BMS, GTB, CVC and the Décret BACS: A Building Data Guide

The same building, described in three markets, comes wrapped in different acronyms for identical layers. Lining them up first saves a lot of confusion later:

• BMS (Building Management System) is the English term for the head-end that supervises a building's technical systems. In French it is the GTB (Gestion Technique du Bâtiment); in German, the Gebäudeleittechnik (GLT).
• GTC (Gestion Technique Centralisée) sits one level below the GTB: centralized control of a single subsystem, such as heating or lighting, rather than the whole building.
• CVC (Chauffage, Ventilation, Climatisation) is the French for HVAC: the heating, ventilation, and air-conditioning plant that a BMS spends most of its time supervising.

So a GTB supervises the building, a GTC supervises one subsystem, and CVC is the largest subsystem it watches. Get those three straight and the regulation that follows stops being intimidating.

The Décret BACS is a French regulation that requires Building Automation and Control Systems in tertiary (non-residential) buildings. It implements part of the European Energy Performance of Buildings Directive, so comparable obligations exist across the EU even where the local decree carries a different name.

The trigger is plant power. It applies to tertiary buildings whose HVAC (CVC) plant exceeds 290 kW from 1 January 2025, with the threshold dropping to 70 kW from 1 January 2027. Switching equipment on and off is not enough to comply. A conforming system has to continuously monitor and log energy use, analyze it, flag drift and faults against expected behaviour, and hand operators data they can benchmark and act on.

That last requirement is where most buildings come up short. The BMS controls the plant perfectly well and was never asked to do anything else. Compliance quietly turns a control problem into a data problem, and the data has to leave the controller to be useful.

Building systems are one of the most vendor-locked corners of the industrial world. The head-end talks a proprietary protocol to its own field devices, the licensing ties you to one integrator, and the time series you need for energy analysis lives inside a controller that was specified to run a plant, not to publish data.

The field protocols underneath are usually open: BACnet over IP or MS/TP, Modbus RTU and TCP, KNX, M-Bus, LON. The lock-in sits in the head-end above them, where the supervision software, the historian, and the dashboards hide behind a per-seat license and an export format nobody documents. Pulling a clean, continuous feed for compliance or analytics often means reading the buses directly, around the head-end rather than through it.

An open edge gateway reads the building's field buses itself and publishes a clean, structured stream to wherever it is needed: an energy platform, your IT systems, a compliance dashboard. The BMS never has to cooperate.

Because the field protocols are standardized, one gateway running open software can poll BACnet objects on the HVAC plant, read Modbus registers on the main meters, take M-Bus readings from the sub-meters, and pick up KNX group addresses on lighting, then normalize every value into a single schema with consistent units and timestamps. That schema goes out over MQTT or a REST API. The existing BMS keeps running the building exactly as before. You gain an independent, ownable copy of the data the regulation asks you to analyze.

The shape is the one used across industrial IoT: field devices on standard protocols, an open gateway that reads and normalizes them, and a structured output feeding analytics, alarms, and compliance logs.

You should not have to replace a working control system to satisfy a data obligation. Melqart Systems does not touch your BMS or GTB. We build the open edge layer beside it: the layer that reads the building's standard protocols and gives you a clean, ownable feed of the data.

The compliance records and the energy analytics then stay in your hands, not behind another license. See building management and BMS edge connectivity for how this fits a building estate, and our routers and gateways for the hardware that speaks BACnet, Modbus, and the rest.

What is the difference between GTB and GTC? A GTB (building technical management) supervises the whole building across all its subsystems. A GTC (centralized technical management) controls a single subsystem, such as heating or lighting. The GTB usually sits above several GTCs.

What does CVC mean? In the building context, CVC stands for Chauffage, Ventilation, Climatisation: the heating, ventilation, and air-conditioning systems, the English HVAC. It is the plant a BMS spends most of its effort supervising.

Who is affected by the Décret BACS? Owners and operators of tertiary buildings whose HVAC plant exceeds the power threshold: 290 kW from 2025, then 70 kW from 2027. They must install a building automation and control system that monitors, logs, and analyzes energy performance.

What is a BMS? A Building Management System is the supervisory software and controllers that run a building's technical services (HVAC, lighting, metering, access). GTB and Gebäudeleittechnik are the French and German names for the same thing.