Hard Start Kits: When They Help and When They Hide

The short version

A hard start kit adds a start capacitor and a relay to a single-phase compressor so the compressor reaches running speed faster. The kit is useful when the compressor is mechanically sound but working against a condition that makes starting difficult: low voltage, a long line set, a non-bleed thermostatic expansion valve (TXV), or an aging motor that is slow to get moving. The kit does not reduce the current the motor draws at locked rotor. The kit reduces the time the motor spends there, and that time produces the heat in the windings.

The trap is installing one to make a struggling compressor start instead of finding out why it is struggling. A hard start kit masks the real fault on a compressor with failing windings, a weak run capacitor, or a pitted contactor.

What a hard start kit does

A single-phase compressor motor has two windings: a run winding and a start winding. The run capacitor creates a phase shift between them so the motor develops a rotating magnetic field and turns. That phase shift is optimized for running speed, not for startup. At startup the motor is stationary and draws locked rotor amps (LRA). The run capacitor alone provides limited starting torque. The motor relies on getting past the first fraction of a second by building speed quickly enough that the windings do not overheat.

A hard start kit uses a relay to switch a start capacitor in parallel with the run capacitor, which puts the added capacitance in series with the start winding. The start capacitor is much larger than the run capacitor, typically 88 to 324 microfarads (uF) in common hard start kits. The start capacitor delivers a stronger phase shift during the first few hundred milliseconds. That stronger shift increases the torque the motor produces while the motor is below running speed.

The relay’s job is to disconnect the start capacitor once the motor reaches roughly 75% to 80% of running speed. If the start capacitor stays in the circuit after the motor is up to speed, it overheats the start winding and can damage the compressor. Hard start kits differ in the switching method.

The two types

Most hard start kits use either a potential relay with a start capacitor or a positive temperature coefficient resistor (PTCR) device. HVAC School also describes electronic and timer-type kits that remove the capacitor on a timer. You cannot tell which type a kit is from the wire count: SUPCO sells its potential relay kits in both three-wire and two-wire versions.

Potential relay kits sense the voltage the motor generates in its own start winding as speed builds. That voltage is called back electromotive force (back EMF). When it reaches the relay’s pickup voltage, the relay opens and removes the start capacitor from the circuit. According to HVAC School, the pickup voltage is generated by the motor itself, not by the capacitor.

The relay disconnects at the right moment regardless of load or voltage conditions, because the relay responds to what the motor is actually doing. The relay resets immediately when the motor stops. After the reset, the relay is ready for the next start cycle with no cool-down period of its own. The compressor still has a limit: Copeland suggests a maximum of 12 cycles per hour for its reciprocating compressors.

PTCR kits use a thermistor wired in parallel with the run capacitor or in series with the start capacitor. The thermistor starts at low resistance when cool, allowing current to flow to the start winding. As current heats the thermistor, its resistance climbs and the thermistor effectively removes itself from the circuit. HVAC School describes a PTCR as a resistor that starts off at a low resistance when cool and changes to higher resistance when it gets hot. The limitation is that the PTCR needs several minutes to cool down before it can assist the next start.

Feature	Potential relay kit	PTCR kit
Disconnect method	Senses back EMF from the motor	Thermistor heats up over time
Cool-down required	No, but the compressor’s cycle limit still applies	Yes, typically 3 to 5 minutes
Accuracy of disconnect timing	Responds to actual motor speed	Approximate, does not sense speed
Rapid cycling tolerance	Can assist on back-to-back starts	Misses starts that follow a recent shutdown
Cost	Higher	Lower
TXV systems	Recommended	Not recommended for non-bleed TXVs

Where a non-bleed TXV leaves the pressures on the high and low sides unequal at restart, the compressor needs dependable starting torque, and a potential relay kit is the usual choice. But HVAC School notes that a scroll compressor on a short line set with 230V or better probably needs no kit at all. A PTCR that is still warm from a recent cycle provides little help exactly when the compressor needs it most.

When a hard start kit helps

Five conditions account for most of the installations where the kit does real work.

Low voltage. Copeland rates a compressor with a 208/230V nameplate to run from 197 to 253V, so 208V service by itself is not a fault. Measure the voltage at the compressor terminals instead. When the measured voltage sags below the rated range at startup, the motor has less voltage to work with during the locked rotor period, and the start capacitor provides additional torque to compensate.

Long line sets. When a condenser sits far from the electrical panel, voltage drops across the length of the wire. The compressor receives less voltage at the motor terminals than the panel supplies, and the effect is worst at startup when current draw is highest.

Non-bleed TXVs. A TXV with a hard shutoff closes when the system cycles off, so pressure does not equalize through the valve. The compressor starts against a pressure differential between the high side and the low side. Copeland’s application bulletin AE4-1329 states that pressure differentials as low as 2 PSIG can prevent compressors from starting without start assist.

Reciprocating compressors. Reciprocating models have reed valves that stop reverse flow through the compressor itself, so the high side has to equalize through the metering device instead. Reciprocating compressors require more torque to start against pressure than scroll compressors do. Scroll compressors can typically start against unbalanced pressures because the scrolls separate briefly on startup and allow equalization.

Aging compressors with higher mechanical resistance. A compressor with worn bearings or tighter tolerances from years of thermal cycling may need more torque to break free on startup. A hard start kit can keep a mechanically sound but aging compressor running reliably. An aging compressor is where the line between helping and hiding is thinnest: the kit is appropriate when the compressor is otherwise healthy and just needs more starting torque, not when it is on its way out.

When it does not help

Inverter and variable speed compressors. These compressors have their own electronic starting circuits that ramp voltage and frequency. Adding a hard start kit to an inverter compressor interferes with the drive board and can damage both the board and the compressor.

Compressors with failing windings. A compressor that trips on overload or draws excessive amps at steady state has a fault that no start assist fixes, and the fault is not always the windings. An overload usually means the motor is carrying more load than it was built for, and the cause can be mechanical or a system condition. Each additional start on damaged windings makes the eventual failure worse.

A bad run capacitor. A run capacitor that has drifted out of tolerance or failed open reduces the phase shift the motor needs to run, not just to start. The compressor may struggle to start, and a hard start kit can push it past that struggle, but the compressor runs poorly once it is up to speed. Replace the run capacitor first and then evaluate whether a hard start kit is still needed.

A pitted or worn contactor. Voltage drop across corroded contactor contacts shows up as low supply voltage. The fix is a new contactor, not a device to compensate for the voltage the contactor is wasting.

Short cycling from a thermostat or control problem. If the compressor is cycling on and off rapidly because of a control fault, a hard start kit does not solve the control fault. A PTCR kit is actively harmful on a system that short cycles, because the thermistor does not cool between starts and the compressor loses its starting assist on the very cycles where startup conditions are worst.

Sizing

A hard start kit sized wrong damages the compressor. A start capacitor with too few microfarads does not produce enough torque to help. A start capacitor with too many microfarads drives excessive current through the start winding and overheats it. Both outcomes are worse than no kit at all.

Aftermarket kits like the 5-2-1 Compressor Saver line are sold by tonnage or horsepower range, and they simplify sizing by matching the capacitor and relay to a tonnage band. CPS Products rates the CSRU1 for 1 to 3 ton scroll and reciprocating compressors and the CSRU2 for 3.5 to 5 ton units. Start from the compressor model number when you select a kit. Copeland has technicians cross-reference the model number in its Online Product Information tool or Copeland Mobile app, along with the application, the refrigerant, and the operating conditions, to find the matching components. Do not install a kit rated for a larger compressor on a smaller one because you have that kit on the truck.

For a potential relay kit assembled from separate components, the relay’s pickup voltage must match the back EMF the specific compressor produces. The start capacitor and the relay are specified together as a combination for that compressor, not chosen separately. Use those specifications rather than a generic pairing.

Hard starts and soft starts are different products

A soft start reduces voltage to the motor gradually during startup, which reduces the current the motor draws on its way up to speed. A hard start adds current to the start winding for a burst and then removes it. They solve different problems.

HVAC School makes the distinction clearly: a hard start gets the motor to full speed as quickly as possible, while a soft start carefully controls the voltage and therefore the current, providing lower initial current during start and slowly increasing up toward full speed. A soft start reduces inrush current substantially. A hard start does not reduce the peak current at locked rotor. It shortens the time spent at locked rotor.

A PTCR device is sometimes called a soft start. HVAC School states directly that a PTCR is a start device, but there is nothing soft about it. A PTCR does not ramp voltage. It sends a spike of current into the start winding through a thermistor that has not yet heated up, without the phase shift a start capacitor adds.

Soft starts are useful when the goal is reducing light dimming, lowering the load on a generator or inverter, or decreasing mechanical shock to the compressor and piping. They cannot produce the large torque boost that a hard start delivers, which means they are unlikely to start an old, stuck compressor.

Common mistakes

Verify that the run capacitor tests within tolerance before installing a hard start kit, because a failed run capacitor is the more likely cause of a startup problem on a system that previously started fine
Check whether the equipment manufacturer calls for start assist on a non-bleed TXV system, since pressures may not be equal at restart
Read the compressor model number off its nameplate and look up the approved components, rather than guessing from condenser nameplate tonnage
Check the contactor contacts for pitting and measure voltage at the compressor terminals during startup to rule out voltage drop before adding a kit
Confirm the compressor’s amp draw at steady state after installation to verify the motor is running normally once it starts

Don’t

Install a hard start kit on an inverter or variable speed compressor, because the kit interferes with the drive electronics
Use a PTCR kit on a system that short cycles, since the thermistor needs minutes to cool before it can assist the next start
Treat a hard start kit as a long-term fix for a compressor with failing windings, because the kit masks the fault while the windings deteriorate further
Install an oversized start capacitor to get more starting torque, because excess current through the start winding overheats it and damages the compressor
Skip checking the run capacitor, the contactor, and the wiring before deciding the compressor needs a start assist device

Frequently asked questions

Does a hard start kit reduce the amp draw on startup?

A hard start kit does not reduce the peak current the compressor draws at locked rotor. HVAC School states directly that a hard start device can do nothing to reduce the current a stalled motor draws. The kit changes the start winding branch, adding current and phase shift there so the motor accelerates to running speed faster and spends less time at high current. A clamp meter that averages over half a second or more may show a lower amp draw with a kit installed, but that reading reflects the shorter duration of the inrush, not a lower peak current.

Will a hard start kit void my compressor warranty?

Whether a hard start kit voids the compressor warranty depends on the manufacturer and the kit. Some manufacturers sell their own branded hard start kits and approve specific aftermarket models. Others state that any aftermarket modification to the electrical circuit can affect warranty coverage if it contributes to a failure. Check the warranty terms for the equipment you are working on and use the manufacturer’s approved kit when one exists.

Can I install a hard start kit on a scroll compressor?

Scroll compressors can typically start against unbalanced pressures without a start assist, because the scroll set starts unloaded rather than running backwards, which Copeland treats as a fault the motor protector stops. An approved kit will not harm a healthy scroll compressor, but HVAC School warns that an unnecessary kit can do more harm than good. Adding a kit to a scroll compressor that is stalling from liquid migration can override the compressor’s own protection and break the scroll set.

A hard start kit helps in conditions like low voltage, long line sets, or a 208V supply that sags under load. It is less commonly needed on a scroll compressor than on a reciprocating compressor, but the decision depends on the starting conditions rather than the compressor type alone.

Is a PTCR the same as a soft start?

A PTCR is not a soft start. A soft start ramps voltage to the motor gradually, reducing inrush current. A PTCR sends a spike of current through a cool thermistor, but HVAC School notes that the PTCR does not create the phase shift a start capacitor does. HVAC School states that a PTCR is a start device, but there is nothing soft about it. The confusion comes from technicians using the term soft start for any device that is not a traditional potential relay kit.

How do I know if a compressor needs a hard start kit versus a new compressor?

A compressor is a candidate for a hard start kit when it is struggling at startup and the capacitor, the incoming voltage, and the wiring all check out. The compressor also has to start and run normally once it gets going, with amp draw within nameplate ratings and no tripping on overload. A compressor has a problem the kit cannot fix when it draws excessive amps at steady state, trips on overload after running for a few minutes, or shows winding resistance out of specification. Measure the run winding and start winding resistance with a multimeter and compare them to the manufacturer’s specifications before deciding between a kit and a new compressor.

Glossary

Back EMF: the voltage a motor generates in its own windings as it spins, proportional to speed
Hard start kit: a device that adds starting torque to a single-phase compressor and disconnects once the motor is up to speed, most often a start capacitor switched out by a potential relay, and in other designs a PTCR or a timer
LRA: locked rotor amps, the current a motor draws when it is stationary at startup
Microfarad (uF): the unit of capacitance, printed on both run and start capacitors, and the capacitance determines how much phase shift the capacitor produces
Non-bleed TXV: a thermostatic expansion valve with a hard shutoff that closes during the off cycle, preventing pressure equalization through the valve
Potential relay: a relay that opens its contacts when the back EMF from the motor reaches a specific pickup voltage, used to disconnect the start capacitor
PTCR: positive temperature coefficient resistor, a thermistor used as a two-wire hard start device that increases in resistance as it heats up
Soft start: an electronic device that ramps voltage to the motor gradually during startup, reducing inrush current, a different product from a hard start kit
Start capacitor: a high-capacitance capacitor designed for brief duty during motor startup, removed from the circuit by a relay once the motor reaches speed

Drafted with AI assistance and reviewed by the author.

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