We fitted a power shower using the existing mixer shower pipes. The shower is great, but if anybody turns on the hot water elsewhere in the house while the shower is in use, it causes an air lock in the whole hot water system. This is easily fixed by blowing through with the cold water using the washing machine pipes. I think that when the pressure drops air is being sucked in through the vent pipe off the hot water tank, would a non-return valve fitted onto this pipe stop this problem?