Start Assist Kits vs. OEM Hard Start Kits

To start off, why do we even need start gear on a compressor in the first place? It’s a good question and I will try to answer it.

A compressor is designed with a certain amount of starting torque which allows it to come up to proper speed and start pumping.  When there are conditions present at the job site that don’t allow the compressor to come up to proper speed quick enough, the compressor stays in “locked rotor amps” longer than it should and eventually trips out on its internal overload.

So what are the conditions that can cause the compressor to not come up to speed quick enough?

  1. Unequal pressures  between the suction and liquid side of the compressor
    1. usually found when a TXV is installed on the system
    1. Even though compressors are rated at 208/230 volts, they operate best at the 230 volt range.
      1. At 208 volts or lower, the torque is reduced and it takes longer to come up to proper speed.
  3. LONG LINE applications (over 75 feet)
    1. because of the added liquid refrigerant that has to be pushed through the system, the compressor has additional torque applied to it.
  4. The use of low ambient controls (fan cyclers).
    1. Starting any compressor in low ambient conditions should always have a crank case heater and start kit.

Now we know why hard start kits are needed, what type of start kit should you use?

The BEST start kit is to always use the recommendations of the compressor manufacturer or OEM kit.  This consist of a potential relay and a start capacitor sized for the starting torque requirement of that specific motor in the compressor. The potential relay has specific pickup and drop out voltage so it comes in properly and drops out the capacitor within the specification of the manufacturer.  Most contractors shy away from these because it does require doing some wiring of the components into the circuit.

So to avoid doing extra wiring and to make the installation easier, contractors use start assist kits that just “piggy back” onto the run capacitor.  There are many types of these kits out there and you need to look, again, at the application.  Some increase starting torque 300%, 500% or 600%. So which do you choose?

For the average residential application with a TXV on the indoor coil, you can probably use the one that increases the torque 300% for most.  But when you have TXV’s, long line set, or any of the other applications above, you should go to at least the 500% or 600% torque increase.

We handle Supco SPP start assist kits through our Source 1 parts.  Attached is information on the 3 different SPP start assist kits available through Source 1. 

Keep in mind, the BEST is still the OEM compressor manufacturer’s recommended Hard Start Kit but, if you choose the proper start assist kit, in most cases, you should not have problems starting compressors.

Start Assist Kits Specs


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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