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I've got a 2.5 ton HVAC system. I've been looking at adding one of the 5-2-1 Hard Start kits to reduce the wear on the system. I see that they have recommended sizes for 1-2-3 ton, 4 ton, and 5 ton systems.

Is there a reason I can't use the 4 ton kit on my 2.5 ton unit? Would it not just supply more support?

Does your compressor start now? If it does, adding a hard start kit does absolutely nothing beneficial for you and in fact might be harmful. The original intent of a hard start kit was to compensate for voltage drop in the wiring by the time it gets out to a compressor, especially in areas where the utility supply may drop during high use days (i.e. all of your neighbors are home using their AC systems). But for the people who make them, that was not enough sales volume, so they "invented" a problem needing a solution. Their claim is that a compressor starts with high current and that this is damaging the compressor; which is an utter falsehood. The 5-2-1 sales literature starts out with a LIE! It states that "Amps x Volts = wattage" and that wattage is what you pay for. So per their concept, when the starting current goes to 600% (they say 700% witch is an exaggeration), the watts go up so high that the motor over heats. The lie is that watts = amps x volts x POWER FACTOR, and when a motor first starts, the power factor is extremely low. So for example if your 230V motor draws 10A when running, and the PF = 80%, the watts used is 230 x 10 x .8 or 1840W. On starting though, it is going to be 60A x 230V, but the PF is .2, so the watts are 2760, not 13,800 as they would claim.

What the "hard start" kit does is add another capacitor to the motor only for start-up, which compensates for the low power factor, then automatically switches it out of the circuit when the motor gets to 80% speed, usually less than 1 second. It ALSO increases the voltage, which increases the torque, which can be potentially DAMAGING to the compressor. They don't tell you that part!

Hard start kits have a legitimate purpose, but making your compressor last longer when it is not having trouble starting is bogus.

As to your question; yes, if you use a hard start kit intended for a larger motor, it can INCREASE that potential for excess torque damage.

The overload relay is a protection device used in the compressor circuit. Power is applied to the compressor motor windings through the overload device, and the relay is used to add the start winding. The start capacitor provides increased starting voltage to the compressor windings, until the compressor is at running speed.

The sizing of both the start and run capacitors is critical to motor efficiency. There is a maximum of +10% tolerance in micro farad rating on replacement start capacitors. A capacitor that is too large can cause energy consumption to rise.

Starting up a compressor requires 4–8 times more electric current than running it. Over time, this jolt of power, which creates a surge of heat, can damage the compressor.

The name "Hard Start Kit" implies that it makes your system start harder. When in fact, it is designed to repair a system that is already hard starting. A hard start kit shortens the startup period of the compressor and reduces the amount of electricity it takes to start your AC. In fact, with a hard start kit, the electrical efficiency of the startup process is much higher than without one. This efficiency reduces heat and wear on the compressor and other important AC components. Most manufacturers of scroll compressor systems have hard starts already installed in them.

There are many opinions regarding a hard start. Some believe that the excessive power can damage the compressor motor by providing too much power. Others believe the excessive power actually causes movement in the motor. But there are some convincing test results that you can find that prove the 5-2-1 actually does a great job.

If your compressor is near the end of its life, then a hard start kit may prolong its life for a short time, but shouldn’t be viewed as a long term fix.

Signs that your compressor is hard starting:

• AC starts, then quickly shuts off after you turn it on: This is
  called short-cycling, and it means there’s something wrong with your
  compressor.


• You hear clicking noises when the compressor starts: A clicking sound
  points to a problem with your compressor, which is using too much
  power to turn on (hard starting)


• Your compressor trips your circuit breaker: This means your air
  conditioner is using too much energy to start, so your circuit
  breaker trips before any serious electrical damage occurs. This is a
  symptom of hard starting.


• Your lights flicker when you turn your AC on: This is normal to a
  certain extent, but if you notice a dramatic flicker then it means
  your AC is drawing too much power to start, which is a symptom of
  hard starting.

The starting capacitor is the largest difference in the various 5-2-1
devices. You don't want too large of a starting capacitor. You need
to use the correct one for your application.

FYI: Multiply the load amps by 2,650. Divide this number by the supply voltage. The resulting number is the capacity of the capacitor you need in microfarads (µF). Add 10% of this number for the maximum microfarad