I have a Rheem 20 instant. Since installation, when a cold tap is turned on it always runs for a few seconds and then turns off.

This is irritating and probably emits a puff of gas each time. With cold input appliances that do not use the hot water line at all now like the dishwasher and washing machine it will trigger frequently. I have minimised our gas consumption elsewhere so it's time to deal with this minor annoyance.

As I said it has always done this since installation. Is this a minor adjustment required to an existing input valve or do I need to install backflow prevention or non-return or something additional on the cold input?