I had a cylinder in an airing cupboard that was bypassed and removed a few years ago when I had a combi fitted. There are 2 x 22mm pipes which are now joined together- i assume flow and return from boiler. But there is also a 15 mm pipe that is tee'd into the 22mm pipe - the 15 mm has a flamco vent on it. Does anyone know what the 15mm pipe is likely to be?