HVAC Hard Start Kits: How Does It Work?

HVAC Hard Start Kits: How Does It Work?

The AC's compressor is the heart of the air conditioning system. However, sometimes, ACs won't turn on or turn off again after several minutes due to the lack of electricity or a lack of energy to start up the AC. Air conditioning systems then require a hard start kit, which is basically a capacitor -- an electronic component that stores and releases electricity. Hard start kits provide the HVAC Systems with an extra boost of energy when the power turns on.

There are many homeowner videos saying that the capacitor sends a big jolt of energy to help the AC start. It might be true for DC systems, which are direct electron currents flowing in a single direction, however, an AC on the other hand changes its electron direction periodically. DC systems work in cars or stereo systems when a capacitor sends a big chunk of energy to start, but AC circuits behave differently wherein currents switch phases to create a start to the torque.


Start compressors do not send a storage charge to the capacitor. That is because, on the top of the capacitor, the bleeder resistor works to bleed the charge of the capacitor. So in a matter of a few seconds, the bleeder will bleed off the charge off the capacitor.

The bleeder resistor through a relay will bleed off any charge that is in both the start and run capacitor. The start capacitor causes an increase in phase angle (AC) between the start and the run winding creating a greater starting torque. So it is not sending a big charge to the compressor, it’s actually causing a phase shift between the windings to start the torque. Then it will pick up the voltage. It is important to not forget to mount the relays on the correct side up. Capacitors are not made to heat up and run for a long period of time.

A couple more notes on HVAC hard start kits:

  • AC circuits do not behave like DC circuits.
  • Two-wire hard start kits with potential relay are good to use. However, do not use two-wire PTC (positive temperature coefficient), such as the Supco SPP6. Once they get hot once, they do not provide a reliable restart. Suggestion: Two-wire kits made by Kickstart work fine, they only run on two wires that go on the run capacitor and are easy to install.
  • Three-wire hard start kits with potential relay (like the 5-2-1, etc.) are also good to use.

In conclusion, the capacitor does not have a big stored charge to the capacitor to the compressor because there is no charge due to the bleeder resistor. It is the behavior of AC circuits to create a phase shift between the windings and create the starting which is then taken by the relay.

Check out what technicians say about hard start kits.