Check Valves. Check valves allow ﬂow in one direction and pre- vent ﬂow from reversing. Check valves are installed in hydronic systems to protect equipment that can be damaged by reverse ﬂow, and to prevent reverse ﬂow upon system shutdown (typically seen at pump discharge and steam traps). The three types of check valves commonly used in hydronic systems are swing, double disc, and silent.
Swing check valves use gravity to close a swinging disc. They can be installed in horizontal pipes or vertical pipes with upward ﬂow. They cannot be installed in pipes that ﬂow downward. They are most commonly speciﬁed in applications up to 2 in. (51 mm) pipe size. One disadvantage of swing check valves is that they close slowly and can “hammer” if ﬂow reverses.
Double disc check valves have a center post, two pivoting discs, and a spring coiled around the center post. Double disc check valves are commonly used in 2½ in. (64 mm) pipes and larger. They have the lowest pressure drop of the common check valve types, and close fast enough to prevent water hammer in nearly all applications.
Silent check valves are also known as center-guided check valves and are commonly used in applications in reverses. The main disadvantage of silent check valves is their high pressure drop.