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

Why a Freeze-Stat Is Not Just Another Temperature Sensor

A freeze-stat is fundamentally different from a standard duct temperature sensor. Where a point sensor measures temperature at one location, a capillary-type freeze-stat uses a vapour-filled sensing element — typically 3–9 metres of copper capillary tube — that responds to the coldest point anywhere along its length. This is critical because coil freeze damage is a localised phenomenon: a small pocket of sub-freezing air from a leaking outdoor air damper can rupture one section of a coil even when the bulk air temperature downstream is well above 0°C.

### Correct Placement

The freeze-stat capillary must be distributed across the entire face area of the heating coil, mounted 50–100 mm (2–4 inches) downstream where the air is nominally mixed. A serpentine pattern is standard — horizontal runs across the width of the coil, spaced at 150–200 mm vertically, to ensure no cold spot goes undetected. The sensing element must be positioned to cover areas most vulnerable to stratification: the bottom of the coil (where cold outdoor air falls after entering through the dampers) and the coil edges near the AHU casing (where bypass air can short-circuit the heating section).

### Trip Setpoint and Control Sequence

The freeze-stat is typically set to trip at 1.7–3.3°C (35–38°F) — just above freezing with a margin for sensor tolerance. Upon trip, the control sequence must:

1. **Stop the supply fan immediately** — continued airflow over a frozen coil with no heating will accelerate freeze damage.

2. **Fully close the outdoor air dampers** — cutting off the source of freezing air. Spring-return actuators are mandatory here; the dampers must close even on power failure. 3. **Fully open the heating coil valve** — the spring-return actuator should drive to the fully open position, flooding the coil with hot water to thaw any ice formation. 4. **Annunciate a critical alarm** — the BMS must alert operators immediately; freeze-stat trips are never nuisance alarms.

### Code Requirements

Per the IMC and ASHRAE 90.1, freeze protection is mandatory on all water coils exposed to outdoor air in climates where the winter design temperature falls below 0°C. In Australian terms, this covers all climate zones south of approximately Brisbane — essentially any location with a heating design day below 2°C. The Australian Standard AS 1668.1 (mechanical ventilation) references similar requirements for air-handling systems serving occupied spaces.

### Testing and Maintenance

Freeze-stats must be functionally tested at least annually before the heating season. The test involves spraying a refrigerant or cold pack on a section of the capillary to verify that the controller trips, the fan stops, the dampers close, and the heating valve opens. A freeze-stat that fails to trip during testing is a critical deficiency — replace it immediately.