Flow Switch (Flow Status Sensor)
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The Paddle in the River
Imagine a small paddle placed in a river. When water flows — the paddle moves. When water stops — paddle returns to rest. That movement closes or opens an electrical contact.
A flow switch works exactly this way — a simple, reliable device that tells the BMS whether fluid is flowing or not. Two states only: flowing or not flowing.
Flow Switch Applications in BMS:
CHILLED WATER PUMP PROTECTION:
Flow switch in chilled water pipe
Chiller must not operate without water flow (coil damage)
No flow → DI = 0 → BMS stops chiller immediately
CONDENSER WATER FLOW CONFIRMATION:
Flow switch confirms cooling tower water is circulating
No flow → cooling tower fans stopped → chiller protected
AHU COIL FREEZE PROTECTION:
Flow switch in chilled water return
No flow in winter → pipe could freeze
BMS activates heat trace or bypasses valve
Flow Switch vs Flow Sensor:
| Feature | Flow Switch | Flow Sensor |
|---|---|---|
| Output | ON/OFF (DI) | Continuous (AI) |
| Information | Flowing or not | Exact flow rate |
| Use | Protection, confirmation | Measurement, control, BMS energy |
| Cost | Lower | Higher |
Related Topics
- What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
- How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
- What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
- What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
- Browse all Sensors topics — more from this section of the EnSmart BMS Library
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