Go with the one under the sink first, see how you go after all this is a process of elimination, not a garaunteed fix. Depending on the brand of the aav it will have various ways to connect them, and usually have a bush or an adaptor to suit different pipe sizes. When you buy it, and your other fittings, open it up and see what you need to connect it to the pipe. The bloke at your supply shop should be able to help, maybe, pehaps.....................
