My air conditioner compressor wasn't functioning properly recently. The compressor wouldn't start when the start relay energized. A HVAC technician diagnosed the problem nebulously as "It needs a hard start kit because the compressor is getting old and tired."

The compressor is a machine. It doesn't get tired. I am asking for a precise description of the components in a hard start kit and how they're connected to the compressor.

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    There is another product term, "soft start", which can be used in solar PV or generator situations. Maybe this is the same thing, but I don't know. The idea is to reduce the in-rush current that occurs when the compressor starts up, and limited power supplies (like those mentioned) can fail to work with AC compressors.
    – donjuedo
    Aug 6, 2019 at 13:18
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    hard start kits are more expensive and thus have more profit than just replacing a $10 capacitor, that being said it takes about 5 times the power to start an AC unit than it does to run it, and the additional heat generated in the motor is not good for it, so you want it to start as fast as possible Aug 6, 2019 at 23:56
  • @RichieFrame I understand your comment on price & profit motive. I also understand start inrush current (though I understand the first moment's inrush to be many more times higher than five). The joule heating resulting from the integral of the current over time says minimize the overall power (the current and the time the current is applied).
    – ndemarco
    May 18, 2020 at 14:10

2 Answers 2


Residential A/C compressors are generally single phase motors and all single phase motors do not naturally rotate on their own, they need something to START them rotating. There are numerous ways to do that, each with its own set of advantages and disadvantages. For A/C compressors, they use what's called a "Capacitor Start" method wherein a capacitor is put into the motor circuit for the first second or so in order to get it to start rotating, then once it has begun rotating in the correct direction it is taken out of the circuit again with a device called a "Potential Relay" (in most cases) that knows when the motor gets to speed.

A "Hard Start" kit is just a bigger capacitor for the motor and new a Potential Relay suitable for that bigger capacitor. The working principle behind his recommendation is that your existing motor starting capacitor was sized for when the motor was new and fresh, now the bearings are wearing and there is more friction, so that original capacitor is not strong enough to make the motor begin rotating. Some people also seem to believe that capacitors get "weak" over time (they don't; they either work or they fail, there is no "weakening").

There is some debate as to the validity of that concept though, because motor people know that over sizing a capacitor does not make up for bad mechanics and in fact comes with a new set of problems, otherwise the motor mfr would have just used a larger capacitor to begin with. The original purpose of a "Hard Start" kit was actually intended to deal with situations where your line voltage is too low for the motor to start, so the larger capacitor gave it a boost. Sometimes it can be true that when your A/C was first installed, the line power was higher and now so many people in your area have added A/C units and/or other big loads that it is causing a voltage drop that you didn't used to have. So my theory is that because adding a Hard Start kit will fix that problem sometimes, some HVAC technicians have taken to looking at them as the "cure-all" for any sort of problems. But if your compressor is truly wearing out, a Hard Start kit will, at best, only prolong the inevitable. It might get you through the summer, but be prepared for having to have the compressor replaced sooner than later.

Unless you can determine that your line voltage is indeed lower than normal. Your utility or an electrician can tell you that.

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    Capacitors certainly can get weak over time; I don't know if motor capacitors do, but there's a reason people often have to replace them in very old electronic equipment. Aug 6, 2019 at 6:23
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    To elaborate on @immibis' comment, some capacitors in audio equipment have a liquid or a gel as one electrode (called electrolytic capacitors) which slowly dries over time (1, 2) which in turn leads to gradual decrease in capacity. If you have an electrolytic capacitor in your starter, it may suffice that you replace it with a fresh one of same specification.
    – Pavel
    Aug 6, 2019 at 13:39
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    The comment "Capacitors certainly can get weak over time" drove me to ask a separate question on that. Can they? If they do, what is their effect on a motor? Does a weaker (lower value?) capacitor cause a motor to run hotter?
    – ndemarco
    Aug 7, 2019 at 12:17
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    @Pavel: Electrolytic capacitors which are left holding a voltage near their withstand voltage will build up a dielectric layer that will have the effect of increasing their withstand voltage while decreasing their capacitance. From what I've read, this is in many cases part of the manufacturing process (start with low-voltage high-capacity caps, and then build up the dielectric layer until the parts behave as required).
    – supercat
    Aug 7, 2019 at 15:41
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    Electrolytic capacitors "deform", meaning the oxide layer on the film dissolves back into the electrolyte, but only if they have not been energized for a long period of time, as in 2 years or so. Every time they are energized, that oxide layer is formed anew. The danger with old capacitors is that the oxide layer, if already dissolved, takes too long to re-form and when energized fully before that happens, the layers of the capacitor film will burn through. But on a motor starting capacitor where the motor is started frequently, they can last years with no significant decrease in performance.
    – JRaef
    Aug 7, 2019 at 19:56

"Tired" is a figure of speech, it means that performance has sagged from as-new state, which is an inevitable behavior of most machinery. Age alone is a factor (particularly for capacitors) but usage is also a factor. It's possible that what's "tired" is the capacitor, because that is normal for capacitors.

Since a hard-start kit goes where a capacitor goes, it might be the case that the capacitor is simply "tired" and he is unwittingly obscuring this fact by going straight for the hard-start kit. It might be that simply replacing the capacitor would have the same effect. Those are sold by Galco, Grainger and others.

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