Relays are one of the most common—and often overlooked—components in commercial HVAC and building automation systems. While they may seem like simple electrical devices, they play a critical role in ensuring equipment operates safely, efficiently, and in the proper sequence.
At their core, relays act as electrically controlled switches. They allow a low-voltage control signal from a thermostat, building automation system (BAS), or controller to safely operate higher-voltage equipment such as motors, pumps, fans, and lighting circuits. This electrical isolation protects sensitive control devices while enabling reliable operation of larger mechanical systems.
So what does that look like in the field? Here are six of the most common applications where relays keep commercial buildings running every day.
One of the most common uses for relays in HVAC systems is controlling fan and blower motors.
When a thermostat or BAS sends a 24VAC signal, the relay energizes and closes its contacts, allowing power to reach the fan motor. This simple process happens thousands of times throughout the life of an HVAC system.
Typical applications include:
Why relays are used:
Every time a commercial HVAC fan starts, stops, or changes stages, there’s a good chance a relay is making it happen.
Learn more about the APC relay solutions: MR-700 Series and PAM Series.
Proper sequencing is essential in HVAC systems, especially when dampers and actuators are involved.
Relays are commonly used to ensure outside air dampers open before supply fans begin operating or to control actuator power based on system conditions. This sequencing helps prevent equipment damage and ensures proper airflow throughout the building.
Typical applications include:
Why relays are used:
For example, a relay can prevent a supply fan from running until an outside air damper is fully open, helping protect the fan while maintaining proper ventilation.
Learn more about the APC relay solutions: MR-200 Series (DPDT).
Relays aren’t limited to HVAC—they’re also widely used in commercial lighting control.
Building automation systems and occupancy sensors frequently use relays to switch lighting circuits on and off based on schedules, occupancy, or daylight conditions.
Typical applications include:
Why relays are used:
By allowing low-voltage controls to operate higher-voltage lighting circuits, relays help facilities reduce energy consumption while simplifying building management.
Learn more about the APC relay solutions: MR-800 Series.
Hydronic HVAC systems rely on relays to control the movement of water throughout the building.
Whether operating heating hot water pumps, chilled water pumps, or control valves, relays help ensure equipment runs only when needed and in the correct sequence.
Typical applications include:
Why relays are used:
Relays play a key role in maintaining comfort, maximizing energy efficiency, and protecting hydronic equipment.
Learn more about the APC relay solutions: MR-100 Series.
Most thermostats operate using low-voltage control signals, while the equipment they control often requires much higher voltages.
Relays bridge this gap by allowing a 24VAC thermostat signal to safely switch equipment operating at 120, 240, or even 480 volts.
Typical applications include:
Why relays are used:
Without relays, today’s thermostats and control systems couldn’t safely communicate with much of the equipment found in commercial HVAC systems.
Learn more about the APC relay solutions: MR-100 Series and PAM Series.
Many of the relays in a building are hidden inside control panels, quietly translating BAS commands into real-world actions.
When a BAS controller sends a digital output, a relay converts that signal into a switched electrical circuit capable of controlling equipment throughout the facility.
Typical applications include:
Why relays are used:
While building occupants may never see them, relays are constantly working behind the scenes to turn digital commands into physical actions that keep buildings operating efficiently.
Learn more about the APC relay solutions: MR-700 Series, MR-800 Series, and MR-200 Series.
Although relays perform a relatively simple function, selecting the right relay is essential for long-term system performance.
Factors such as contact ratings, coil voltage, switching frequency, electrical isolation, and environmental conditions all affect reliability. Choosing the wrong relay can result in premature failures, nuisance service calls, equipment downtime, and increased maintenance costs.
That’s why engineers, contractors, OEMs, and system integrators rely on relays specifically designed for HVAC and building automation applications. The right relay not only improves reliability but also simplifies installation and helps ensure consistent system performance for years to come.
Relays may be small components, but they perform some of the biggest jobs in commercial buildings. From starting fans and pumps to sequencing dampers, controlling lighting, and executing building automation commands, relays quietly keep HVAC systems operating safely, efficiently, and reliably.
Understanding where relays are used—and selecting the right relay for each application—can improve system performance, protect valuable equipment, and reduce maintenance over the life of the building.
At Air Products and Controls (APC), we engineer relay solutions specifically for the demands of commercial HVAC and building automation. From versatile multi-voltage relays to compact field-mounted solutions and panel-mounted control relays, APC offers dependable products designed to simplify installation, improve reliability, and support the long-term performance of your systems. Explore our complete line of relay solutions to find the right product for your next project.