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Anatomy of an AC Failure

A project log for When should I replace my A/C air filter?

How can I spend >$100 and >100 hours to save $1 a year on air filters? They say to replace them every month, but is that really true?

Tim RightnourTim Rightnour 08/22/2019 at 01:121 Comment

So right up front, I have multiple AC units.  This failure was on my second unit, not on the one with the test installed.  This has nothing to do with my tests.  It just so happened that it failed yesterday and my automation system let me know..  though.. in a roundabout way...  So this is unrelated to the experiment, but still interesting in the context.

So the secondary AC failed around 6am (that I noticed).  What is interesting, is how.  I've managed to capture data on two different AC failures over the years.. and each one has been really interesting from a data point of view.  (Way less so from a heat and money point of view).  My original capture log is here:

May 2014 Failure

As I later discovered, that failure was a capacitor failure.  This one is very different:

In this image, we see the blue (the big AC unit with the experiment running on it) and the yellow, the smaller unit.  You can see how the previous morning, it was running a bit often, but mostly okish.  Then around 2pm or so, we see a sharp downward spike in power usage.  At which point, it seems to not shut off really.  As we get later into the evening, it runs less and less, and then in the morning, it turns on, spikes up, and then never shuts off.


I'll know tomorrow, but I believe this to be a fan failure.  I think the downward spike is the fan shutting off entirely, and the unit just tries in desparation to cool, but it cannot, because no airflow.  I tried turning on just the fan, but no power usage, no blowing, dead.

And before you ask, yes I change the filters on that one.

The more failures I encounter, the better I'm getting at diagnosing them with just the power logs.  I'll let you know tomorrow what the official word is.

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Tim Rightnour wrote 08/22/2019 at 21:01 point

Update.  Yep, air handler fan failure. Motor blew out under warranty.

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