Guide to HVAC Hard Start Kits

The compressor in your air conditioner undergoes significant strain each time it kicks in to start your AC. Installing a hard start kit can provide the necessary boost to ease its workload, potentially extending its lifespan.

If your AC lacks a hard start kit or you notice it struggling to start, exploring the option of a hard start kit could provide you with additional time before needing to replace the entire unit.

In this guide, we’ll discuss what HVAC hard start kits are, how they work, and the benefits of adding one to your HVAC system.

What are HVAC Hard Start Kits?

Some experts believe the air conditioner uses 4 – 8 times more energy starting up than it does to continuously run. The extra electrical charge required to start up the unit can create heat that causes premature failure of the compressor and other AC parts.

A hard start kit helps your AC kick on with less electrical force. This makes starting the unit easier and can potentially increase startup efficiency. Once started, it is much easier for fans and pumps to keep running, but going from a high temperature to the desired low temperature puts a lot of pressure on the system.

If you install a kit on your compressor, you get more torque upon startup but put less stress on the AC unit itself. After starting up, the capacitor has a relay that removes it from the circuit.

Hard start kits are only for single-stage air conditioners, though. You likely have a single-stage air conditioner if your home is at least ten years old. However, a hard start kit is unnecessary if you have a two-stage air conditioner because they have a different electrical setup.

Benefits of Using Hard Start Kits

There are many benefits of using an AC hard start kit, including the following:

Higher Compressor Efficiency

Compressors use the most energy when starting your air conditioner. This extreme energy use is bad for the environment and puts more pressure on the system itself. A hard start kit helps the AC turn on faster, therefore using less energy and keeping your repair bills more affordable.

Lower Utility Bills

When you use less energy, you have lower utility bills. A hard start kit helps your AC turn on faster, so you use less energy overall.

Extends the Life of Your AC

An AC hard start kit prolongs the compressor’s life, which is the heart of your AC system. When the compressor goes, you typically need to replace it along with other major parts, sometimes making AC replacement a better option. Installing a hard start capacitor can prevent premature AC failure.

Less Noise

When compressors are struggling, they can be very noisy upon startup. You may hear clicking noises, loud whirring, or excessive vibration. With a capacitor kick, you can decrease the noise and have more peace of mind about your AC’s operation.

Fewer Maintenance Costs

Since the kit helps your compressor work more efficiently and puts less pressure on it, you may decrease your AC repair or maintenance costs. While we always recommend an annual inspection for your AC unit, knowing that it runs more efficiently and may have fewer problems can keep more money in your pocket.

Determining if Your HVAC System Needs a Hard Start Kit

It’s a good rule of thumb that if you purchase a new single-stage AC unit, you should also purchase a hard start kit. Saving your compressor from the start can give you the most time out of your AC unit before it needs repair or replacement.

If you have an existing AC unit, here are signs it may be hard starting and could use some support:

  • Turns on and off frequently: If your AC doesn’t stay on or turns on and off frequently, it’s a sign of a faulty compressor that needs help.
  • Clicking noises upon startup: Your AC unit should be somewhat quiet. If you start hearing clicking noises when it starts up, it could mean the compressor is under too much pressure and needs support.
  • Tripped circuit breaker: If your circuit breaker trips frequently when the AC starts, it’s a sign the unit uses too much energy to start up.
  • Flickering lights: Your AC unit may not trip the circuit breaker, but if it causes the lights in the house to flicker each time it turns on, it could be using too much energy. 

If you’re unsure if your AC unit could benefit from a hard start kit or if you should install one on a new air conditioner you are purchasing, consult with an HVAC professional for a complete assessment.

Types of Hard Start Kits

Your air conditioner manufacturer will likely specify the type of hard start kit your AC can handle. Here are the two most common options:

  • PTC Hard Start Kit: The positive temperature coefficient (PTC) kit consists of a ceramic component and two wires. It heats up quickly because its low temperature and resistance allow more current to flow through it.
  • MPR Hard Start Kit: The mechanical potential relay hard start kit uses electro-motive force to start the AC unit but removes itself once the compressor is at 80% capacity.

It’s best to let an HVAC professional determine which option is best for your AC unit.

Installation Process

Hard start kit installation can be a DIY project if you’re handy and can carefully follow the manufacturer’s instructions. However, we always recommend consulting a professional to ensure it’s done right.

If you decide to DIY the installation process, here’s a high level process:

  1.   Shut off the power to the AC unit
  2.   Discharge the capacitor on the outside of the condensing unit
  3.   Remove the capacitor’s wires and remove it from the mounting bracket
  4.   Install the new hard start kit by attaching it to the mounting bracket in the old capacitor’s place
  5.   Connect the new wires from the relay to the compressor motor
  6.   Connect the wires from the start capacitor to the relay
  7.   Connect the final wires to the power source
  8.   Turn the power back on

Before testing it, give the compressor time to cool down. When ready, start the AC unit and listen for any unusual noises or difficulties with starting the unit. If all goes well, you’ve successfully installed a hard start kit. 

Maintenance and Troubleshooting

While hard start kits can temporarily revive an aging air conditioning system, they also require some maintenance and may present their own set of issues that need troubleshooting.

Maintenance Tips

  • Check the hard start kit capacitor annually and replace it if it shows signs of bulging, leaking, or deformation .
  • Ensure all electrical connections to the hard start kit are secure and free of corrosion.
  • Keep the area around the hard start kit clean and clear of debris or obstructions.

Common Issues and Troubleshooting

  • Hard start kit not engaging: This could be due to a faulty capacitor, loose wiring, or an issue with the potential relay. Check connections and test the capacitor with a multimeter. Replace components as needed .
  • Compressor still struggling to start: If the compressor labors excessively even with the hard start kit installed, there may be a deeper mechanical problem requiring compressor repair or replacement.
  • Excessive noise or vibration: Loose mountings or an unbalanced compressor can cause the hard start kit to vibrate noisily. Tighten mountings and inspect the compressor for issues.
  • Premature hard start kit failure: Electrical spikes, brownouts, or incorrect sizing can damage the capacitor or relay. Ensure proper sizing and consider installing a hard start kit with surge protection .

Regular maintenance and prompt troubleshooting of any hard start kit issues will help maximize the lifespan of your AC system. However, keep in mind that a hard start kit is only a temporary solution before a full system replacement is required.

Cost Considerations for Hard Start Kits 

Hard start kits are typically affordable and cost much less than compressor replacement. On average, homeowners pay $300 – $350 for the kit and installation. It’s a good idea to get quotes from a few HVAC professionals to determine who offers the best price.

Because the investment in a hard start capacitor is so low, it can provide an incredible return on your investment because you don’t have to worry about compressor replacement, at least for the time being.


If you’re looking for ways to make your AC unit run more efficiently and last longer, consider installing a hard start kit. These kits only work on single-stage air conditioner units and can be installed after you’ve had the unit for a few years or when it’s brand new.

The key is to get an opinion from an HVAC professional who can determine if your unit will benefit from a hard start capacitor and how much life it will give you before replacing your compressor or AC unit.


What does a hard start kit do?

A hard start kit bypasses the compressor, giving it more support upon startup. This helps lengthen the life of your compressor and AC unit as a whole. A compressor costs an average of $2,500 to replace, so prolonging it as much as possible is key.

Do you really need a hard start kit for AC?

Newer AC units (under 5-7 years old) with properly sized and functioning components generally do not require hard start kits. If you have a single-stage AC unit, a hard start kit can extend its life. The average new AC unit costs $5,900, so anything you can do to extend its life is worth it.

Can a hard start kit damage a compressor?

If the compressor is already damaged or faulty, a hard start kit may not be able to overcome the issue and could potentially put extra strain on the compressor.

If the hard start kit is improperly installed, it can damage your compressor. To ensure it works properly, it’s best to leave the installation to professionals. 

How long will a hard start kit last?

You can usually buy a few more years of life on your AC unit after adding a hard start kit. For very old units (15+ years), installing a hard start kit may only temporarily extend the system’s life before full replacement is necessary. However, how long it lasts depends on your current unit’s overall health and age. 


George Brazil HVAC 

Indoor Temp 

911 HVAC 

Leonard Splaine

Air Care AZ 

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