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HVAC Differential Pressure Switches
Filter alarms, fan monitoring and water-circuit switching for HVAC systems from Produal and Beck.
An HVAC differential pressure switch trips an alarm contact when the pressure difference across a filter, fan or water circuit crosses a set point, without needing power or a controller. This page covers the Produal PS and DPI-24 for AHU filters, fans and ducts, and the Beck 908 for water and heating circuits, adjustable from 40 to 1,000 mbar.
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HVAC Differential Pressure Switches Guide
What a differential pressure switch does in an AHU
Across a filter, a rising pressure drop means it's collecting dirt; across a fan, a missing pressure drop means it isn't moving air even if the motor runs; across a water circuit, a missing pressure drop means a pump has stopped or a strainer has blocked. Each is a safety or maintenance interlock: the switch contact starts an alarm or stops downstream equipment before damage occurs.
How to choose an HVAC differential pressure switch
- Medium: the Produal PS and DPI-24 are built for air and ducts; the Beck 908 is built for liquids, including chilled and heating water circuits.
- Switch type: the PS is a mechanical switch with field-selectable switching points, a low-cost option for simple filter or fan alarms. The DPI-24 is electronic, doubles as a multi-function transmitter, and includes a flow switch, on a 24 VAC/DC supply.
- Range: the Beck 908 is adjustable from 40 to 200 mbar or 150 to 1,000 mbar depending on order code; match the range so your alarm point sits mid-scale.
- Wetted parts: for water circuits, choose the diaphragm (EPDM, NBR, FKM or silicone) and brass G 1/8 connection to suit the medium and up to 80 °C.
- Signal type: choose a switch for a simple on/off alarm, or the DPI-24 when you also want the actual pressure value on the BMS.
For industrial air and gas duty beyond HVAC, or the wider Beck range, see pressure switches. For water circuits specifically, see the differential pressure switch for water.
FAQ
Common questions
How do I set the alarm point for an AHU filter?
Use the filter manufacturer's recommended final pressure drop as the trip point, or an agreed value between the clean and final drop if you want an earlier warning. The Produal PS has field-selectable switching points, so you can set it during commissioning against a manometer reading.
Switch or transmitter for filter monitoring?
A switch gives a simple pass or fail alarm at one set point. A transmitter, or the Produal DPI-24 which is both switch and transmitter, shows the actual pressure drop so you can trend it and plan filter changes before the alarm trips, rather than reacting to it.
Can I use the same differential pressure switch for air and water?
No. Use an air-rated switch such as the Produal PS or DPI-24 for ducts and filters, and the Beck 908 for water and heating circuits. The Beck 908's brass connections and diaphragm materials are chosen for liquid duty, not for duct-mounted air sensing.
What does a fan proving switch actually prove?
It confirms that a fan is moving air by sensing the pressure difference the fan itself creates, rather than just reading the motor's run signal. A seized belt or reversed rotation can leave a motor running with no airflow, which a differential pressure switch catches and a motor status contact does not.
What differential pressure range does the Beck 908 cover?
It's adjustable from 40 to 200 mbar on one order code or 150 to 1,000 mbar on another, for a maximum operating pressure of 10 bar and media from -10 to +80 °C. Tell us your expected pump or fan pressure drop and we'll confirm which range fits.
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