I plan to install a water softener by breaking into a horizontal pipe run. The easiest installation will result in the highest point in the new pipework being about 400mm higher than this pipe. I know that airlocks are associated with low pressure systems but surely the height of the local high point also matters. Might I get an air lock with a 400mm high in a mains pressure system and if so, what kind of air vent would I need for mains pressure operation?