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A hard start kit gives a single-phase compressor a short burst of extra starting torque, then gets out of the way. It's one of the most useful parts on a service truck. It can bring back a compressor that hums and trips on its overload, quiet a unit that dims the lights every time it kicks on, and help an aging system start reliably when line voltage sags on a hot afternoon.
This category covers the main types of starting devices: PTC (positive temperature coefficient) kits such as the Supco SPP series, potential relay and start capacitor combinations, electronic hard start kits such as the SPP-E line, and 3-wire kits that connect directly to the compressor's start winding. Each one works a little differently, and picking the right type matters as much as picking the right size.
A hard start kit is not a cure for a mechanically failed compressor, and it shouldn't be used to cover up a weak run capacitor, low voltage from undersized wiring or a refrigerant problem. Used correctly, though, it reduces starting stress on the compressor windings and the electrical system and can add real service life to equipment that is otherwise sound.
Most residential compressors are PSC (permanent split capacitor) designs. The run capacitor creates the phase shift needed to turn the motor, but it only provides moderate starting torque. That's fine when pressures have equalized and voltage is healthy. It isn't enough when the compressor tries to start against unequal head and suction pressures, after a short cycle, or when voltage drops under load.
A hard start kit adds a start capacitor in parallel with the run capacitor for the first moment of startup. The extra capacitance increases current through the start winding and greatly increases starting torque. Once the motor is up to speed, the kit has to take the start capacitor out of the circuit. Start capacitors are rated for intermittent duty only and will fail quickly if left in the circuit. How the kit removes the capacitor is what separates the types:
Before adding a kit, rule out the usual suspects. Test the run capacitor under a meter that reads microfarads. Check line voltage at the contactor during startup. Inspect the contactor points. Confirm the compressor windings aren't grounded or open. A hard start kit on a grounded compressor will not save it.
Hard start kits are sized by compressor horsepower or tonnage range and by voltage. Most residential kits are rated for 208-240V single-phase compressors. Check the compressor's nameplate for voltage, rated load amps (RLA) and locked rotor amps (LRA), and choose a kit whose range covers that compressor. The supported range for each kit is listed on its product page.
PTC kits are simple and inexpensive, but their cool-down period means they may not boost on a quick restart. A unit that short cycles, or one that is frequently restarted after a power interruption, will be better served by an electronic or potential relay kit.
Two-wire kits connect in parallel with the run capacitor, at the HERM and C terminals, and are quick to install. Three-wire kits take more wiring but give a potential relay a direct reading of the start winding. If the OEM specifies a particular start assist or a potential relay with a specific pickup voltage, follow that guidance.
| Kit type | How it drops the start cap | Wiring | Strengths | Limitations |
|---|---|---|---|---|
| PTC (e.g. SPP series) | Thermistor heats and resistance rises | 2-wire across run cap | Simple, fast install, no moving parts | Needs cool-down between starts |
| Potential relay + capacitor | Relay senses back-EMF and opens | 2-wire or 3-wire | Matches OEM start assist method, repeatable | Relay must suit the compressor |
| Electronic (e.g. SPP-E) | Solid-state switching | Typically 2-wire | Handles closely spaced starts | Electronics sensitive to surges |
| 3-wire kit | Potential relay on start winding | 3-wire to C, run, start circuit | Most precise relay sensing | More wiring, more time |
Kits are designed for single-phase PSC compressors. They are not used on three-phase compressors. Some scroll and variable-speed compressor manufacturers have specific guidance on starting devices, so check the OEM literature first. Inverter-driven compressors, including most modern mini splits, start through their drive electronics and don't use a hard start kit at all.
Supco has built starting devices and capacitors for the HVAC trade since 1945, and its hard start kits are a common sight on service trucks. Product pages include ratings, compressor size ranges and spec sheets so you can match the kit before you order, and items in this category ship in 3-5 business days.
It will if the problem is insufficient starting torque and the compressor is mechanically and electrically sound. It won't fix a seized compressor, a grounded or open winding, a bad run capacitor or a failed contactor. Diagnose those first.
A correctly sized and wired kit reduces stress by shortening the time the compressor spends at locked rotor current. Problems come from wrong sizing or a start capacitor that stays in the circuit, which is why the switching device has to work properly.
A PTC kit uses a thermistor that heats up and blocks current to the start capacitor. A potential relay kit uses a relay that opens when the compressor reaches speed. The relay doesn't need a cool-down period, so it handles quick restarts better.
Yes. The hard start kit only assists at startup. The run capacitor stays in the circuit the entire time the compressor runs and must be the correct MFD rating.
Most current mini splits use inverter compressors that start through a variable-speed drive, so a hard start kit doesn't apply. Some older or fixed-speed units use a PSC compressor, so check the unit's wiring diagram and the manufacturer's guidance.
Two-wire kits are faster and cover most residential applications. Three-wire kits give a potential relay direct sensing of the start winding and are preferred when matching a specific OEM start assist or when you want the most precise dropout.
Service life depends on how often the system cycles, line voltage quality and heat inside the control panel. If the compressor starts struggling again, test the start capacitor and switching device as part of your diagnosis.
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