Relays-General Switching

Various HVAC relays are used in heating and cooling systems, including the following:

• Fan relays control indoor blower motors, cycling them on and off in response to thermostat signals or control board outputs. They are among the most frequently replaced relay types in residential service. Time delay relays introduce a controlled pause before or after a switching event, used to stagger compressor starts, prevent short cycling, or keep blowers running after a cycle ends. 
• Potential relays (voltage relays) and current relays assist single-phase compressor motors in starting by removing the start capacitor from the circuit once the motor reaches sufficient speed. The distinction between them lies in whether they sense back-EMF or current draw. 
• Definite purpose relays are rated for the high inrush currents generated by compressor and fan motor loads; control and switching relays handle general circuit switching across a range of voltages and contact configurations. Current inventory also includes thermal sequencer relays and relay overloads. Brands in stock include Tyco, Honeywell, Service First, Omron, Schneider Electric, Potter & Brumfield, Trane, Lennox, Carrier, American Standard, York, Mars, Reznor, and ICM.

Four specs must match on any relay replacement: coil voltage (the control-side voltage that energizes the relay, typically a 24V relay, 120VAC, or 208-240VAC in HVAC applications), contact configuration (SPDT, DPDT, or SPST), contact amp rating (must meet or exceed the load being switched), and relay subtype (fan, time delay, potential, start, etc.). Coil voltage must match exactly. A 24VAC coil on 120VAC will overheat and fail; a 120VAC coil on 24V will not energize; the control voltage is labeled on the existing relay or on the equipment wiring diagram. 

For potential relays, matching goes beyond coil voltage: pickup voltage and continuous voltage ratings are motor-specific and not universally interchangeable, so cross-reference the OEM relay number or the compressor manufacturer's spec sheet. When the failed part has no legible markings, pull the equipment model number, work from the OEM parts list, and filter by coil voltage, amp rating, and contact configuration to find a compatible replacement.

A time delay relay HVAC technicians reach for adds a controlled pause before or after a switching action: an ON-delay relay waits a preset time after control voltage is applied before closing its contacts, while an OFF-delay relay holds contacts closed for a preset period after control voltage is removed. ON-delay relays stagger compressor starts in multi-stage systems, reducing simultaneous inrush current and protecting electrical components from load spikes. 

OFF-delay relays keep indoor blower motors running after a heating or cooling cycle ends, clearing residual heat from heat exchangers or pushing remaining cool air from evaporator coils into the conditioned space. Both functions improve efficiency and reduce equipment wear. Time delay relays also provide short-cycle protection, enforcing a minimum off-time before a compressor can restart; current inventory includes 24VAC time delay relays from Mars, JARD, and allied brands in both thermal and solid-state versions.

A potential relay compressor (also called a voltage relay) uses the back-electromotive force (back-EMF) generated across the start winding as the motor accelerates; when that voltage reaches the relay's pickup value, the normally closed contacts open and remove the start capacitor from the circuit, and the relay stays open as long as the motor runs. A start relay single-phase compressor that technicians use (current relay) operates on the same principle but senses the drop in current draw through the start winding as the motor comes up to speed rather than measuring back-EMF. Current relays are typically used on smaller-horsepower, lower-torque compressors. 

Both types assist single-phase hermetic compressor motors in generating enough starting torque to overcome system pressure at startup, but they are not interchangeable; each must be matched to the specific compressor motor's electrical characteristics. Potential relays in current inventory include units from Danfoss, ICM, and Copeland-compatible sources. Always cross-reference to the OEM part number or compressor model before ordering.

SPST (Single Pole Single Throw) has one set of contacts that opens or closes a single circuit. This is the simplest configuration, common in basic switching applications such as fan relays controlling a blower motor. SPDT (Single Pole Double Throw) has one input and two outputs, a common contact plus both a normally open (NO) and normally closed (NC) contact, allowing it to switch between two circuits; this is one of the most versatile configurations in HVAC controls and appears across many relay types in this collection. 

DPDT (Double Pole Double Throw) controls two separate circuits simultaneously with a single coil actuation, offering the same NO/NC flexibility as SPDT on two independent poles, used where two separate loads must switch together, such as reversing motor direction or switching both line legs of a 240VAC circuit. Contact amp rating matters as much as configuration: a relay with the correct SPDT layout but an undersized contact rating for the motor's inrush current will fail early. Confirm the amp rating is appropriate for the load before ordering.

All relays and electrical components in this inventory are brand-new manufacturer overstock, not pulled from equipment, not refurbished, and not tested-and-returned. Items arrive in original packaging from the source manufacturer. Before ordering, confirm coil voltage, contact configuration (SPDT, DPDT, or SPST), contact amp rating, and relay subtype match the failed component; check available quantity on the product listing, as surplus inventory sells as stock lasts and is not always replenished in the same configuration. Coil voltage and contact specs are listed on individual product pages; for items where full spec details are not visible online, the Surplus City team can assist with part verification at 765-482-7000.

The relay collection includes 83 SKUs from over 30 brands across multiple subtypes. Filtering by coil voltage (24V, 115-120V, or 208-240V), relay type, or brand via the on-page filters is the fastest way to narrow to the right part before ordering. Orders ship to all 48 contiguous states with options including ground, LTL, and freight depending on order size and destination; orders are processed within 3 to 5 business days with tracking provided once shipped. Check online for in-person pickup availability at our various locations. For large orders or custom shipping needs, the team is reachable at 765-482-7000.