The airflow switch (AFS) has many uses and is a control that can either be a safety switch or an operating control. For all intents and purposes, it’s nothing more than a switch, just like one on your wall at home to turn on a light. This switch will turn on (make contact) or turn off (break contact) automatically.

The AFS operates off of one of three pressure points: negative pressure (low), positive pressure (high), or differential pressure.


An AFS works off the principle of pressure — air pressure — and is measured in inches of water column (in. wc). Sometimes that pressure is referred to as static pressure.

It’s this pressure or difference in pressure that will make or break the contacts of the switch inside the body of the AFS. The switching side (contact of the switch) can have one (or more than one) set of contacts. These contacts are normally closed (NC) or normally open (NO). The normal position of these contacts occurs when there is no pressure applied or measured from the AFS. Once pressure is applied or measured, then the NO contacts will close and the NC contacts will open.

How Do These Contacts Make and Break On Pressure?

Picture a simple balloon. It’s flat as it sits in your hands; now begin to blow into the balloon. As you blow into the balloon, it will expand. You are adding positive pressure to the balloon. In an AFS, there is a balloon (better known as a diaphragm) that will do the same thing as pressure is applied to it. As the diaphragm expands it will push the contact settings of the switch so that those contacts that were NO are now closed and those that were NC are now open.

In the AFS, the reverse can occur if you were to create negative pressure out of the balloon. This is the case with the original use of the AFS on burners — not the atmospheric type, like in residential boilers, but in the industrial end of our trade.

The AFS was attached physically to the side of the blower wheel assembly or the negative (low) pressure side (the side where the air goes into the blower wheel). Each time the burner was enabled (turned on) the blower wheel started up and its inlet dampers would open full (also known as prepurge), at which time the AFS diaphragm would be pulled inward which would close its contacts. So that means that the contacts were NO. In this case, the AFS was acting as a safety switch. It would prove to the burners controller that the blower wheel and its motor were indeed running and that the dampers did open as required for proper ignition of the burner. If the dampers had never opened, then the AFS would have never closed and the burner would have never come out of prepurge and would have never tried to fire.

Where the Airflow Switch Is Used

  • The AFS can be used as a differential pressure switch to prove if the supply fan motor or return fan motors are running on an air handler.
  • The same type of AFS may also be used to determine the status of the filters in an air handler. As a fan proving switch, the low side would be piped into the inlet side of the blower wheel and the high side would be piped into the outlet side of the same blower wheel. Then you would calibrate the AFS to make contact when there is a difference in pressure greater than one inch (1 in.) of static pressure (in. wc). This would prove that the blower wheel is spinning.
  • Understand that this is not always a dependable way to prove that a motor is actually running because a blower wheel can spin naturally, without its motor running. This is why it is very important that you calibrate your AFS to make at a high enough pressure, but not too high, so that the pressure difference would require the motor to be actually spinning the blower wheel, to make the AFS.

    Above we mentioned a setting of 1 in. static pressure. That pressure may vary and is usually set by the engineer. On the filter side of the unit, you would pipe the low side of the AFS to the inlet side of the filters, and face the tube end into the flow of air. Then you would pipe the high side of the AFS to the outlet side of the filters and face the tube away from the flow of air, or against the filter. Again, you would calibrate your AFS to make at a certain difference in pressure across the filters.


    To calibrate an adjustable AFS, you will need a manometer to measure static pressure, a digital ohmmeter, a pressure pump (hand pump), and some pneumatic tubing. The AFS that you are about to calibrate will have a knob located on its front body, below the switching terminals, that you will adjust (spin in or out) to complete your calibration of the switch. Let’s say you want to calibrate the AFS to make at 1 in. wc.

    You would:

    1) Put your hand pump onto the high side of the AFS and have the manometer piped in parallel with the two.

    2) Hook up the ohmmeter to the common terminal and the NO terminal of the AFS and have the ohmmeter on continuity test. With the pump tight you apply pressure to the AFS by squeezing the pump (squeeze the pump gently). As the pressure begins to rise in the AFS, you will be able to measure it from the manometer.

    3) You pump up the AFS until the NO contacts close, which will cause the ohmmeter to ring proving contact (use your hearing). At that point you look at the manometer to see at what pressure those contacts closed (use your sight).

    If it were greater than 1 in. wc, then you would adjust the AFS outward to be more sensitive. If it were less than 1 in. wc, you would adjust the AFS inward to be less sensitive. Once you have the AFS set, pull your hoses, then reattach and do it again. Why? Because the bellows (diaphragm) of the AFS can and will hold air as you pump it up and relieve it, which can cause you to get false readings after a while. So it is good to relieve it and start over before you call it “calibrated.”

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


    Examples of the many names for the airflow switch:
  • Differential pressure switch (DPS)
  • Differential flow switch (DFS)
  • Negative pressure switch (NPS)
  • Positive pressure switch (PPS)
  • Filter status switch (FSS)
  • Examples of the many uses for the airflow switch
  • Proof of slow switch for air handlers;
  • Filter status;
  • Proof of damper switch for emergency generators;
  • Proof of draft switch for combustion air dampers, supply air dampers, return fans, exhaust fans, fire dampers and doors; and
  • High static alarm switch.
  • Publication date: 06/18/2001