What Is a Hard Start Kit? Does Your Air Conditioner Need a Hard Start Kit?

Did you know it takes 4 to 8 times more electrical current to start your central AC unit than is required to run it? The excess heat generated during startup is damaging to your central air conditioning system compressor. A hard start kit can benefit you if:
  • You want to extend the life of the HVAC system
  • Your air conditioning system is aging
  • You have a home generator
  • Low voltage or inconsistent voltage
  • Lights dim when AC unit starts
  • Compressor won't start/Humming
  • AC system has a TXV metering device
  • Multiple AC systems running at once
  • Air conditioner trips breaker on startup
  • Long refrigerant line sets ( multi-story buildings etc.)

What is a Compressor Hard Start Kit?

A hard start kit is a start capacitor and a potential relay or PTC ( positive temperature coefficient ) device. They are designed to give your air conditioning compressor an electrical boost upon startup.

What Does a Hard Start Kit Do?

A hard start kit helps your AC compressor start up to 10 times faster and more efficiently. It helps overcome the mechanical inertia upon startup. The first couple of seconds for any electrical motor/compressor is when the most strain electrically is placed upon it. Amperage spikes momentarily on startup then drops to normal operating levels. It is this amperage spike that tends to create problems. The excess amperage equals excess heat that can be damaging to your electrical system in your air conditioning system.

How Does an HVAC Hard Start Kit Work?

A hard start kit provides greater starting torque and reduces the duration of inrush current to the single-phase compressor. The start capacitor of the hard start kit is wired in parallel with a normally closed potential relay or PTC thermistor with the compressor run capacitor. Once the circuit is energized, the start capacitor alters the phase angle ( think of it as electrical leverage) between the start and the run winding. Once the compressor is running, the start capacitor drops out of the circuit leaving only the run capacitor to keep the compressor running. The start capacitor is only in the circuit for a fraction of a second. ** Before any hard start kit is installed, the compressor run capacitor must be checked to confirm it is still within its designed, rated specifications.

Types of Air Conditioning Hard Start Kits:

Air Conditioning Hard Start Choices
  • PTC Hard start kit - This style of hard start uses a ceramic element to take the start capacitor out of the circuit. As electric current flows through the PTC element, it begins to heat up to approximately 250 degrees. The electrical resistance in the device increases and opens the circuit, and the start capacitor is taken out. All this happens in less than a second. There are a couple of potential problems with this style of hard start kit. Every time the PTC hard start is energized, it must cool down ( 2 to 3 minutes ) before it can energize the start capacitor again. Also, this style device is "dumb" meaning it provides a fixed boost - not sensing if the compressor actually started and may stay in the circuit a little longer than necessary or drop out prematurely.
Example of PTC Hard Start Kit

Supco Hard Start Kit with PTC Relay

  • Mechanical Potential relay hard start kit - ( Recommended ) This style hard start kit uses a potential relay to take out the start capacitor of the circuit upon startup. Once the AC compressor is at approximately 75% to 80% its full operating speed, back EMF (electro-motive force) generated by the compressors rotor creates a magnetic field that opens the relay, taking the start capacitor out of the circuit. Once the compressor stops- the relay closes again and is ready for the next cycle. Potential relay hard starts are more precise than PTC hard starts because it drops the start capacitor out of the circuit predictably every time and doesn't need to cool down to reset.
Example of Mechanical Hard Start Kit

Hard Start Kit with Mechanical Relay

If a Hard Start Kit is so Important, Why Aren't They Installed at the Factory?

Years ago, every single phase residential air conditioning unit did have hard start kits from the factory. But in efforts to reduce the cost of manufacturing, they have been eliminated in most units.

Considering getting a hard start kit? Not sure if your system already has one? 

Air Zero Sells AC Hard Start Kits

Contact the HVAC professionals from Air Zero. 727-392-6111 Air Zero is located in Largo FL. and provides air conditioning services to all of Pinellas County Florida.