It's a safety outlet for excess pressure. Blocking it removes the system's protection against over-pressure.
The usual fixes — recharging or replacing an expansion vessel, or fitting a new relief valve — are routine repair jobs. The exact cost depends on your boiler, so a proper look comes first.
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