The freeze stat is a safety switch that is used in almost all commercial and industrial air-handling units, cabinet unit heaters, unit ventilators, and blower units that have chilled water and/or hot water coils.

This switch will open or close its contacts when the temperature across the water coil drops below a preset setpoint. That setpoint typically is 38?F.

The freeze stat may also have more than one set of contacts in itself. Those contacts do not always have to be normally open (NO). They can be normally closed (NC).

The freeze stat is easy to understand on paper, because it is nothing more than a switch that will open if the air moving across the water coil should drop below a given setpoint. But there are so many brands and types of freeze stats, it is important that you understand them and how they are wired into the circuit to function properly.


Let’s take a walk back in time and try to envision how the first freeze protection devices worked. Back when pneumatic controls were king, we used low-limit thermostats that passed the signal air pressure if the air across the water coil dropped below the adjustable setpoint. Notice, it was the signal air that passed; when that happened, the other pneumatic controls would all be affected.

How? All control systems are designed to fail and they always will. We don’t wish for it to happen, but it does. When the low-limit thermostat (freeze stat) passed air, because the discharge air across the coil was cold, the hot water valve would lose pressure, which would cause it to open the full flow of water to the coil.

Can you picture that happening? The low limit passed air and the hot water valve went full open. Add more to your picture, the cold water valve is going to close off flow to its coil and the outside air dampers and return air dampers are all going to lose their signal air, as well as the pneumatic relay located in the starter for the fan motor.

When this happened:

  • The outside air dampers would close off fully.
  • The return air dampers would open fully.
  • The supply and return fans would shut down.
  • All of this happened because when the controls were installed, they were done this way in case the system failed.

    These low-limit stats are still in use. In fact, most government facilities still have pneumatics, so they still have low-limit controls.

    You may be a bit confused because the low-limit sensor only has one pneumatic line to it. That pneumatic line is just “teed” into the signal line. That is all that is needed. We want it there to pass the signal air in case the mixed air or discharge air should become cold enough to possibly freeze the water coils.


    Fast forward to today’s controls and the use of electric freeze stats. We now have electrical contacts that close or open when the air temperature across the water coil should drop below an adjustable setpoint.

    A simple freeze stat can have only one set of contacts that are normally closed (NC). You wire this freeze stat into your safety circuit to shut the system down should the air temperature drop below the setpoint in the freeze stat.

    One very important thing to remember about all freeze stats is that their sensors are averaging sensors, not remote bulb. The sensor may look like a bulb-type sensor, but it is very important that you install all of the sensor into the airstream against the water coil. Why? Because it is an averaging sensor, which means that if you only put half of the sensor into the unit airstream and leave the other half out of the unit, all coiled up nice and neat, if there is a problem, your coil will freeze and the freeze stat will never know it. Only half of your sensor was in the coil or airstream, while the rest was outside the unit, nice and warm.

    Today, most freeze stats have more than one set of contacts, which can be either NO or NC. It is far cheaper to install one multi-contact freeze stat into your unit than to install several single-pole freeze stats because you need the contacts. With current control systems and the way they are wired, the freeze stat is an efficient safety control to install in any type of unit.

    Picture this: You have an air handler with hot and cold water coils.

  • The freeze stat would be installed in between the two coils, and it would be installed so that it stretches across the entire coil at least once, if not twice.
  • You need two sets of contacts for safety in this freeze stat. For the first set of contacts, you want NC and you are going to wire the safety circuit from the fire alarm NC circuit over to the starter for the fan motor in your air handler.
  • You are also installing a direct digital control (ddc) system, so you will need to use an NO set of contacts from the same freeze stat, and you will wire those contacts back to your controller’s input.
  • So far you have not installed any wires from the freeze stat to either of the control valves, hot water or cold water. These valves have been piped into the system so that if they should fail, the hot water valve will open fully to flow into the coil, and the cold water valve will close off flow to the coil. These valve actuators are being controlled from the same controller you wired your freeze stat into.
  • Now picture this: You are doing your start-up on this unit and its controls.

  • You physically trip the freeze stat. (By the way, all of the different types and brands can be tripped for testing purposes.)
  • Upon tripping the freeze stat, the NC contacts open, which will shut your fan down, just like you wanted it to. Also, the NO contacts close, which will complete the circuit back into your controller’s inputs.
  • At the time that your controller receives the input that the freeze stat tripped, it will stop sending its signal to your valve and damper actuators, which will have them all FAIL to their CLOSED positions.
  • The hot water valve will go fully open, the cold water valve will go fully closed, the outside air dampers will have closed off completely, the return air dampers will have failed fully open, and the supply fan will have shut down.
  • One little touch of the reset button on your freeze stat, and everything returns to normal. The test is complete.
  • One final note: The freeze stat may have more than two sets of contacts — they can have several — and like a relay, those contacts could either be NO or NC. So, always remember to truly understand how the freeze stat you are about to use works.

    Contributed by John Williams Jr., CM. Williams is a controls technician for Carrier Corp. He can be reached at johnwilliamsjr1@aol.com (e-mail).

    Publication date: 07/23/2001