Abstract: This invention relates to self-priming devices for indirect domestic water heating systems which device acts as expansion and contraction chambers for primary and secondary water in the system. The device consists of a closed housing divided internally by a wall intermediate its ends to form axially disposed first and second chambers. A open ended tubular member extends from the wall into the second chamber to provide communication between the chambers and an inlet and outlet is provided in the second chamber adjacent the wall. At least one inlet is provided in the first chamber at the end remote from the wall. The first chamber is connected to a primary water circuit and the second chamber is connected to the secondary water circuit to separate the water in one circuit from the water in the other circuit. The volume of the first chamber is substantially 4 to 4.