Differential Pressure Switches & Transmitters for HVAC Explained
Where a flow switch answers “is fluid moving,” a differential pressure device answers a related but distinct question: “what’s the pressure difference across two points, and has it crossed a threshold that matters?” That measurement drives filter alarms, fan status confirmation, and pump protection throughout an HVAC system. Jaideo Automation supplies the GLPS-D Differential Pressure Switch, DPSN Series Air Pressure Switch, DDCM Differential Pressure Switches, and Water Differential Pressure Switch.
How a Differential Pressure Switch Works
A differential pressure switch measures the pressure difference between two connection points and trips an electrical contact once that difference crosses a set threshold — either rising above or falling below it, depending on the application. Unlike a flow switch, which detects flow directly, a DP switch infers system status indirectly through the pressure signature that flow or blockage creates.
Common HVAC Applications
- Filter status monitoring — pressure drop across a filter increases as it loads with dust, and a DP switch alarms once the filter needs replacement
- Fan proving — confirms a fan is actually running by detecting the pressure differential it creates, rather than just trusting the motor start signal
- Pump protection — similar principle applied to pump discharge/suction pressure differential
- Duct and air system monitoring — detecting blockages or dampers in the wrong position through unexpected pressure readings
Switch vs Transmitter: What’s the Difference?
A differential pressure switch gives a simple on/off contact signal at a fixed setpoint. A differential pressure transmitter instead outputs a continuous, proportional signal representing the actual pressure value across its range — useful where a control system needs to know not just whether a threshold was crossed, but the actual current reading, for modulating control or trending.
Air, Water, and Vacuum Applications
DP switches and transmitters are specified differently depending on the media — air-rated switches for duct and filter monitoring, water-rated switches for hydronic system monitoring, and vacuum-rated switches for negative-pressure applications like extraction systems.
Differential vs Absolute/Gauge Pressure Sensors
Not every pressure measurement in HVAC compares two points. A gauge pressure sensor measures pressure relative to atmospheric pressure at a single point — used for things like confirming refrigerant charge pressure or monitoring hydronic system static pressure at a specific location, rather than the pressure difference across a filter or component. An absolute pressure sensor references a fixed vacuum rather than atmosphere, occasionally used in specialized applications like altitude compensation. In practice, most HVAC monitoring and control points call for differential measurement — the comparison between two points is what indicates filter loading, fan operation, or flow status — but gauge sensors have their place wherever a single-point reading against atmosphere is what’s actually needed, such as verifying static system pressure hasn’t dropped due to a leak.
Where This Fits
DP switches often work alongside flow switches for complete system verification — see our Paddle Water Flow Switches guide. For the full sensor and switch selection picture, see our HVAC Sensor & Flow Switch Selection Guide.
Frequently Asked Questions
What is a differential pressure switch used for in HVAC?
Differential pressure switches are commonly used for filter status monitoring, fan proving, and pump protection by detecting when the pressure difference across two points crosses a set threshold.
What is the difference between a DP switch and a DP transmitter?
A DP switch gives a simple on/off signal at a fixed setpoint, while a DP transmitter outputs a continuous, proportional signal representing the actual pressure value, useful for modulating control or trending.
How does a differential pressure switch detect a dirty filter?
As a filter loads with dust, the pressure drop across it increases; a DP switch monitors this differential and trips an alarm once the pressure drop crosses the threshold indicating the filter needs replacement.