November 14, 2025
In modern HVAC & Building Automation Systems, wet-to-wet differential pressure (ΔP) sensors play a critical role in monitoring pump performance, coil health, valve authority, and overall hydronic balance.But one topic that confuses many engineers is this:Should you choose a Unidirectional sensor or a Bidirectional sensor?Let’s break it down in a simple, practical, BMS-friendly way 🟢 What Is a Wet-to-Wet DP Sensor?A wet-to-wet DP sensor measures the difference between two water pressures in a hydronic system.Both ports (High & Low) are water-submerged, making them suitable for:1. Chilled water systems2. Hot water systems3. Condenser water loops4. Bypass circuits5. Pump monitoring6. Coil ΔP measurement7. Valve authority calculation🟢 Unidirectional Wet-to-Wet DP Sensors - Measures pressure only in ONE directionHigh port > Low portIf the differential reverses, the sensor does not measure negative DP.Where to Use Perfect when flow direction is fixed and guaranteed, such as:1. Across a cooling coil2. Across a heating coil3. Across a strainer or filter4. Across a pump (discharge to suction)🟢 Bidirectional Wet-to-Wet DP Sensors - Measures pressure in BOTH directionsIt outputs positive or negative DP, depending on which side has higher pressure.Where to Use These are essential when pressure may flip due to system dynamics:1. Across 3-way valves2. Across bypass lines3. Across decoupler lines in primary–secondary pumping4. In district cooling systems where direction reversals are possible5. Across condenser water systems exposed to tower load variations🟢 How to Select the Right Type?Choose Unidirectional when:1. Flow is always from point A -> B2. You need stable and inexpensive sensing3. Typical application: coils, pumps, strainersChoose Bidirectional when:1. Flow direction may change2. You have 3-way valves or bypass circuits3. You operate a variable primary system4. You want reliable data even under transient or low-load conditions🟢Final ThoughtWet-to-wet sensors may look simple, but choosing the right directionality is a high-impact engineering decision.A small change in DP logic can completely transform how your hydronic system behaves—especially in today's variable flow, energy-optimized, sensor-driven HVAC world
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