Hard start kit

For homeowners

A hard start kit is a small electrical add-on that gives an aging or struggling air conditioner compressor an extra boost of starting torque. It consists of a beefy “start capacitor” and a small electromechanical relay. When the compressor first tries to start, the kit puts the start capacitor in parallel with the existing run capacitor — effectively giving the compressor more electrical “kick” to break free of the static pressure inside it. Once the compressor is spinning, the relay drops the start capacitor out of the circuit so it doesn’t overheat.

For homeowners, this is something to know about but not to install on a whim. It’s the right fix in specific situations: a compressor that hums and won’t start, an aging system that’s slowly losing starting capability, or a unit with a TXV (a particular type of refrigerant valve) that doesn’t equalize pressure well. It’s the wrong fix as “preventive maintenance” on a healthy system — forcing extra current through a compressor that doesn’t need it shortens its life rather than extending it. Cost is around $40-$80 for the part and half an hour of labor. Have a competent tech tell you whether your unit actually needs one before agreeing to the install.

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For technicians

The principle of a hard start kit comes down to a single fact: the start capacitor needs to be in the circuit for only the first fraction of a second of compressor startup, and then it has to get OUT of the circuit before it burns up. A run capacitor can sit in the circuit forever because it’s sized to handle continuous current; a start capacitor cannot. Start capacitors are physically much larger — 100 to 250 microfarads versus a run cap’s 40 to 50 microfarads — but they’re designed for intermittent duty, not continuous. Leave a start cap in the circuit longer than a few seconds and the dielectric overheats, the cap fails, and depending on construction either it pops open through its pressure relief or in worst cases it ruptures.

So the engineering problem is: how do you put the start cap in the circuit for exactly the right duration, no longer? The answer is the potential relay — a device that uses the compressor’s own behavior to decide when to disconnect.

How it works. A single-phase induction motor like the one in a residential compressor has three windings: start, run, and common. During startup, current flows through both the start and run windings, with the run capacitor providing the phase shift that makes the motor rotate. Once the motor is up to speed, the start winding starts generating a back-EMF — a voltage induced by the rotating rotor cutting through the start winding’s magnetic field. This back-EMF is the key. At rest, no back-EMF. Spinning up, back-EMF rises proportionally to speed. At full running speed, back-EMF is significant — typically 200 to 400 volts on a 240V system.

The potential relay is wired to sense this back-EMF. Its coil is connected between the start winding terminal (S on the compressor) and the common terminal (C). At rest, the coil sees zero volts because the motor isn’t spinning. The relay contacts sit in their normally-closed position. As the motor accelerates, the voltage across the relay coil rises. When it crosses the relay’s pickup threshold — typically somewhere between 150 and 350 volts depending on the specific relay model — the coil energizes, the armature pulls in, and the contacts OPEN. The start capacitor is now disconnected from the circuit. The compressor continues running on just the run capacitor.

The potential relay’s NC contacts are wired in series with the start capacitor. The start cap is wired in parallel with the existing run capacitor (both connecting between the S terminal and the contactor side of the line). So when the contacts are closed at startup, both caps are in the circuit together — total capacitance is the sum, and the boost in starting torque is dramatic. When the contacts open, only the run cap remains, and the system is back to its original configuration. The drop-out event happens about half a second to a full second after the contactor closes.

Pin numbering on potential relays is non-intuitive and varies by manufacturer. The most common residential relay (Supco SPP series, MARS series, and the standard 5-2-1 kit) uses pins 1, 2, and 5. Pin 5 and pin 2 are the coil terminals — coil between them. Pin 1 and pin 2 are the contact terminals — NC contact between them. So pin 2 is shared between the coil and the contact, which is why the diagram has two pin 2s. Wiring it up: pin 5 ties to the S terminal of the compressor, pin 2 ties to the C terminal (common), and pin 1 ties to one side of the start cap. The other side of the start cap also ties to S.

The bleed resistor. Most modern start capacitors include a resistor of 15,000 to 20,000 ohms wired permanently across the two terminals. This resistor’s only job is to drain the residual charge from the cap after the compressor shuts off. Without it, the start cap can hold a charge of several hundred volts for hours or days after the system shuts down — a real shock hazard for service techs. The bleed resistor draws a tiny current continuously during operation (essentially zero) but ensures that when the system is off, the cap discharges within about thirty seconds.

PTC thermistor type. The simpler, cheaper, and inferior version. Instead of a separate start capacitor and potential relay, it’s a single device — a disc of ceramic semiconductor material — wired in parallel with the run capacitor. When power is first applied, the thermistor is cold and its resistance is low, so it acts roughly like a wire and dumps a phase-shifted current pulse into the start winding. As current flows through it, the thermistor heats up due to its own resistance, and PTC behavior means its resistance increases dramatically with temperature. Within a few seconds, the thermistor’s resistance has climbed so high that essentially no current flows through it — it’s effectively removed itself from the circuit. The motor continues running on just the run capacitor.

PTC kits cost less than half what a relay kit costs, install in two minutes (just clip across the run cap terminals), and have no moving parts. Downsides: the boost is less precise, the thermistor needs to cool down between startup attempts (so if the compressor short-cycles, the PTC isn’t ready for the next start), and they don’t provide as much actual starting torque as a true potential relay kit with a properly sized start cap. For marginal residential cases where a compressor is “almost” starting reliably, a PTC kit may not be enough. For genuinely struggling compressors, you want the relay kit.

When to install — the right reasons:

The compressor is showing signs of struggling to start. Hum on energizing, internal overload tripping after a few seconds, breaker tripping on startup, very slow spool-up, or audible “rattle” during the first second of operation. These are signs of insufficient starting torque, and a hard start kit can extend the compressor’s life by reducing the time spent at locked-rotor current.

The system has a TXV (thermal expansion valve) instead of a fixed-orifice metering device, AND it doesn’t have a hard start kit from the factory. TXV systems don’t equalize pressure as well during the off cycle as fixed-orifice systems do — there’s a residual pressure differential across the compressor when it tries to start, and it has to work against that.

Long line set (50+ feet) where pressure equalization takes longer. Hard start kits compensate for the extra load.

Low voltage at the disconnect. If the system is at the end of a long run from the panel and seeing 220V or below under load, the compressor isn’t getting enough voltage to develop full starting torque. The right fix is bigger wire and proper voltage. The practical fix on a homeowner’s budget is often a hard start kit.

The wrong reasons:

“Preventive maintenance.” A healthy compressor with a healthy run cap does not benefit from a hard start kit. You’re forcing extra current through the start winding for no reason. Over years, this stresses the start winding’s insulation and can shorten compressor life rather than extend it.

“It’ll lower my electric bill.” It won’t, except in the most marginal case where the unit was previously drawing locked-rotor current for several seconds per cycle. Once the compressor is running, the hard start kit is out of the circuit and contributes nothing to operating efficiency.

“My compressor is making noise.” Hard start kits don’t fix noise problems. Noise is bearings, scrolls scuffing, mounting bolts loose, refrigerant slugging — none of which are addressed by improving starting torque.

Installation. Pull the disconnect outside. Open the access panel. Identify the existing run capacitor and the contactor. Mount the hard start kit assembly somewhere in the cabinet — usually on the cabinet wall with self-tapping screws, or hung from the existing wiring harness with the included cable tie. Connect three wires from the kit: one to the S terminal of the compressor (or the wire going to S), one to the C terminal of the compressor, and one to the R terminal. The exact wiring varies by kit manufacturer — every kit comes with a diagram.

The most common installation mistake is wiring the kit to the run cap’s terminals instead of directly to the compressor terminals. This sort of works but creates additional wire length and voltage drop, and on some systems with internal control boards it causes the board to misread the start winding’s behavior.

Failure modes:

The potential relay coil burns out. The start cap never drops out of the circuit, so the start cap burns up within hours. Sometimes spectacularly. Replace both the start cap and the potential relay. Don’t just replace the cap and reuse the relay — the failed coil is what caused the cap failure.

The relay contacts weld closed. Less common but happens. Same result as a burned-out coil. Replace both.

The start cap fails first. The cap dies of normal aging or manufacturing defect. The relay is fine but the kit is no longer doing its job. Replace just the cap.

The PTC thermistor cracks. PTC kits fail more often than relay kits, especially in coastal Florida humidity. The ceramic disc develops microcracks over thermal cycles and eventually the device either fails open (no boost at all) or fails as a short (puts continuous current through the start winding, eventually killing the compressor).

Diagnostic test. With the unit off and the disconnect pulled, check the start cap’s microfarad value across its terminals. Should match the printed value within 6%. Off-tolerance, the cap is failing. Check the relay’s coil resistance across pins 5 and 2 — should read in the range of 1500 to 15000 ohms depending on the specific relay. Check the relay’s contact across pins 1 and 2 — should read near zero ohms at rest (NC). If you have a known-good 24V test source, apply it across the coil and verify the contacts open.

Cost. A residential 5-2-1 style hard start kit retails for $40 to $80. PTC thermistor kits retail for $15 to $25. Installation labor for either is half an hour or less. Cheap insurance for a marginal compressor — but only if the compressor actually needs the kit. On a healthy compressor, the same money is wasted at best and shortens life at worst.