Pipe dripping from boiler
What is the metal pipe?
This is generally called the safety discharge pipe which is attached to a component called the pressure relief valve in the boiler or hot water tank. Its job is to release any excess pressure in the central heating system, boiler or hot water tank.
Why is the metal overflow pipe dripping?
If the boiler pressure rises too high the excess water will leak out through the metal pipe.
Metal pipe from the boiler leaking all the time
The metal pipe should only leak if the pressure in the system is above 3bar. If the pressure gauge is reading less than 3bar and the pipe is still constantly dripping you will need a replacement pressure relief valve.
Turn off the central heating so the radiators are cool or cold. Then check the pressure gauge. When cold it should read between 1 and 1.5 bar.
If the pressure is above 1.5 bar when cold try to reduce the pressure by bleeding a radiator.
Metal pipe from the boiler leaking only when the heating is on
If the metal pipe only leaks when the heating is on mean the boiler is not dealing with the water expanding in the system properly. There are two main reasons:
Faulty expansion vessel
The expansion vessel keeps the pressure steady in the system. As the water heats up and expands the extra volume moves into the vessel keeping the system pressure steady.
From time to time the air bubble inside the vessel has to be recharged. This should be done as part of the service and for most boilers isn’t a DIY job.
Blocked expansion vessel pipe
The pipe to some expansion vessels can block up giving the appearance of a faulty expansion vessel.
This is not a DIY job to fix. To diagnose this fault empty all the pressure out of the expansion vessel then set the system pressure to 1bar.
Now re-inflate the expansion vessel. The pressure in the system should rise as you pump up the vessel.
If the system pressure stays the same as you pump up the vessel the pipe is blocked.
Pressure set too high when cold
When the radiators are cold the pressure should be set somewhere between 1 and 1.5 bar. If it is set higher as the radiators heat up the pressure will get too high causing the safety valve to open.
What to do next
If the pressure is above 3bar when the system is cold there is pressure from the cold water mains getting into the system when it shouldn’t be. See the following pages:
filling loop passing when closed
fixmyplumbing 
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