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?

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    The capacitor is creating a phase shift, the size of the cap needs to match up with the motor size to large of a cap can cause problems also.
    – Ed Beal
    Feb 20, 2019 at 21:35
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    I was curious, so I'm just going to leave this here. The 5-2-1 name comes from the standard numbering of the relay terminals on the relay this type of device uses. Similar to the terminal numbers on automotive relays (85-86-87-30).
    – JPhi1618
    Feb 20, 2019 at 22:02
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    Just curious, where did you even get the idea that this is something you should do? Feb 21, 2019 at 1:15
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    @R.. The last time I had my HVAC serviced the tech (a guy that has always been very honest with me, I know him outside of the HVAC stuff) suggested it to me. He said that he could install it or if I felt like saving $100 I could just pick one up myself. He showed me what to do. Being a handy kind of guy I started looking into it. Feb 21, 2019 at 14:21

2 Answers 2


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

  • After some research it appears that Carrier makes an OEM Start Capacitor that has the same rating as the 5-2-1 kit for systems up to 3 tons. The OEM kit was $76 versus the $30 for the 5-2-1 kit. Since both kits include a start capacitor with the same micro farad ratings and a potential relay it was a no-brainer. After I installed the kit my heat pump started up like it was brand new and it was extremely quiet. There weren't any whining sounds like there were previously. I'm a believer! Mar 5, 2019 at 14:37

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.

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    They also say that a worn run capacitor can make the compressor use 10-20% more electricity vs a new capacitor. Seems like if that was true, installing a new run capacitor would be a regular maintenance task like changing an air filter (albeit less frequently...). Now I'm really curious if there's any truth to the "old capacitor" logic.
    – JPhi1618
    Feb 20, 2019 at 21:56
  • @JPhi1618 Testing the run capacitor is recommended as part of regular service visits.
    – user71659
    Feb 21, 2019 at 3:10
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    Do I understand it correctly that adding a capacitor just for the start actually increases the electricity bill, as it functions as a sort of power factor corrector? (in other words the capacitor gets charged up with a higher power factor than with which the motor would be starting up without the capacitor present?)
    – Pavel
    Feb 21, 2019 at 6:52
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    @Pavel, the capacitor itself has a low power factor as well, only the combination of motor and capacitor has a high power factor if they are well matched. From a cost point of view, this is a Tragedy of the Commons — if everyone kept their power factor low to pay less, the grid would be rather unstable, which is why commercial power contracts usually stipulate a minimum power factor. Feb 21, 2019 at 10:01
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    I am loving this post! Just wanted to add most compressor failures are due to insufficient vacuum at installation. Or blocked coils. I pull down to below 50 microns and hold below 200. Most manufacturers recommend 200 and 1000 respectively. I have never had a compressor fail in 20 years. I only put hard starts on if a TX valve is installed and the manufacturer requires it to maintain warranty, or if I'm buying a season for a customer who can't afford a new one. With the information on this post I'm going to read up a lot more on this subject. Great stuff.
    – Joe Fala
    Feb 21, 2019 at 19:17

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