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What is HLI in BMS?

December 5, 2025

In Building Management Systems (BMS), HLI – High Level Interface – is a digital communication method that connects HVAC equipment to the BMS using protocols like BACnet, Modbus, OPC, or API.

 

It replaces traditional hardwired signals and provides rich, real-time data.

How HLI Works ?
HLI creates a software-level connection between equipment controllers and the BMS.
Flow: Equipment Controller -> Protocol (BACnet/Modbus/API) -> BMS

Through this single digital link, the BMS receives:
1. Temperatures, 2. Pressures, 3. Status, 4. Alarms, 5. Energy consumption
6. Run hours, 7. Setpoints.

This is why HLI is faster, cleaner, and smarter than hardwired I/O.

Where HLI is Mostly Used in Buildings?
HLI is widely used in areas where equipment has built-in controllers that can communicate digitally:

1. Chillers 2. VRF / VRV Systems 3. VFDs 4. AHUs & FCUs with factory controllers 5. Energy Meters & BTU Meters 6. Pumps with smart controllers
7. UPS / DG / Fire Alarm Panels (via Gateway) 8. IoT Sensors & Cloud APIs

These systems generate a large amount of data, which hardwired I/O cannot handle efficiently — so HLI becomes the preferred method.

Why Is It Called “High Level Interface”? Why Not Another Name?
It is called High Level because:
1. It exchanges high-level digital data, not simple ON/OFF or analog signals.
2. It communicates directly with the equipment’s brain (controller).
3. It provides hundreds of data points, not just basic I/O.
4. It operates at the software/application layer, not wiring level.
5. It can support advanced features like alarms, trends, diagnostics, energy 6. data, and cloud connectivity.

“High Level” differentiates it from:
Low-level wiring (DI/DO/AI/AO)
Field level networks
That’s why the industry standard name is HLI.

Names like “Digital Link” or “Protocol Interface” were never adopted because ASHRAE, OEMs, and BMS vendors standardized the term “High Level Interface” decades ago, and it became globally accepted.

Why BMS Needs HLI
HLI is essential because:
1. Hardwired I/O cannot provide deep equipment data
2. It reduces wiring and installation cost
3. Integration becomes faster and more accurate
4. Enables remote monitoring & troubleshooting
5. Supports cloud, IoT, analytics, and digital twins
6. HLI essentially upgrades a normal BMS into a smart, connected, data-driven platform.

Conclusion: HLI is one of the most important parts of modern BMS.
It provides high-quality digital data, improves building performance, reduces costs, and enables cloud and IoT capabilities.

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