Relay vs. Contactor: HVAC and BAS Selection Guide

When it comes to relay vs. contactor selection for HVAC and building automation work, the wrong call usually isn't immediately obvious. It shows up six months later as a failed contact, a tripped circuit, or a callback that wasn't in the budget. Both devices switch loads. Both use an electromagnetic coil. But they’re designed for fundamentally different jobs, and using one where you need the other can create real problems.

This guide breaks down what each device is built for, where they overlap, and how to choose the right one for your application.

What Is a Relay?

A relay is an electrically operated switch that uses a low-power control signal to open or close contacts that switch a separate circuit. RIB® relays are designed to interface low-voltage control signals with higher-voltage equipment loads, keeping the controller separated from voltage spikes, electrical noise, and unintended feedback on the load side.

RIB® (Relay-In-a-Box®) relays from Functional Devices are enclosed, prewired relay assemblies designed for fast field installation in HVAC, lighting, and BAS applications. They ship as complete units—coil, contacts, housing, and leads—ready to mount and wire without additional assembly.

What Is an Electrical Contactor?

An electrical contactor is a heavy-duty switching device designed to handle high-current, high-voltage loads—primarily motor loads, compressors, and other inductive equipment. Like a relay, it uses an electromagnetic coil to open and close contacts. Unlike a relay, it's built to withstand the arc energy, heat, and inrush current that come with switching those loads repeatedly.

Contactors are physically larger than relays, typically more expensive, and designed with arc suppression, heavier contact materials, and often auxiliary contact blocks for status feedback.

Key Differences: Load Capacity, Coil, and Application

When to Use a Relay in HVAC and BAS

A relay is the right choice when you need to interface a low-voltage control signal with a load that requires switching. In HVAC and BAS work, that covers a very wide range of applications.

BAS Controller Outputs

BAS controllers output binary signals to switch equipment: fans, valves, dampers, lighting. Those outputs are typically 24VAC or a dry contact. A RIB® relay translates that signal into a switched circuit, keeping the controller isolated from line voltage. This is the core BAS relay application.

Fan Coil Unit Control

A thermostat or BAS output sends a 24VAC signal to the relay coil; the relay contacts switch line voltage to the fan motor. The RIB2401B is a common choice here—a 20A SPDT relay with 24VAC coil that handles this scenario cleanly across a wide range of fan coil configurations.

Damper and Actuator Switching

Spring-return damper actuators draw modest current when energized. A RIB® relay handles this load comfortably and provides the electrical isolation that protects the controller output.

Lighting Control

RIB® relays are commonly used for scheduled or occupancy-based lighting switching via a BAS or panel controller. Models with appropriate contact ratings handle LED and fluorescent fixture loads, including banks of fixtures on a single circuit.

Make-Up Air and Exhaust Fan Control

For single-phase fan motors up to the relay's horse power (HP) rating, a RIB® relay is a straightforward and code-compliant solution. Check horse power (HP) ratings on the datasheet when the load is a motor.

Thermostat-to-Equipment Bridging

A thermostat sends a 24VAC signal to call for heating or cooling, but the equipment requires line voltage to operate. A relay like the RIB2401B bridges that gap, drawing only 83mA from the thermostat output while switching a 1HP 120VAC motor load.

When a Contactor Is the Right Choice

A contactor belongs in the circuit when the load is a motor, compressor, or other high-inrush inductive device that exceeds what a general-purpose relay is rated to handle safely and repeatedly.

Compressors

Residential and light commercial HVAC compressors are almost universally switched by contactors. The startup inrush current on a compressor—often 6–8 times the running current—requires a device rated for that duty. A general-purpose relay is not.

Chiller and Rooftop Unit Compressor Staging

Larger tonnage equipment with multiple compressors uses contactors at each stage. The contact ratings, arc suppression, and mechanical durability are all sized for the application.

Electric Heat Strips

Resistance heating elements can draw high steady-state current. Contactors are standard in electric furnace and heat pump auxiliary heat applications.

High-Cycle Applications

If the device switches a motor load thousands of times per day, a contactor's heavy-duty contacts and arc suppression may be better suited for that duty cycle. A relay contact can wear prematurely in the same application.

One important nuance: some RIB® relay models carry a pilot duty rating, which means they are rated for inductive load control applications within that rating. Always check the datasheet for the general-use contact rating, any published pilot duty rating, and horse power (HP) rating before specifying a relay for those types of loads. If the load exceeds any of those ratings, a contactor may be required.

Frequently Asked Questions

What is the main difference between a relay and a contactor?

The main differences are load capacity and intended application. Relays are designed for low-to-medium current loads and control signal switching, with typical contact ratings up to 20–30A. Contactors are designed for high-current motor and compressor loads, with ratings from 25A up to several hundred amps and heavy-duty arc suppression built in for inductive load switching.

Can I use a RIB® relay instead of a contactor?

For loads within the relay's contact and pilot duty ratings, yes. For compressors, large motors, 3-phase loads, or any high-inrush inductive load that exceeds the relay's rated capacity, no—a contactor is required. Using a relay for a load it isn't rated for will cause premature contact wear and eventual failure.

What is a pilot duty rating on a relay?

Pilot duty is a contact rating that indicates the relay is suitable for switching inductive loads within that rating. It accounts for the inrush current which occurs at startup, which can be several times the running current. Always verify the pilot duty rating on the datasheet before using a relay to switch an inductive load.

Are RIB® relays UL Listed?

Yes. Enclosed RIB® relay models are UL Listed as complete, field-ready assemblies. Always verify the specific listing standard on the product datasheet for your application.

Can I use a contactor where a relay would work?

Technically, yes—but a contactor is typically larger, more expensive, and overkill for low-current control and switching applications. For BAS output switching, fan coil control, and similar applications, a compact UL Listed RIB® relay is the appropriate and cost-effective choice.