HVAC Control Standards Demystified — BACnet, Modbus, and the Open Protocol Market

The protocol landscape is already settled

There is a common assumption in UK building services that HVAC integration is complicated because of too many proprietary systems. That was true before 2000. It is not true now. The market has converged on two open protocols covering the overwhelming majority of new and recently installed plant:

• BACnet (ASHRAE 135 / ISO 16484-5) — 38–40% of all BMS installations globally. Self-documenting: when a BACnet device reports "temperature," the meaning and units are standardised with no vendor documentation needed. Over 800 certified manufacturers. Zero licensing cost.
• Modbus (IEC 61158) — ~25% of BMS installations. The oldest industrial protocol (1979), used universally for PLCs, energy meters, chillers, and legacy equipment. Simplest master-slave architecture. Zero licensing cost.

The practical implication: build BACnet/Modbus integration and your software or gateway works with over 800 hardware vendors without a custom driver for each one.

Control signals — 100% standardised

Signal	Standard	Application
Analog 0–10V DC	Universal	Proportional control — valve position, damper angle
Analog 4–20mA	Universal	Industrial control, long cable runs
PWM	Standard	Triac/relay outputs for heating/cooling stages
24VAC / 24VDC	Industry standard	Device power supply

Belimo actuators — the global market leader — expose BACnet/Modbus objects and accept 0–10V or 4–20mA signals. Any BMS can control any actuator via these signals, regardless of manufacturer.

PID — the universal control algorithm

Every HVAC controller from every vendor uses PID (Proportional-Integral-Derivative) control. The mathematics are identical across Honeywell, Siemens, Trane, and Chinese DDC manufacturers. What varies is default tuning parameters — and modern controllers auto-tune, so even that difference is largely abstracted away.

Where standardisation breaks down — and why it doesn't matter

Physical actuator characteristics are not standardised: torque ratings, stroke lengths, and mounting interfaces vary by manufacturer. A Belimo mounting is not a Siemens mounting. But this is a mechanical contractor's concern during installation — not a software or integration problem. Once an actuator is physically installed and wired to a BACnet network, the BMS software sends a command; the actuator executes. Torque ratings are irrelevant to the software layer.

The open vs proprietary shift

Pre-2000 BMS systems were heavily proprietary: Johnson Controls ran proprietary Metasys protocol; Honeywell ran proprietary Excel 5000. Customers were locked in for the life of the building. That model has collapsed under three pressures: customer demand for vendor independence, regulatory pressure (some government projects now mandate open protocols), and economics — open systems reduce service costs by an estimated 30–40% over a building's lifecycle.

Today every major BMS vendor — Siemens, Honeywell, Trane, Schneider — offers BACnet-compliant controllers. 62% of facilities globally have at least one BACnet application. This is the baseline expectation in new commercial construction and major retrofit.

The software-only integration strategy

For IoT platforms and smart building software looking to add HVAC control capability, the recommended architecture is software-only:

• Build a BACnet/Modbus gateway exposing your sensor data (occupancy, temperature, air quality) as standard BACnet objects
• Implement a BACnet client library to read and write to any installed actuator — valve position, damper angle, setpoints — using open-source stacks (BACnet Stack, libmodbus)
• Add occupancy-based scheduling and energy analytics as the differentiation layer
• Let existing or commodity actuators handle PID execution

This approach reaches market in 6–9 months, works with 800+ hardware vendors, meets open-protocol requirements of virtually every system integrator and FM operator in the UK, and requires no hardware certification beyond CE marking on any gateway device.

Chinese manufacturers — a practical note

Chinese HVAC hardware vendors have moved rapidly toward BACnet/Modbus compliance. Manufacturers including E-Top Controls (Xiamen), Saswell (Shenzhen), and Hotowell Technology now produce BACnet MS/TP and Modbus RTU controllers and actuators with CE, RoHS, and ISO 9001:2015 certification. A BACnet actuator from a Chinese ODM costs £40–90 versus £120–250 for a comparable Belimo unit — a significant consideration for pilot deployments and price-sensitive retrofit projects, provided the specifier verifies BTL listing and CE documentation.