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After running either just the central HVAC fan, or the central HVAC AC for just a few minutes, the Bryant thermostat (Model TSTATBBPB101, C Wire attached) screen momentarily shows "PF", and then goes blank (it normally shows the current and target temperatures).

1 minute later (if running just the fan), or 5 minutes later (if running the AC), the thermostat's screen comes back on, and everything looks good.

The fan (or AC) will then turn on, and work fine, until the above cycle repeats after just a few minutes.

I then installed a new LUX TX9600TSa thermostat, and the problem persists. Without batteries, but with C wire connected, the thermostat will show "Low/Battery Low" after running the AC for 5 minutes, and then go blank. With batteries installed, and the C wire still connected, the fan (or AC, if it is selected) will simply stop after a few minutes, even though the fan is still turned on (or in the case of the AC, the target temperature has not been reached).

1 minute later (if just running the fan), or 5 minutes later (if running the AC), everything will reset and look fine, but the problem persists: I can no longer get the HVAC fan or AC to run for more than a few minutes at a time.

Just to ensure it's not a clogged filter, I put a brand new MERV 16 filter into the HVAC system, but the results did not change.

What is likely the problem, and is this something I can fix myself?

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HVAC is losing power.

  • Cause 1 overflow shutoff switch.
  • Cause 2 bad transformer.

My first inclination is that you have a condensate shutoff switch activated, due to a full pan due to over condensation and/or a clogged overflow pipe. If you had a really nasty old filter, or no filter for some time, then this is even more likely.

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    Thanks for your answer, Ted. Go ahead and add your location and whatnot in your profile if you like. Signatures aren't really ideal in answers on Stack Exchange. We're different from a discussion forum in some ways. – isherwood Jul 22 at 13:36
  • Thanks Ted. I checked the condensate overflow, and it is empty. The filter I removed was still good, but I replaced it with a new one anyways, just to rule it out. Still can't get the fan or AC to run for more than a couple minutes. – RockPaperLizard Jul 23 at 7:51

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