HVAC controls in Illinois serve a four-season climate. Heating equipment runs through cold winters and springs, and cooling equipment runs through hot, humid summers and autumns. Where a building automation system (BAS) output has to switch a line-voltage or higher-current load—an enable circuit, a heat stage, a pump starter—a relay is the interface that achieves this. Functional Devices, Inc. manufactures the RIB® (Relay-In-a-Box®) Trifecta—relays, current sensors, and power supplies—for building automation, including the enclosed relays specified across heating and cooling control work.
The Heating Side of Illinois HVAC Work
Temperature regulation through Illinois winters and summers requires controls work. The cooling side (compressor and condenser fan staging) follows the same pattern covered in our guide to HVAC controls in California. What an Illinois winter adds is the heating side: boilers and rooftop units cycle daily on night setback and morning warm-up schedules, heat stages are brought on and off as loads move, and circulation pumps run through the season.
Boiler enable circuits, pump starters, gas heat stages on rooftop units (RTUs), and unit heater fans are all loads that a BAS commands on and off through the heating season. Each one needs isolation between the controller output and the load circuit it drives.
HVAC Relay Applications in Illinois Heating Systems
Boiler Enable
A BAS binary output energizes the relay coil, and the relay contact closes the boiler's enable circuit. The relay keeps the controller electrically isolated from the boiler control circuit.
Rooftop Unit Heat Stages
With staged gas heat on an RTU, each stage is wired as its own switching point. Capacity is brought on incrementally, one relay per stage, as heating demand changes.
Pump and Fan Control
Hot-water circulation pumps and supply fans cycle under BAS control throughout the heating season. The relay contact carries the motor starter or contactor coil circuit.
Damper Actuators
Outside-air dampers are driven open during occupied hours and closed upon shutdown. Actuator loads are light-duty switching, typically handled by a general-purpose relay.
Unit Heaters
Warehouses, loading docks, and parking structures are heated by unit heaters controlled by a thermostat or a BAS output through a relay.
Matching the Relay to the Load
Relay selection depends on the answers to two questions: what voltage is the control circuit, and how much current does the load draw? Most BAS binary outputs are 24 Vac, and the coil has to match this. (Universal-coil models accept a range of control voltages.)
For light-duty switching, such as damper actuators and pilot duty, the RIBU1C (10 A SPDT, 10–30 Vac/dc or 120 Vac coil) is a widely specified general-purpose relay. The RIB2401B (20 A SPDT, 24 Vac/dc or 120 Vac coil) is specified for fan coil control and RTU enable circuits where higher contact ratings are required. For higher-current loads, or applications that switch two circuits from a single control output, the RIB24P30 (30 A DPDT, 24 Vac/dc coil) is specified. Each is supplied as a prewired, enclosed assembly.
Changeover and Freeze Protection in Illinois HVAC Systems
Two-pipe systems in older Illinois buildings serve heating and cooling through the same piping, and changeover between the two is facilitated via a controls sequence. The BAS repositions valves and starts or stops pumps, with relays switching each circuit.
Freeze protection is the other winter sequence. An air handler draws outside air through a damper and moves it across a hot-water or chilled-water coil with a supply fan. In an Illinois winter, that air can be cold enough to freeze the water inside the coil, so a low-limit safety trips when the temperature at the coil approaches freezing: the fan is shut down and the damper is driven closed. A relay with a spare contact can report that trip to the BAS as a status point.
The interface pattern is the same on the heating and cooling sides alike: the BAS commands the load on or off, the relay does the switching, and the controller is kept isolated from the load circuit.
Frequently Asked Questions About HVAC Controls in Illinois
What coil voltage do most BAS outputs use?
Most BAS binary outputs are 24 Vac, and the relay coil has to match the control circuit driving it. Universal-coil models such as the RIBU1C accept 10–30 Vac/dc or 120 Vac, which covers most control circuits found on a mixed job.
Which RIB® relay is used for RTU control?
The RIB2401B (20 A SPDT) is commonly specified for RTU enable circuits and fan coil control. Light-duty points such as damper actuators use the RIBU1C, and higher-current or dual-circuit loads use the RIB24P30.
Can one relay switch two loads at once?
Yes, if the relay has DPDT contacts. The RIB24P30 switches two independent circuits from a single control output, which suits paired loads that start and stop together.
Why do outside-air dampers close when the fan shuts down?
Closing the damper stops cold outside air from moving across the coil while the fan is off. In an Illinois winter, outside air below freezing can freeze the water in a hot-water or chilled-water coil, so the damper is driven closed as part of the shutdown sequence.
Where can I buy RIB® relays in Illinois?
RIB® relays are stocked by authorized Functional Devices distributors across Illinois. The distributor can confirm availability of a specific model and advise on selection for the load.
Source RIB® Relays in Illinois
Functional Devices manufactures the relays, current sensors, and power supplies used across Illinois heating and cooling control work. To specify a relay, browse the RIB® relay lineup or find an authorized Functional Devices distributor in Illinois.