Technology Talk: Differential Pressure Switches vs. Current Switches


Differential pressure switches are the traditional way to monitor a fan status on fan coil units but did you know you can also use current switches?  

Current switches provide the most reliable and cost-effective way to monitor positive proof of flow for a fan motor in fan coil units (FCUs).

current switch operation graphAccording to the Theory of Current Sensor Operation, in a constant volume fan motor, about 60% of the energy consumed is used to move the air, leaving 40% or less of the energy consumed to turn the motor. When the fan motor has a mechanical failure or belt breakage the motor amperage drops roughly 60%.

Current switches monitor fan motor status by reading the amperage in the conductor supplying the motor. A mechanical failure in the motor causes a drop in amperage that is instantly detected by the current switch, which sends an alert to the control system. This allows for a cost-effective and reliable way to monitor positive proof of flow. Below you will find a more in-depth review of the traditional approach and the more modern choice the current switch.

The Traditional Approach: Differential Pressure Switch Technology

Differential pressure (DP) switches monitor fan motor status by sensing the difference in pressure between the supply and discharge sides of the fan. This is accomplished by penetrating the duct work and installing pick-up tubes on both sides of the fan. The tubing is connected to a mechanical switch that has a single adjustable trip point. When a change in the differential pressure between the pickup tubes is detected, the switch opens or closes accordingly.

fan coil unit diagram

The technology is effective, but it has drawbacks. DP switches are mechanical, and, therefore, require routine service. Clogged tubing and pitted or corroded contacts make the switch unable to respond to an alarm condition. Additionally, the pick-up tubes are external and can be subject to damage during installation of the FCU. Finally, standard industry DP switches do not always have sufficient sensitivity to monitor systems with low airflows, such as those found in fan coil units.

Current Switches for Fan Coil Units: Application Advantage

Fan coil units put out a much lower air volume than packaged air handling units, making DP switches undesirable due to low accuracy at low airflow rates. So rather than measure flow using differential pressure, an installer can monitor the amperage draw on the motor to determine its operational status.

H808 wiring example

Current switches have a low turn-on current and can detect a motor’s amperage down to 0.15 A (on select models), making them ideal for the low air flow of fan coil units. Multiple wraps of the conductor through the current switch center window allows them to monitor conductors with even lower amperages.

Current switches are 100% solid state, with no mechanical parts to fail or to require service beyond a one-time calibration at the time of installation. A self-calibrating feature (on select models) reduces the setup time even further and ensures reliable and accurate readings. Current switches are installed on the conductors inside the FCU, so they are protected from damage during FCU installation.


Interested? Check out the H808 current switch by Veris. This switch offers high performance, with a wide array of amperage range options. This product can accurately detect belt loss, coupling shear, or other mechanical failure on loads from 1/5 to 100HP.



  • Detecting belt loss, coupling shear, and mechanical failure
  • Verifying lighting circuit and other electrical service run times
  • Monitoring status of industrial process equipment
  • Monitoring status of critical motors (compressor, fuel, etc.)

High-performance devices in solid-core housings

  • Small size…fits easily inside small enclosures
  • Status LEDs for easy setup and local indication
  • 1 Amp status output for increased application flexibility
  • 100% solid state and polarity insensitive, with a 5-year warranty

Click here for more information on the H808 Current Switch 

Interested in Veris’s H808 Current Switch? Contact Tower Equipment Professionals today for pricing, information, or with any questions you may have at:



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