Ask Rick: How does a freezestat work?

Q: How does a freezestat work?

A: You should use a freezestat to protect your water coils from freezing. Rooftop units with water coils should always have freezestats, even if using a water/glycol mixture.

Typically the heating coil would be upstream of the cooling coil. Usually the freezestat is attached on the leaving side of the heating coil. The freezestat won’t trip when cold air hits the Hot Water coil unless the coil isn’t generating enough heat to maintain Leaving Air Temperature (LAT) above the freezestat setpoint.

Typically freezestats are made of 1/8”OD flexible copper tubing which is filled with a temperature sensitive gas vapor (as temperature drops, gas vapor pressure drops). The tubing is connected to a bellows cup and a control box. There is usually a selectable temperature set point on the control box (35F is typical). The freezestat will “trip” if any section of the temperature sensor element drops below the setpoint. The sensor element can be various lengths, often up to 20ft long,, and will trip if any 12” to 18” length goes below the setpoint.

When a freezestat trips, the main contact opens and a button pops out on the control box. An auxiliary contact closes at the same time. Freezestat trips are usually hard-wired into the air-handling unit controls to stop the unit fans and open the Hot Water valve to full open or some minimum % open. Freezestats are designed to be reset manually by someone physically pushing the button back in on the control box.

Annexair uses Johnson Controls A70HA-1C freezestats


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