Combis and some other boilers should have a METAL Pressure Relief Pipe going out through the wall. These are sometimes WRONGLY referred to as the boiler’s “Vent” or “Overflow”. It is usually turned back to the wall or taken to a low level.
It comes from the Pressure Relief Valve on the boiler which usually have a red top marked “3 bar”.
It can be leaking water because:
1) The PRV has been opened, perhaps to let water out of the system. Once these valves have let water out they tend to keep leaking because dirt gets stuck on the valve seat. If you twist the valve it will open and “clunk” shut. That may help, or it may make the leak much worse.
2) The system pressure has gone above 3 bar and the valve is doing its job. The pressure can go too high because of a lack of expansion space in the boiler’s pressure vessel, which needs attention. See BOILER PRESSURE PROBLEMS. See faq 31
3) It can also happen due to a boiler fault such as a failed pump.
4) Water coming out of a PLASTIC pipe is probably condensate - from a condensing boiler and normal. It must go to a drain and not be allowed to dribble on the ground!
ANY Pressure Relief Valve which has let water out is likely to need replacing!
PRVs in boilers nearly always have red tops, old Vaillants’ are grey.