I live in the ground floor flat of a converted Victorian House and have recently come across a rogue pipe that shoots water out sporadically into an external drain kind of like a toilet flush swoosh sound, but the water is clean. I have no idea where this is from or what it could be - Can anyone shed some light? It is a white plastic pipe that comes from under the house (there is a basement, but it is tanked). I initially thought that it was noise from an upstairs toilet of a flat above, but saw it pump out with my own eyes today ( and subsequently make the drain at the side of the house overflow!) I hope someone can help - thanks
If this pipe is just above ground level and is spewing clean water out onto the ground (or is otherwise unchecked), it is more than likely coming from a sump. This sump could be from a drain tile or the A/C drain. If you have access to the basement, this can easily be determined by tracing the pipe back from where it goes through the wall/structure, unless it is all behind sheetrock.
(NOTE: I didn't see Tester101 and Fiasco Labs comments prior to writing this. I'm basically agreeing with them.)
Recently, plumbing codes have begun to require "trap primers" or an extra deep trap for floor drains to keep their traps full of water.
I would propose that is what you have here. To keep the external drain's trap filled with water the trap primer is occasionally pumping water into the drain.
More common is a primer that is connected to the potable water supply and activates when pressure fluctuations are sensed, such as the flushing of a nearby water closet.