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Contactors and Relays for HVAC

Contactors and relays switch HVAC loads such as compressors, fans and heaters from a low-voltage control signal. This sheet covers definite-purpose contactors, general relays and how to read their ratings for replacement.

Ref HV-0020UL 60947-4-1UL 508

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A contactor is a heavy-duty switch with an electromagnetic coil. When the thermostat or control board energizes the coil (usually 24 VAC in residential and light commercial equipment), the contacts close and send line voltage to the compressor and outdoor fan. Definite-purpose (DP) contactors are built for HVAC and refrigeration motor loads and are rated in full-load amps (FLA) and locked-rotor amps (LRA) as well as resistive amps. A relay is the same idea at lower current. It switches smaller loads or pilot circuits, such as a blower, a heat strip sequencer or another contactor.

How to identify

Read the side label. It shows the coil voltage, number of poles, FLA, LRA, resistive amps, and the maximum line voltage. Single-pole contactors often have a shunt (a bar that stays closed across one leg). Relays show the coil voltage and contact arrangement, such as SPDT or DPDT, with ratings for each contact.

Contactor and relay basics

Item	Contactor	General-purpose relay
Typical load	Compressor, condenser fan, heaters	Blower, pilot circuits, other coils
Coil voltage	24 VAC common; 120 and 240 V also made	24 VAC, 120 V, 240 V
Contact ratings	FLA, LRA, resistive	Amps by load type, often pilot duty
Poles	1-pole with shunt, 2-pole, 3-pole	SPST, SPDT, DPDT and others
Typical failure	Pitted contacts, burned coil, ants or debris	Welded or burned contacts, open coil

Key points

Match the coil voltage exactly. A 24 V coil on 240 V burns out at once; a 240 V coil on 24 V will not pull in.
The replacement's FLA and LRA ratings must equal or exceed the motor's ratings on the equipment nameplate.
Match the pole count. A 2-pole cannot always replace a 1-pole with shunt without wiring changes.
Pitted or burned contacts cause voltage drop and heat at the compressor. Replace the contactor; do not file the contacts.
Single-pole contactors leave one leg live when open. Treat the equipment as live.

Common mistakes

Assuming the unit is dead because the contactor is open.
Picking a contactor by resistive amps only.
Reusing burned wire terminals when swapping a contactor.
Ignoring ants, which are drawn to energized contacts and jam them.

Governing standards: UL 60947-4-1 (contactors and motor starters); UL 508 (industrial control equipment). Equipment nameplate and maker data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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