Hard Start Kits: When a Compressor Needs One and How to Pick It

By HVAC Cheat Sheet Team · 4 min read

A hard start kit is a start capacitor plus a device that switches it out once the compressor is turning. It gives a PSC compressor extra starting torque for a fraction of a second, then gets out of the way so the run capacitor carries on alone.

When a compressor actually needs one

Copeland's guidance for its single-phase scroll compressors is that the scrolls start unloaded even if system pressures aren't balanced, so low-voltage starting is good and start components are not normally required. It recommends start assist for:

  • Weak power supplies. Starting current causes a voltage sag of about 300 to 400 milliseconds. That cuts starting torque, lengthens the start and can dim lights and buzz conduit. Start components shorten the start.
  • System designs that need high starting torque. Start capacitors are used there.
  • Milder voltage drops, or a compressor that hesitates on start. A PTC device is the lighter option.

Older reciprocating compressors, and systems with a TXV that doesn't bleed pressure off during the off cycle, start against unequal pressures and are the classic hard-start candidates.

What a hard start kit won't fix

Before you install one, rule out:

A kit on a failing compressor may buy time, but it hides the diagnosis. Note it on the invoice if you install one as a stopgap.

Types of hard start kit

Type How it drops out Good for Watch out for
PTC (thermistor) Heats up and goes to high resistance Mild hard-starting, piston/cap-tube systems that equalize Needs to cool between starts; short off cycles defeat it
2-wire potential relay kit Relay senses start-winding voltage Most residential PSC compressors; TXV systems Must match the compressor's voltage and HP range
3-wire potential relay with matched start cap Relay picks up at a specific voltage Exact OEM-style replacement Relay pick-up and drop-out must match the compressor
Electronic "soft start" Electronics control the start current Generators, weak supplies, limiting inrush Different job from a hard start kit; follow the maker's charts

How the potential relay works: at startup the relay contacts are closed, putting the start capacitor in the start-winding circuit. As the motor speeds up, the voltage across the start winding (C to S) rises. When it hits the relay's pick-up voltage, the coil opens the contacts and removes the start capacitor. Copeland lists a specific relay (pick-up, drop-out and coil voltage) and start capacitor for each compressor family, so check the compressor maker's data when you're replacing OEM start parts.

Wiring it

Follow the kit's instructions. In general:

  • 2-wire kits connect in parallel with the run capacitor: one lead to HERM, one to C (common) on the capacitor. Nothing else changes.
  • 3-wire kits connect the relay to the compressor's common, start and run circuits and put the start capacitor in series with the relay contacts.
  • PTC devices also connect in parallel with the run capacitor, across HERM and C.

Power off, lock out, discharge capacitors first. Mount the kit so it can't touch the fan or the hot discharge line.

After installing

  • Watch several starts. The compressor should come up to speed quickly and smoothly, with less light dimming than before.
  • Clamp amps on the start: the inrush should be shorter than before.
  • If it still struggles, the problem was something else.

Before adding a kit, rule out the capacitor and contactor: see AC outdoor unit humming but not running.

FAQ

Do scroll compressors need a hard start kit?

Copeland says its single-phase scrolls don't normally need start components because they start unloaded. They're recommended mainly for weak power supplies or a compressor that hesitates on start.

Can a hard start kit hurt a compressor?

A correctly sized kit drops out as soon as the motor is near speed. Start capacitors are built for short duty only, so a mismatched or stuck relay that leaves one in the circuit can fail the capacitor and overheat the start winding. Use a kit rated for the compressor's voltage and size.

Will a hard start kit fix a humming compressor?

Only if the cause is low starting torque. A hum from a dead run capacitor, low voltage or a locked rotor needs that fixed instead.

PTC or potential relay?

PTC devices need time to cool between starts and suit systems where pressures equalize. With a TXV that doesn't bleed off, or short off cycles, use a potential-relay kit.

Sources

Parts:CompressorRun Capacitor
Symptoms:Outdoor Unit Won't StartBlown Fuse or Tripping Breaker

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