Water dripping from a boiler pressure-relief pipe outside usually means the sealed heating system has discharged water or the safety valve is no longer sealing properly. The boiler may have become over-pressurised, the expansion vessel may not be controlling pressure correctly, the system may have been overfilled, or debris may be holding the valve open.
Do not cap the pipe, operate the safety valve or keep topping up the boiler. First confirm which pipe is dripping, record the pressure when the system is fully cold and again while heating, and arrange a Gas Safe registered engineer if the drip continues.
First make sure it really is the boiler pressure-relief pipe
A boiler can have more than one pipe leaving the property. The pressure-relief discharge is commonly a metal pipe that passes through an outside wall and turns towards the wall or ground. Its job is to carry water away safely if system pressure rises too far.
A condensing boiler also has a condensate pipe, which is usually plastic and may release water while the boiler is operating. Normal condensate is not the same as a leaking pressure-relief valve. A separate unvented hot-water cylinder can have its own safety discharge and a visible tundish, which must be assessed under the cylinder manufacturer’s instructions by a suitably qualified person.
If you are unsure which outlet is wet, photograph the pipe and show the image to an engineer. Do not trace it by removing boiler covers or opening boxed-in pipework.
What the dripping tells you
The pressure-relief valve, often shortened to PRV, is a safety component. It should remain closed during normal operation and open when pressure reaches its designed safety threshold. Water at the outside outlet therefore deserves investigation even when the boiler appears to work.
Worcester Bosch advises checking the outside pressure-relief pipe when a heating system repeatedly loses pressure. Baxi says that debris in the valve can allow it to drip, while expansion-vessel problems are another possible cause of low pressure after discharge.
Notice when the pipe drips
It drips only while the heating is hot
Record the pressure from fully cold through a normal heating cycle. If it rises sharply as the radiators warm and the outside pipe starts dripping, the system is not accommodating expansion normally. An engineer may need to assess the expansion vessel, filling arrangement and safety valve.
It drips continuously, even when cold
A valve may have failed to reseal or debris may be trapped on its seating. Worcester Bosch explains that using a PRV to drain a system can carry contamination through the valve and leave it dripping afterwards. Do not twist or operate the valve in an attempt to clean it.
It started after repressurising
The heating system may have been filled beyond the model’s cold-pressure range, a filling control may not have fully closed, or a pressure-control problem may already have been present. Check only that the normal user filling controls are in the closed position shown in the exact boiler manual. Do not force a stiff valve or use tools.
It started after servicing or heating work
Tell the company that carried out the work. The timing may be relevant, especially if the system was drained, refilled or components were changed. Do not assume the previous work caused the problem, but preserve photographs and pressure readings.
The drip has stopped but boiler pressure is now low
The system may have lost enough water through the discharge to leave a low cold reading. Repressurising once may restore operation if the exact manual describes the user procedure, but a recurring fall shows that the cause remains. Do not enter a cycle of filling and discharging.
Common causes an engineer may investigate
Expansion-vessel pressure or vessel failure
The expansion vessel provides space for heating water to expand. If it loses its air charge, its diaphragm fails or its connection is restricted, pressure can climb rapidly as the system heats. The PRV then opens to protect the system, after which the pressure may be low when everything cools.
Testing or recharging an internal expansion vessel involves boiler work. It is not a homeowner procedure. Do not remove the casing or press an internal valve.
A pressure-relief valve that will not reseal
Scale, corrosion products or magnetic debris can prevent a valve from closing cleanly after it has operated. The valve or its seat may also be damaged. Baxi advises that a dripping safety valve with low boiler pressure needs a Gas Safe registered engineer to clean or replace it.
Overfilling or a passing filling control
If too much water is added, the cold pressure starts too high and has less room to rise safely as the system heats. A filling control that does not close fully can also continue admitting mains water. Compare the cold reading with the exact manual and tell the engineer if pressure rises while the boiler is off and cold.
A system blockage or circulation problem
Restricted circulation can contribute to abnormal temperature and pressure behaviour. Useful clues include noisy operation, uneven radiator heating, rapid temperature changes or a boiler that stops while the outside pipe discharges. Do not change pump settings or add chemicals without a diagnosis.
The wrong pipe has been identified
Condensate, an unvented-cylinder discharge, an overflow and a boiler PRV pipe require different responses. A clear photograph of the outside outlet, boiler model and any nearby cylinder or tundish helps a professional identify the route safely.
Safe checks you can make
- Let the heating cool fully and photograph the pressure reading.
- Photograph the outside pipe and note whether the water is dripping, running or leaving only a stain.
- Run the heating normally only if there is no active leak near electrics, no unusual noise and pressure is within the model’s stated range.
- Record the highest pressure reached while the system warms.
- Check that accessible user filling controls are in the manual’s closed position.
- After the system cools, record the pressure again without topping it up repeatedly.
These observations do not diagnose the component, but they show whether the discharge is linked to heating expansion, continuous mains filling or a valve that drips regardless of temperature.
What not to do
- Do not cap, plug, clamp or redirect the discharge pipe.
- Do not operate the PRV as a test or use it to drain the system.
- Do not remove the boiler casing.
- Do not keep repressurising whenever the gauge falls.
- Do not stand beneath an active discharge or touch the water; it may be hot.
- Do not confuse the outlet with a cylinder safety discharge and follow boiler advice blindly.
The outlet is part of a safety route and must remain open. Gas Safe Register advises against DIY work on gas appliances.
When should you switch the boiler off?
Use the normal accessible user control to switch the boiler off, if it is safe to reach, when pressure rises rapidly, the pipe is running rather than occasionally dripping, water is close to electrical parts, the boiler is making heavy noises or the display shows a fault. Keep people away from the outlet.
Arrange prompt help if water could damage a neighbouring property, a public path or the building fabric. A continuing safety discharge is not routine condensate and should not be left indefinitely.
What information will help the engineer?
- the boiler make, model and approximate age;
- the fully cold and hottest pressure readings;
- whether the drip is continuous or only happens during heating;
- whether the system was recently filled, drained or serviced;
- how often you have needed to top up pressure;
- whether radiators have recently been bled; and
- photographs of the outlet, gauge and any staining.
That pattern helps distinguish overfilling, uncontrolled expansion, a passing filling control and a valve that cannot reseal.
If you rent the property
Report the dripping pipe and pressure readings to the landlord or managing agent promptly. GOV.UK says landlords are normally responsible for heating and hot-water systems. Include photographs and explain if heating or hot water has stopped or the discharge could affect another property.
Do not attempt internal repairs or repeatedly refill a system that is discharging. Give the landlord reasonable access for the engineer.
Frequently asked questions
Is a dripping pressure-relief pipe an emergency?
A slow drip is not always an immediate emergency, but it shows that the system has discharged or the valve is leaking. A running discharge, rapidly rising pressure, water near electrics or heavy boiler noise requires prompt action.
Can I stop the drip by turning the valve?
No. Operating the PRV can carry debris onto its seat or damage it. Leave the safety component alone and arrange inspection.
Why does boiler pressure keep dropping as well?
Water leaving through the discharge reduces the sealed system’s water volume. The low reading may therefore be the result of an earlier high-pressure event rather than a separate leak.
Could the expansion vessel be the cause?
Yes, particularly when pressure rises sharply while heating and falls very low after cooling. An engineer must test the vessel and the rest of the pressure-control system.
Is the dripping pipe just boiler condensate?
Possibly, if the wet outlet is the normal plastic condensate route. A metal PRV discharge and a cylinder safety outlet serve different purposes. Ask a professional to identify the pipe if you are unsure.
Related boiler-help guides
- Boiler pressure rises when the heating is on
- Why does my boiler keep losing pressure?
- How to tell a condensate pipe from a pressure-relief pipe
- Boiler pressure low after bleeding radiators
Is the outside discharge pipe still dripping?
Book a fixed-price boiler diagnosis before repeated discharge causes further pressure loss.