Trouble with AC Condenser

Sense has helped me identify that our central AC is having issues. I have found that when our AC is on, the outside condenser turns on/off a lot and the condenser fan only works intermittently; however, the AC is still blowing cold air inside. I will note that the issue really only happens in the late afternoon/early evening when the house is under the most intense heat. I DO NOT see the same issue in the mornings or overnight hours when the air/house is cooler.

I’ve had an AC tech out to the house and they have confirmed that the dual capacitor and Freon level are both good. They are ordering a new control module and fan motor that will arrive next week.

See below for what Sense is detecting when the AC is acting up:

Do any HVAC experts have any ideas on what is going on with my system?

Thanks for your help

Do you have a single-stage, two-stage, or multi-stage/variable AC system ?

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Hi Kevin - I’m pretty certain that I have a single-stage AC System. My furnace is multi-stage, but my AC doesn’t work that way.

If it is single-stage, then it looks like the compressor suddenly starts consuming more power, who knows why, but then some kind of thermal or power protection cutoff in the controller stops it. Each time it tries to restart, it jumps to a much higher operating power level that it should and the same protection kicks in. Have seen a similar pattern with sump pumps that lock up.

I’m guessing that more AC-literate folks might have some theories on what could cause substantial added draw to a single-stage AC unit.

Thanks Kevin. I believe that there is some kind of thermal protection cutoff because this has been happening for the last 5 days or so (Sense first alerted me to the issue last weekend), but ONLY during the extreme heat part of the day (late afternoon/evening).

I’m not all that confident that the AC tech I had to the house really knew what is going on and is simply throwing parts at it. It’s really hard in my area to find emergency HVAC help for AC’s. It’s a lot easier in the winter when we have “no heats”.

@lavignerj1, the interesting question for me, is why does a single-stage unit suddenly start demanding 2x the power to keep it going ? I’m not an expert, but my take is that the events go like this:

  • Temp goes up during the day
  • AC compressor cycles get longer and longer, plus more frequent
  • Something happens in your compressor unit that greatly increases the load (maybe icing)
  • Controller on the compressor sees an abnormal increase and trips thermal protection
  • Controller / thermostat goes through waiting period to restart, then restarts
  • Load is still too high and thermal trips again
  • After a number of ineffective cycles like this the ice melts (or whatever) and the system can run normally again.
  • After another normal running period, things ice up again, and the run/trip cycles start again.
  • Eventually it gets cool enough that your compressor doesn’t need to run for an hour at a stretch.

It would be interesting to see a screenshot of normal cooling cycles to see how long they are.

My single-stage window-unit air conditioner does require more power when exterior temperatures are high. The graph below is from the master bedroom, which we typically don’t run during the day, but I found a day in which we did run it some. I interpolated these measurements with red dotted lines which follow the sinusoidal pattern of exterior temperatures where I live.

The compressor works harder to liquefy refrigerant when exterior temperatures are high. The ratio is 25% in my case, which does not match the double you note. I bring this up as it might be a partial explanation. However, I recognize that there is more going on in this case.

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