Relay + Contactor Types Reference
Why this matters
Relays + contactors are the muscle of any control system. Low-voltage signal in → high-voltage load out. The most-replaced HVAC service part is the contactor on the outdoor unit. The tech who knows what to look for, what to test, + what to replace saves the customer + the equipment. This is the field card.
Relay vs contactor (the distinction)
Relay: typically lower load (≤ 30A). General purpose. Multiple poles common.
Contactor: higher load (30A+); used for motor + compressor starting. Heavy-duty contacts; arc-suppression.
Solenoid valve: similar concept; mechanical valve opens/closes on signal.
Coil voltage (control side)
- 24V AC: HVAC + appliance most common
- 120V AC: control board on furnace
- 240V AC: heavier equipment
- 24V DC: some industrial controls
- 12V DC: automotive, low-voltage controls
- Coil + contact voltages can differ (24V coil controlling 240V contacts is typical contactor)
Contact configurations
SPST-NO (single-pole single-throw, normally open): one switch, open at rest, closes when energized.
SPDT (single-pole double-throw): one common, two outputs; one closed at rest, other open.
DPDT (double-pole double-throw): two SPDT in one device; both poles switch simultaneously.
3PST-NO (3-pole single-throw, NO): three power lines switched simultaneously (3-phase contactor).
NC (normally closed): contact closed at rest, opens when energized. Less common.
Contactor selection
Coil voltage matches control voltage (24V on AC; 120V on some commercial).
Contact rating (FLA) ≥ load FLA + locked-rotor (LRA) handling typical 5 - 8x FLA for motors.
Pole count: single (120V), double-pole (240V), 3-pole (3-phase).
Auxiliary contacts: extra NO/NC contacts for signaling status, interlocks.
HVAC contactor common ratings
Residential 3-ton AC outdoor unit: 25 - 40A contactor, 24V coil, single-pole OR double-pole.
Heat pump: similar; sometimes additional 24V relay for reversing valve.
Furnace blower: 15 - 20A relay; integrated into control board for newer units.
Inducer motor (high-efficiency furnace): small dedicated relay.
Defrost board (heat pump): combines clock + sensor + relay for defrost cycle.
Common HVAC contactor failures
Pitted contacts: arcing burns metal off; resistance increases; contactor overheats + fails.
Stuck closed (welded): contacts fused together; compressor runs continuously; bypasses safety.
Burned coil: open winding; multimeter reads infinity. Doesn't pull in.
Vibration loosens screws: contact intermittent.
Insect nest (especially honey bees): blocks armature movement.
Diagnostic procedure (HVAC contactor)
- Power off; lock out
- Visually inspect contacts (pitted? burned? clean?)
- Multimeter at contactor coil terminals: 24V AC with control calling? Coil energized?
- Multimeter across contacts (line side L1 + load side T1): 0 ohms when contactor closed = good; some resistance = pitted; infinite = burned out
- Test coil: disconnect; measure coil resistance (typical 10 - 50 Ω); infinite = burned coil
- Test armature: push manually with insulated tool; should pull in smoothly; sticking = mechanical failure
Common relay types in HVAC
Time-delay relay: blower delay (on/off) for hot/cold air production.
Anti-short-cycle relay: prevents compressor restart within 3 - 5 min after shutoff.
Sequencer (electric furnace): stages 4 - 6 heating elements 30 sec apart.
Defrost relay (heat pump): cycles unit into defrost mode.
Aux heat relay: brings in strip heater on heat pump.
Other control devices
Pressure switch: closes/opens at preset pressure. Common: low pressure cutout (loss of refrigerant), high pressure cutout (clogged condenser).
Temperature switch / klixon: closes/opens at preset temp. Common: limit switch on furnace plenum (140 - 200°F shuts off burner if airflow loss).
Float switch: water level. Common: condensate overflow shutoff.
Door switch: open/close detection (fridge, dryer).
Flame sensor (rod in flame): senses flame current; safety on gas furnace.
Thermal fuse: blows at specific temp; one-time; protects against overheat. Replace + investigate cause.
Solid-state relay (SSR)
Semiconductor switching: silent, no mechanical contacts, fast switching. Used for: PID temperature control, modulating loads, frequent cycling.
Pros: silent, fast, long life.
Cons: heat dissipation requires heat sink; more expensive; not as fault-tolerant.
Motor starter
Combination of: contactor + overload protection + manual disconnect.
Used for: 3-phase motor starting + protection. Common commercial. NEMA + IEC standards.
Overload heater: thermal device that trips contactor when motor draws too much current too long.
Common control circuit problems
Burned 24V control transformer (overload from shorted thermostat wire). Open coil winding (relay/contactor). Welded contacts. Defrost cycle stuck (relay won't release). Sequencer failure (electric heater doesn't stage properly). Float switch (condensate) stuck open → AC shuts down on "high water".
Replacement workflow (HVAC contactor)
- Photo of original wiring before disconnect
- Power off + lock out
- Disconnect wires; label per photo
- Remove old contactor from mounting
- Install new contactor (match coil V + contact rating + pole count + auxiliary contacts)
- Reconnect wires per photo + labels
- Re-energize + test contactor function (call from thermostat = contactor closes; release call = contactor opens)
- Verify motor / compressor starts normally
Truck stock
Universal 30A, 40A, 50A single-pole + double-pole HVAC contactors (24V coil); 24V transformer (40VA + 75VA + 100VA); time-delay relay (anti-short-cycle); float switch + condensate pump assembly; assorted limit + pressure switches.
Energized vs de-energized testing
Energized resistance test: contactor closed, measure across contacts: 0 Ω = good; some Ω = pitted.
De-energized test: contactor open, measure across contacts: infinite Ω = open, good; 0 Ω = welded shut.
Coil ohms test: disconnect coil power; measure coil resistance. Compare to nameplate OR similar unit.
Common contactor mistakes
Wrong coil voltage (24V coil on 120V control = burned coil). Wrong contact rating (undersized for motor LRA = burns out). Skipping LRA check on diagnosis. Replacing contactor when actual issue is upstream (24V control miss). Reusing contactor with burned contacts (won't last past warranty). Not torquing terminal screws (eventual loose connection).
References
- UL 60947 (electromechanical contactors)
- NEMA ICS standards (industrial control)
- NEC Article 430 (motors + motor circuits)
- Manufacturer technical data (Mars, Packard, Honeywell, Square D)
- Manuall internal: Diagnostic Test Procedures, Common Gauges + Meters