Advances in Freeze Protection: Low-Temperature Cut-Outs for Unit Ventilators

March 13, 2007 By: Trygve Behny Sensors

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While unit ventilators are still the dependable choice for providing conditioned outside air in classrooms, unit ventilators and air handling units have changed, with better components and improved designs to meet changing building codes and air-quality standards. However, they still rely on the old-style gas-filled capillary tube for protection against coil freeze-up. Facility owners these days expect more—they want easier maintenance and lower lifecycle costs. This is why they may want to consider the type of low-temperature cut-out they are using.

This article compares two low-temperature cut-outs—the conventional gas-filled capillary and the new, solid-state combination sensor—and discusses their functionality, durability, and installation process.

The Freezestat
Freezestats go by a variety of names—thermostat, temperature switch, low-temperature cut-out control, low-temperature detection thermostat, thermal cut-out, and gas-filled capillary bulb and control. They have been used in unit ventilators for decades to sense coil temperature and prevent damage due to freezing.

A mechanical device, the freezestat consists of a diaphragm that moves as the pressure inside a capillary tube changes with temperature, closing a switch at a specific temperature to shut down the flow of outside air when in an alarm state. This capillary tube, filled with refrigerant gas, is constructed from small-diameter, thin-walled metal tubing that is susceptible to kinking.

Because of the use of refrigerant in the capillary tube, some shippers are requiring MSDS sheets when shipping freezestats. Additionally, some states have enacted restrictions on the use of freezestats based on environmental concerns.

The low-temperature cut-out is typically used in conjunction with an averaging temperature sensor for temperature control. This is either a continuous sensing element or a series of point sensors, packaged inside some bendable copper tubing that's typically 0.125 in. dia. with 0.030 in. wall thickness.

Combination Sensor

The solid-state combination sensor, as the name suggests, combines multiple low-temperature cut-out sensors to protect coils from freezing and an averaging temperature sensor for control. The sensor's relay and control circuitry are integrated into the sensing probe using an aluminum can housing crimped to the end of the unit. Newer versions use a handy utility box in place of the aluminum can housing.

The sensing probe contains IC temperature sensors every 16 in. along its length, allowing the device to detect cold spots, independent of the temperature throughout the remaining portion of the probe. The state of each sensing element is continuously monitored through an advanced processing algorithm embedded in a microcontroller. If the system detects an alarm condition, the device will change the state of the relay contacts and provide the user with a visual indication of its status. Figure 1 compares freezestat and combination sensor specifications.



Combination Sensor


Mechanical, snap-acting contacts



Not specified



35°F–45°F (1.6°C–7.2°C),

30°F–44°F (1.1°C–6.7°C),


Manual or Automatic

Manual or Automatic

Sensing element type

Vapor-charged bulb (low temperature cut-out only)

Two elements:
1. Low-temperature cut-out: IC sensors soldered to flexible cable
2. Averaging temperature sensor: resistance thermometer

Sensing element dimensions

0.125 in. dia. x 20 ft.

0.375 in. dia. x 8 ft.
Lengths up to 50 ft.

Temperature sensitive zone

12 in. or 16 in. segments of sensing bulb

1. IC sensor: 6 in. segment
2. Averaging sensor: 2–12.5 ft. segments

Visual display of status



Figure 1.

Simplified Installation
Successful operation always depends on proper installation. Because current capillary-style freezestats are more fragile and have more particular installation requirements, their installations are more demanding and labor intensive than the combination sensor's installation. A percentage of freezestats are damaged during installation and spares have to be swapped in before the system is checked out. Or, even worse, a sensor, damaged during installation, may slowly leak refrigerant and eventually fail without notice. Combination sensors were designed to simplify the installation and prevent problems associated with freezestat installation. Figure 2 lists conditions that commonly occur during installation and how they affect the freezestat and combination sensor.

Installation condition


Combination Sensor


Kinks or sharp bends cause poor flow in the sensing element
and limit its effectiveness.

Element is unaffected by kinks and bends.

Vertical mounting

Sensing element must be mounted horizontally across face
of the coil with vertical portions kept to a minimum. Vertically mounted
portions of the sensing element create undetected cold spots. When too
much of the element is mounted vertically improper operation results.

Orientation of the sensing element has no affect on its
function. It installs directly on coils horizontally, vertically, or

Thermostat case location

Thermostat operates ineffectively when its case is mounted
in the same ambient temperature as the sensing bulb. The thermostat diaphragm
must be warmer than the setpoint for effective operation.

Controls are integrated with sensing element and are
unaffected by ambient temperature.

Figure 2.

Near the freezestat, an averaging temperature sensor is usually installed to provide input to the controller. The thermostat control also requires separate mounting. The combination sensor combines a low-limit temperature cut-out and a continuous averaging temperature sensor, which can provide the thermostat
control in the same aluminum case.

Figure 3. Photograph of Minco's Chill-Out combination sensors

In Conclusion
The Chill-Out combination sensor (Figure 3) reduces
the cost of installation, eliminates the problems associated with freezestat
installations, and offers facility owners a lower total cost of ownership.
The integration of a low-temperature cut-out and an averaging sensor into a
single device cuts installation labor in half. The use of sensing technology
that functions regardless of kinking eliminates the problems caused by rough
handling during installation.

Trygve Behny, BSM, can be reached at Minco, Minneapolis, MN;

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