Pneumatic Thermostats come in low-volume (one-pipe and/or restricted air) and more typically, high-volume versions. (two-pipe, free air, regulated to a working pressure of 15 to 25 pounds and unrestricted).
A three-pipe stat is a high-volume thermostat with main and branch connections and is designed for day/night applications. Accordingly it also has an “indexing switch.” The additional 3rd pipe simply mirrors the supply pressures for day and night-time operations and the 3rd pipe terminates at the other end into a pneumatic electric cross-over device, typically then, a P/E, the cut-in and/or cut-out will correspond to a pressure that will restore a day operation at night time such as starting a stopping/starting a fan in a unit ventilator or fan-coil etc. Purely in night mode, the same units may have radiation ability as opposed to full operation. (with fan).
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