New and existing sprinkler systems shall be maintained for sprinkler use only, and connections to such systems for any other purpose shall be prohibited, except that a house service water-supply connection may be tapped from the sprinkler water-supply connection to the water main, provided the house service tap is not more than one and one-fourth inches for a four-inch sprinkler connection to the main, and not more than two inches for a six-inch or larger sprinkler connection to the main. Where a connection for house service water supply is made to the sprinkler water supply such connection shall be made on the supply side of the check valve in the service main.