Differential Pressure Switch Lockout Relay

Today’s Tech: Differential Pressure Switch Lockout Relay

Problem: Recently I had a request for a Differential Pressure Switch Lockout Relay. The customer wanted the HVAC unit to stay off once the Differential Pressure Switch (DPS) measured above the acceptable limit across an air filter. He wanted a Manual Reset via a momentary pushbutton. However, the customer also wanted override capability of the DPS so that he could test the system periodically.

Solution: I suggested the following (see drawing below):

  1. Use the Normally-Open contact of an RIBU1S in parallel with the DPS contact.
  2. Make sure the DPS contact is rated for the RIBU1S coil voltage and current. In this case, the RIBU1S draws 46mA at 24Vac, so make sure the DPS contact is rated for more than that.
  3. Make sure the Manual Reset pushbutton is also rated for the RIBU1S coil voltage and current.
  4. Use the Normally-Open contact of an RIBU1S in parallel with the DPS contact.
  5. Make sure the DPS contact is rated for the RIBU1S coil voltage and current. In this case, the RIBU1S draws 46mA at 24Vac, so make sure the DPS contact is rated for more than that.
  6. Make sure the Manual Reset pushbutton is also rated for the RIBU1S coil voltage and current.

When you look at the table below the diagram, you can see that under normal conditions (“Time Zero” or t=0), the DPS contact is open, the relay contact is open and the HVAC unit is on. At the next time interval t=1, everything changed because the pressure between either side of the air filter was above the set limit. Because it went above the limit, the DPS contact closed, which energized the RIBU1S coil. Because the RIBU1S coil energized, the RIBU1S contact closed. Because the RIBU1S contact closed, the RIBU1S coil will remain energized (it effectively latched itself on.)

Now the HVAC unit is off, and it will remain off until the Manual Reset button is pressed. Notice how at t=2 the unit is still off even though the air filter was replaced and the DPS contact opened back up. At this point, even though the DPS contact is open, the relay has still locked out the HVAC unit because it was not unlatched by the Manual Reset button. Notice now, that at t=3 the Manual Reset has been opened and everything fell back to the normal condition status (t=0) because the voltage was removed from the RIBU1S coil. The cycle will repeat as soon as the air pressure goes above the set limit again.

What is an alternate solution?

Another concept could include ignoring the lockout function and instead using the RIBU1S contact to control a light, a buzzer or even an RIBW24B-EN3 (an EnOcean® enabled wireless relay). Then you could see, hear or monitor when it is time to change your air filter.

About the Author

David Mackey is an Engineer for Functional Devices. He is a husband of one, a father of three, and a servant of Christ Jesus. David graduated from Purdue University with an Electrical Engineering Technology degree.

He enjoys working with his hands on things like woodworking, cars and, of course, electronics. He is 5' 13" and wears glasses, unless he wears contacts. He is also a self-made hundredaire.