When a metal pipe near a boiler is leaking it 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.
To diagnose the fault follow this process.
Follow the pipe to where it comes from. Does it come from a hot water tank, boiler or central heating system.
If its a hot water tank and has a safety discharge pipe it will be classed as an unvented hot water tank. You need to be a qualified engineer to replace parts on these tanks so call an expert. The only job that can be done is reinstating the bubble on the top of the tank. If its the right kind of tank there will be instructions on a label attached to the tank. If you follow the instructions and the flow of water continues there may be a fault with the system or damaged parts. Either way you need a qualified engineer.
If its boiler or central heating system the process is the same. Turn off the central heating so the radiator 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 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: