Two-way valves located on the return side of coil piping will maintain pump discharge pressure on hydronic coils to enable positive air venting from the coil return header. From a functional perspective, it makes no difference on which side of the coil the two-way valve is located. The more common use of mixing valves over diverting valves is apparently the reason why two-way valves are traditionally placed on the return side of coils (where a mixing valve must go) rather than on the supply side (where a diverting valve would be), as shown in Figure 3-1. In most cases, where three-way valves are desired, they are arranged in the mixing configuration, but occasionally a diverting valve is required. Mixing valves are less expensive than diverting valves and thus are more common. Therefore, it is important that the valve be properly piped and tagged with respect to flow direction, and a mixing valve must not be used for diverting service, or vice versa. Notice that, like two-way valves, the plugs for both mixing and diverting valves are arranged to avoid water hammer (i.e., flow is under the valve seat). With some manufacturers, the valve may be designed so that the common port is the bottom port, with water exiting left and right.
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