Abstract: A ball valve for use in isolating a hydronic circulator in a fluid heating system includes a ball pivotally mounted within a central passage through a housing. When the valve is open, a bore through the ball is aligned with the central passage. On one side of the ball, a pivot pin extends into a socket in the housing and on the other side, a stem connects with the ball for turning the ball into a closed position to block the central passage. A clip with a U-shaped notch interfits with a groove in the stem inside of the housing and acts between the passage and groove walls to keep the stem from being forced out of the housing by fluid pressure. Sealing the ball within the housing is a unitary member including both a retaining plate and a sealing ring which is sandwiched on opposite sides of the plate to provide sealing contact against the ball as well as between the plate and the housing, and externally of the housing for sealing between the plate and the circulator.