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Wet-to-Wet Differential Pressure Sensors: Unidirectional vs Bidirectional — What Every Hydronic Engineer Must Know

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 systems
2. Hot water systems
3. Condenser water loops
4. Bypass circuits
5. Pump monitoring
6. Coil ΔP measurement
7. Valve authority calculation

🟢 Unidirectional Wet-to-Wet DP Sensors - Measures pressure only in ONE direction
High port > Low port

If 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 coil
2. Across a heating coil
3. Across a strainer or filter
4. Across a pump (discharge to suction)

🟢 Bidirectional Wet-to-Wet DP Sensors - Measures pressure in BOTH directions
It 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 valves
2. Across bypass lines
3. Across decoupler lines in primary–secondary pumping
4. In district cooling systems where direction reversals are possible
5. 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 -> B
2. You need stable and inexpensive sensing
3. Typical application: coils, pumps, strainers

Choose Bidirectional when:
1. Flow direction may change
2. You have 3-way valves or bypass circuits
3. You operate a variable primary system
4. You want reliable data even under transient or low-load conditions

🟢Final Thought
Wet-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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