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$23.44Ships in 3-5 Business Days
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$23.44Ships in 3-5 Business Days
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$23.44Ships in 3-5 Business Days
Relays and start devices are the small parts that decide whether a motor or compressor actually gets power, and how it gets up to speed once it does. A thermostat calls, a 24-volt coil pulls in, and contacts rated for real current close to run a blower, a condenser fan or a heat strip. On the compressor side, a start device briefly adds a start winding or start capacitor into the circuit so the motor can break loose against system pressure, then drops it out before it overheats. When any of these parts sticks, burns or opens, the symptom often looks like a bigger failure than it is.
This category covers the relays and starting components that techs and appliance repair people reach for every week: general purpose fan relays, switching relays with DPDT contacts, potential relays such as the Supco 90X series, time delay relays, RCO and URCO style relay-capacitor-overload start kits for fractional horsepower compressors, and PTC solid state starters.
A general purpose relay is an electrically operated switch. A low-current coil, usually 24 VAC in residential HVAC, creates a magnetic field that pulls an armature and closes or opens one or more sets of contacts. Those contacts carry line voltage, typically 120 or 240 VAC, to a blower motor, fan or other load. SPST relays switch one circuit. SPDT relays have a common, a normally open and a normally closed terminal, which is how a single relay can swap a multi-speed blower between heating and cooling speeds. DPDT relays carry two independent poles, so they can switch both legs of a 240-volt load or two separate circuits at once.
Potential relays are used on capacitor-start, capacitor-run (CSR) compressors. The relay coil is wired across the start winding, between terminals 2 and 5, and its contacts between terminals 1 and 2 are normally closed. At startup, the start capacitor is in the circuit through those closed contacts. As the motor speeds up, the voltage generated across the start winding (back EMF) rises. When it reaches the relay's pickup voltage, the coil opens the contacts and drops the start capacitor out. The relay then holds open as long as the motor keeps running. Supco's 90X series potential relays are built around this principle and are selected primarily by pickup voltage, dropout voltage and continuous coil voltage.
A time delay relay adds a timing function to switching. Common types are delay-on-make (the load starts a set time after the control signal arrives), delay-on-break (the load keeps running for a set time after the signal is removed) and anti-short-cycle timers that prevent a compressor from restarting against high head pressure right after it shuts off.
Small hermetic compressors in refrigerators, freezers, water coolers and dehumidifiers often start through a combination relay-capacitor-overload assembly. RCO and URCO style kits package a start relay, an overload protector and, depending on the version, a start capacitor in one unit that plugs onto the compressor's terminal pins. A PTC (positive temperature coefficient) starter does the start relay job with a ceramic thermistor: it conducts when cold, sending current to the start winding, then heats up within a fraction of a second and its resistance climbs, effectively taking the start winding out of the circuit. A PTC needs a few minutes to cool down before it can start the compressor again.
Coil resistance can be checked with an ohmmeter with power off and the relay isolated. An open coil reads infinite. Contacts can be checked for continuity in their de-energized state and then while the coil is energized from a safe source.
The coil voltage is printed on the relay. Most residential HVAC fan relays use a 24 VAC coil, but 120 and 240 VAC coils are common on commercial and refrigeration equipment. A wrong coil voltage will either fail to pull in or burn up.
Contact ratings are usually listed in full load amps (FLA), locked rotor amps (LRA), resistive amps and horsepower at a given voltage. Choose a replacement with ratings equal to or greater than the original. A motor load needs the inductive or horsepower rating, not just the resistive amp number.
Count the poles and throws you actually need. Replacing a SPDT blower speed relay with a SPST part will lose the speed-change function.
Potential relays are not universal by horsepower alone. The pickup voltage must suit the compressor's start winding characteristics. Mounting orientation also matters on many potential relays. Use the compressor manufacturer's specified relay, or cross-reference the OEM number to a Supco equivalent.
For RCO, URCO and PTC devices, match compressor horsepower range, voltage and the pin arrangement on the compressor.
The fastest way to a correct part is the OEM part number or the relay's own model number.
| Device type | Typical use | Key selection specs | Common failure symptom |
|---|---|---|---|
| Fan / general purpose relay | Blowers, condenser fans, auxiliary loads | Coil voltage, contact config (SPST/SPDT), FLA/LRA rating | Fan will not start or will not stop |
| Switching relay (DPDT) | Switching two circuits or both legs of 240V | Coil voltage, two-pole rating, mounting | Partial function, chatter |
| Potential relay (90X series) | CSR compressors with start capacitor | Pickup, dropout, continuous coil voltage; mounting position | Compressor hums, start cap fails |
| Time delay relay | Blower off-delay, compressor anti-short-cycle | Delay type, delay time, coil voltage, contact rating | No delay, or load never energizes |
| RCO / URCO start kit | Fractional HP refrigeration compressors | HP range, voltage, pin fit, included overload | Compressor clicks on overload |
| PTC starter | Refrigerators, freezers, small appliances | Compressor HP and voltage, pin fit | Compressor will not start, especially on quick restart |
HVAC365 stocks Supco relays and start components because Supco has been building replacement parts for HVAC, refrigeration and appliance repair since 1945, and its universal relays are designed to cover a wide spread of OEM applications. Product pages include cross-reference numbers and spec sheet data so you can confirm coil voltage, contact ratings and fitment before you order. These items ship in 3-5 business days, which keeps a truck stock refill or a down unit moving.
Sometimes, but not blindly. A hard start kit adds start torque to a PSC compressor that already runs on a run capacitor. If the compressor was designed as a CSR system with a specific potential relay and start capacitor, the best repair is a properly matched relay and capacitor. Follow the compressor manufacturer's guidance.
It identifies a family of Supco potential relays. Individual part numbers in the series differ by pickup, dropout and continuous coil voltage, so match the specific relay to the compressor rather than treating the whole series as interchangeable.
That click is usually the overload opening and resetting because the compressor is not starting. Common causes are a failed PTC or start relay, a failed overload, low supply voltage or a compressor problem. Test the start device first since it is the inexpensive and frequent culprit.
Generally yes, if the coil voltage, contact ratings and mounting are compatible. You use one pole and leave the other unconnected.
A PTC needs to cool down after each start, typically a few minutes. If a compressor short cycles, the PTC may still be hot and unable to start it, which can look like a failed part when it is actually a cycling problem elsewhere.
Excessive current, usually from a locked or failing motor, a short in the load, chattering from low control voltage or a relay that was undersized for the load. Fix the cause when you replace the relay or the new one will fail the same way.
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