A flow control valve is used (1) to prevent gravity circulation in a hydronic heating system when the circulator is not operating and (2) to permit the summer–winter operation of an indirect water heater. As shown in Figure 10-61, a flow control valve can be installed in either a vertical or horizontal position. Either location will require that it be installed with the arrow on the valve body facing the direction of flow.
When the circulator is operating, water passes through the flow control valve. When the circulator stops operating, the flow control valve remains closed and blocks the gravity flow of the water. A knob on top of the valve allows it to be manually opened to drain the system or to bypass it if there is a loss of electricity and only partial heating is possible. The knob is turned clockwise for the normal position and counterclockwise for the manual-bypass position.
Flow control valves do not require service or maintenance. A defective flow control valve should be replaced.