What Is An AC Hard Start Kit And Why Its Importance?

Posted by: Mas Broto
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Was there ever a moment when your lights dimmed or flickered when you turned on your Air Conditioner? This is because of the intense power surge during an Air Conditioner start-up. In fact, upon starting, the air conditioning compressor requires four to eight times more electrical current than what it needs when it is running.

This sudden outburst of inrush current creates a surge of excess heat. It can cause heavy strain and damage your central air conditioning compressor, terminals, windings, and run capacitor.

Most single-phase air conditioning compressors used nowadays have low starting torque because of the elimination of factory-installed hard start devices. In the past, all single-phase compressors were equipped with hard start devices, but it was discontinued to cut cost (possibly in exchange for efficiency).

What is an AC Compressor Hard Start Kit?

A hard start kit, also called a compressor saver, is an electrical component installed in your air conditioning condenser. It works as a combination of a capacitor and a potential relay or PTC (positive temperature coefficient) thermistor. It provides a strong starting torque to help reduce strain due to excessive inrush current upon the compressors’ start-up.

What does it do?

Hard Start Kits are designed to give your air conditioner compressor an electrical boost during hard starts. When it is struggling to start or shut off abruptly upon starting. This happens under very adverse ambient conditions such as low voltage or high head pressures.

With a hard start device installed, your AC system can start and operate more smoothly, quickly, effectively, and efficiently by using less power during the start-up process. Here, you’re looking at 10 times quicker and 98% more electrically efficient start-up than compressors without a hard start device that has an electrical efficiency rating of less than 50%.

And How does it work?

As mentioned earlier, a hard start device combines a start capacitor and potential relay or PTC thermistor. Once your AC starts and the circuit is energized, the start capacitor changes the phase angle between the start and the run windings.

The start capacitor is then only removed from the circuit as soon as the compressor reaches 80% full speed, leaving only the run capacitor to keep the compressor running.

The voltage generated on the compressor windings will energize the potential relay while ensuring proper pick-up and drop-out voltage. This helps the start capacitor stay in the circuit for the shortest time while also ensuring that the compressor has started.

Benefits of an AC Hard Start Kits

Here are the benefits of having a hard start kit installed in your air conditioner:

Lengthens Compressors Life

By assisting compressors during hard starts and harmful ambient conditions, they can operate easily and quickly. This process reduces the strain on the compressor windings and helps extend their product life.

Protects Your Air Conditioner

With smooth and easy start-up provided by hard start devices, you can expect that not only your compressor is protected. But other parts and components of your AC also aid in extending the life of your unit.

Protects Other Electrical Devices

Installing a hard start kit will help avoid the current shortage on your other electrical devices every time your AC starts. This can be manifested by flickering lights and fluctuating other device’s operations.

Saves Energy Money

Since hard start kits help compressors start efficiently by reducing start-up inrush current, it uses less energy. This will ultimately lower your electric bills and lets you save energy money.

Types of an AC Hard Start Kits

Mechanical Potential Relay Hard Start Kit

This is the most common type of hard start kit. It uses a potential relay to remove the start capacitor on the circuit once the AC compressor is approximately 75% to 80% of its full operating speed.

It works when the compressor’s rotor generates the back EMF (electro-motive force) to open the relay to take the start capacitor from the start circuit.

Once the compressor ceases to operate, it is already ready for the next cycle; no time for cool-down is needed.

PTC Hard Start Kits

PTC hard start kits use a ceramic element to remove the start capacitor from the circuit upon unit start-up instead of using a potential relay.

This works just like the first type, but instead of the EMF opening the relay and removing the start capacitor, the ceramic element heats up to 250 degrees, opens the circuit, and takes the start capacitor out. Because of the great amount of heat generated, PTC hard start devices need downtime to cool down for 2 to 3 minutes before being effectively used again.

PTC hard start kits also do not have intelligent sensing ability. Unlike Mechanical Potential Relay Hard Start Kits, PTC may have the start capacitor stay longer. Even though the compressor has already started or prematurely takes the start capacitor out even before the capacitor starts.

Why do you need a Hard Start Kit?

An AC compressor needs A LOT of electrical juices to move a large number of refrigerants and gases around so your AC can provide your home fresh and cool air. Unfortunately, over time, this so much power (and so much heat) can wear and damage an AC compressor and even other components of your AC.

That’s why you need a hard start kit. A hard start kit provides your compressor that much-needed boost during start-up. Here are some specific examples that require immediate hard start kit attention:

A Frequent Hard Starts

Signs of a frequent hard starts:

  • Short-cycles or when your AC turns off abruptly after starting
  • Loud clicking sounds when the compressor uses too much power while starting
  • The circuit breaker is tripping to prevent further electrical damage. This happens because your AC uses so much electrical current while starting.
  • The lights flicker every time you turn on your AC.

When you notice some of these signs, your compressor is obviously struggling to start and may need a hard start kit to perform better.

If your compressor is very old, installing a hard start kit is only a quick fix and not a permanent solution. The best option is a new Air Conditioner since it’s more cost-effective to do than just a compressor replacement.


As mentioned above, hard start kits are not pre-installed in the factory, unlike before. Therefore, though the AC units have become more affordable, they are not as efficient and cost-effective as those with factory-installed hard start kits.

It is recommended to consider purchasing and installing a hard start kit into your new AC. With that, you can reap countless benefits that will make your life better and easier in the long run.

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Mas Broto
Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.