What is a freeze-stat and where should it be placed?

A freeze-stat is a safety sensor that detects low air temperatures downstream of a heating coil in an air handler — typically set to trip at 35–38°F (1.7–3.3°C) — and shut down the supply fan, close outdoor air dampers, and open the heating valve fully to prevent coil freeze damage. The sensing element should be a capillary tube type that covers the entire face area of the coil, mounted 2–4 inches downstream where air is well-mixed, because localized cold spots from stratification can rupture a coil even when average temperatures are safe. Per IMC and ASHRAE 90.1, freeze protection is mandatory on water coils exposed to outdoor air in climates where entering air temperatures can fall below 32°F (0°C). Spring-return actuators on the outdoor air dampers and normally-open valve configurations for heating coils are also required components of the freeze protection strategy.