Balancing Valve
Balancing valves are used in a hot-water (hydronic) heating system to equalize the pressure drop in multiple piping circuits. The valves are installed on the return side of each circuit. Balancing valves are illustrated and described in Chapter 10, “Steam and Hot-Water Line Controls” in Volume 2.
Backflow Preventer
Sometimes boiler back-siphonage and back pressure will cause the boiler water to mix with and contaminate the domestic water sup- ply. This unwanted mixing of the two water supplies can be pre- vented by installing a backflow preventer. The Bell & Gossett backflow preventer illustrated in Figures 15-69 and 15-70 consists of two independently operated check valves and an intermediate atmospheric air vent contained in the same housing.
When a back-siphonage condition occurs, the atmospheric vent opens to allow air to enter and break the siphon. Leakage is vented
hands or arrows. One hand (usually black) is stationary and is per- manently set when the boiler is filled. The movable hand (usually red or white) is initially set in the same position as the stationary hand after the boiler has been filled with water. Its position will change as the water level in the boiler changes. This movable hand indicates the true level of the water. Efficient operation is being pro- vided as long as the movable hand is directly above the stationary one. In closed hot-water heating systems, automatic valves are used to control boiler water level (see Pressure Relief Valves and Pressure-Reducing Valves in this chapter).