Central Air busted....

I have a cac unit where the transformer works, but I can't get the fan to kick on, or condenser. It worked last summer.

Checked breakers, checked service disconnect was in the on position and the fuse wasn't blown. Swapped tstat with known good, nothing.

The unit is 20 years old and capacitor was replaced 6 years ago.

Should I just buy a capacitor and contactor and replace them? Any other things to have on hand anyway? I've never hvaced. Is the contactor just a relay?

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  1. 4 weeks ago
    Anonymous

    >20 years old
    don't get me wrong I love milking things but I would get a new unit. energy savings alone will pay for it.

  2. 4 weeks ago
    Anonymous

    You haven’t given any useful information to go off of. Is this a split system AC in a residential home? Neither the indoor unit or the outdoor unit are running?
    >Go outside and take the cover off the electrical, mash the contactor in with a sharpie
    If it comes on you’re losing your 24v signal that energizes the contactor. Could be due to a bad board and would explain why nothing is running even though you have power. Hard to say with the piss poor info you gave

    • 3 weeks ago
      Anonymous

      It's a residential air handler/ condenser unit, not a mini split.

      I had a local hvac company come out and diagnose it, the guy was basically saying pull your pants down and bend over.

      >hey anon, your transformer is blown
      >ok, want to replace it?
      >the price of the transform is $679 USD

      I told him to get fricked and ordered one from the local supply house for 30 bucks.

      Installed new transformer, fan now operates normally but when I try to run the AC the output voltage from the transformer goes from 24v to 8 and it's getting super hot, fan shuts off and nothing happens.

      What are the logical next steps outside of total replacement of the outside unit? I'm obviously not calling the HVAC company again.

      • 3 weeks ago
        Anonymous

        you have a low voltage short. check all the 24v wires, you have bare wire touching the metal somewhere. probably a safety on the condenser if it only happens when you have an ac call.

  3. 3 weeks ago
    Anonymous

    It's totally your capacitor. Plus a control wire fell out of a wire nut and is shorting to the chassis.

  4. 3 weeks ago
    Anonymous

    The bundle of low voltage wires going to the outdoor unit is usually brown sheathing. Check all those wire nuts where they connect inside the electrical panel. Check the wires going to the outdoor contactor coil. $675 for a transformer holy shit

    • 3 weeks ago
      Anonymous

      Yeah I'm in business for myself and understand you need to make money, but you can't become a millionaire off every customer. But I live in a rich area surrounded by people who have never picked up a screwdriver so good for them if they get it.

      Thanks for the tips I'll check everything in the morning.

    • 3 weeks ago
      Anonymous

      As you suspected I had a short somewhere.

      There were 3 sets of wires going to the control board and I think my foreign born subcontractors tramped the wiring to my stat when they put a bathroom in, added a new stat cable, but for some reason left the original in.

      Also, the wiring inside the stat was all bare, so i redid that.

      No precipitous voltage sag now when calling for cooling, but the compressor outside hasn't kicked on yet.

      • 3 weeks ago
        Anonymous

        OK, I think I figured it out.

        When the y and c to the compressor aren't connected to the board everything runs fine, once they are connected, nothing works.

        I popped the panel and tested resistance between the contactor leads, and there is none, so it's a dead short on the contactor coil that caused it.

        I'm ordering a contactor and replacing it when it gets here.

        Thanks for all your help anons and other anon who recommending I replace the entire system without even diagnosing it, you're the reason shops that charge $679 for a transformer exist. Enjoy your life of poverty.

  5. 3 weeks ago
    Anonymous

    OP again.

    I disconnected the old contactor, and tried the system with just the contactor connector, same result. Voltage drops to 0 now tho when Y and C and connected to the contactor and we're calling for cooling. Even when the contactor isn't connected, this happens.

    Without the y/c wires connected, I'm getting 26v off the board screws.

    I'm going to jump the contactor inside directly to the board, see if that messes it up, and then start eliminating points of failure on the condenser.

    What points of failure on the condenser should i be looking for?

    • 3 weeks ago
      Anonymous

      >I'm going to jump the contactor inside directly to the board, see if that messes it up, and then start eliminating points of failure on the condenser.

      Report what happened with this first.

      • 3 weeks ago
        Anonymous

        Contactor pulls in directly off the board.

        There's a giant mess of wiring that goes from the red lead back into the house, I think it was for a security system.

        Can just disable that and wire the y/c straight to the contactor?

        • 3 weeks ago
          Anonymous

          Heres the unit, notice red wire not connecting straight to the contactor, goes back in the house to the old security system I think.

          • 3 weeks ago
            Anonymous

            Pic related

  6. 3 weeks ago
    Anonymous

    Op again, disabled all the extra bullshit, put new contactor in, I'm in business. Nothing is shorting and it's getting cold.

    • 3 weeks ago
      Anonymous

      Thanks for the update.
      Glad you got it going.

      • 3 weeks ago
        Anonymous

        Everything is buttoned back up and as every boomer car ad states

        AC BLOWS COLD.

        Thanks for the help, gents.

        • 3 weeks ago
          Prepare

          You might want to get a soft-start for it.

          There's a number of units on amazon. $150 for spartan power for up to 1-1/2 ton 20k btu, to $210 for hyper engineering brand that handles up to a 7 ton, 84,000btu unit.

          Greatly reduce the power surge on start-up.

          • 3 weeks ago
            Anonymous

            I like in the northeast, so run the ac like 3 months a year, if that. I'll roll the dice, but thanks for the tip anon.

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