My AC is 20 years old, and recently stopped working. After some diagnosis I discovered the run capacitor was shot, and ordered a replacement for $20 online, shipping within 3 (very hot) days.

After some research, it appears this is a fairly common component to go out, and had I realized this I would have just replaced it ahead of time.

This is somewhat SOP with cars. We replace functional, but near EOL brakes, water pumps, batteries, and many other components to avoid breakdown at in-opportune times. I have home computers acting as servers, and keep a spare power supply in stock since this is a component that often fails.

Getting the run capacitor was cheap at $20, but it's often hard to find these specialized components locally, especially outside of M-F 9-5 hours. So the question is, what components of a residential AC might it make sense to pro-actively replace, and when? The run capacitor seems like a no-brainier at $20 shipped, but what about the condenser fan, contact relay, or anything else that's cheap to replace ahead of time, but hard to find when you need it now?

These are VERY easy components to replace if you know what you're doing. Honestly it's far easier than even working on a car. So why not maintain these units ourselves, and leave the tough work (entire replacement) to professionals.

  • Most electrical components don't have EOL date. Can keep some things on hand for fast replacement like caps, belts and generic use supplies(wire, connectors). Usually cleaning and lubrication and checking fluids/gases goes further than replacing good components.
    – crip659
    Jun 8, 2021 at 21:03
  • Sounds like more of a procurement problem honestly. Do you know if you hire a pro, they will get the parts much faster than 2 to 3 days. So buy where they do. It seems to me the problem is you don’t know where that is. I would advise HVAC supplyHouses, and Grainger. Or as a last resort, McMaster-carr or GALCO. Jun 8, 2021 at 21:30
  • There you go, ask the pro To sell you the Part. If the pro is backed up with a lot of work, they may be happy for that. Jun 8, 2021 at 21:43

2 Answers 2


The fact is a typical home HVAC system doesn't really have a lot of parts. The capacitor goes out often enough, and its cheap enough that you could make a case for replacing them every 5 years or so. I keep one on hand for quick replacement, but I haven't changed it for no reason. I have got a neighbor's AC running with my spare cap!

The contactor/relay is another part that goes out, but not often enough to replace regularly, and it usually attracts ants or gets dirty, so some basic maintenance could probably keep it working for a lifetime.

The indoor air handler will have a few relays, and a control board, and a fan motor, but I don't think there's anything I would say fails that commonly. Making sure the drain is clean flowing and the air filters are clean is your best bet.


So many different parts to an AC system. Just having someone come out yearly to check Freon and getting the outside coils cleaned or at least checked is a great way to avoid expensive repairs and have your system run to its full capacity. A little maintenance goes a long way when it comes to AC.


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