They work more by water flow/pressure against them than by gravity. Any water flowing in the wrong direction will put pressure against the flapper, closing it. Even if the flapper is hanging down. Many of them also contain springs to help close the flapper in case of water flow the wrong direction.dmopar74;3767671; said:they have a flapper in them, when they are pointed down the flapper is held open by gravity. when they are pointed up the flapper is closed by gravity and water pressure opens it.
now if it was mounted horizontal with the hinge of the flapper on the top in theory it would work.