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I've got an ancient Armstrong Concept 1000 air conditioner (it was old when we bought the house 11 years ago) hooked up to a Nest thermostat. The furnance and blower is much more modern; it's an Aire-Flow model from 2014.

The A/C cools the house fine during the day, but in the evening, the unit in the basement will suddenly start making "bump" sounds, and the house blower will stop working. It seems to start doing it reliably just after sundown; we joke that it's "bump o'clock" at about 9 PM. If I go downstairs, I can hear a relay repeatedly tripping while it bumps. The unit outside appears to be operating normally.

The house's blower motor works fine if I turn off the A/C and just run the fan.

If I turn the A/C off for a half hour or so and let it come back on, sometimes it'll start working again. Sometimes not.

We installed the Nest probably three years ago; the bump sound has been happening for the last year or so.

Any idea what might be happening? Is this a thing where the coils are freezing up? It seems like it's still pretty warm out for that to be what's going on...

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  • What model AC is it?
    – ThreePhaseEel
    Jun 11, 2017 at 22:13
  • Did you have this issue prior to Nest installation?
    – Jeff Cates
    Jun 12, 2017 at 6:01
  • Added info in original post.
    – Nate
    Jun 12, 2017 at 15:41
  • Here's a guess, which I offer in the hope it might be useful but without much confidence. You have an electrical problem. Because the problem occurs at a consistent time it is more likely due to something outdoors than indoors. The "bump" sound is your blower turning on briefly, then losing power, then turning on again (i.e. the loudest and highest-current-draw part of running a motor). Perhaps the electronics in your condenser have a short, and this is interfering with the startup of your blower.
    – Shimon Rura
    Jun 13, 2017 at 22:18

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Okay. I've finally figured this out after two fruitless visits from the HVAC service people, a new logic board, and new blower motor capacitor. Warranty jobs, fortunately.

It turns out the culprit was our Nest thermostat. It was hooked up without a C wire. In this configuration, to charge its battery, it periodically closes the Heat circuit for very short lengths of time. Furnaces see that short signal and generally ignore it. With our old furnace, it was no problem with either heat or AC; everything ran fine.

However, with our new furnace (an Aire-Flow, in case you're wondering), when the AC was blowing, having the Nest close the heat circuit freaked out the logic board, which responded by stopping the blower motor while leaving the AC compressor running. I'm not sure why it seemed to happen more at night — maybe something about the times the battery charged?

Anyhow, the solution was to connect the C wire (for whatever reason, the furnace installers didn't) both in the furnace and in the Nest, and everything seems to be working.

It would be nice if the Nest warned you about working without a C-wire, but hey, it is what it is.

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