The experiment

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 08/17/2019 at 17:064 Comments

So here is the experimental setup:

So lets start with a few bits of data.  First, here are the graphs for the event.  This is an exported grafana dashboard that will let you see the data and zoom in, etc etc.

Live Grafana dashboard of experiment

Now for the filters:

The filters
Different filters

From left to right, brand new 3M, used 3M, super nasty off-brand.  Sadly, I did not have a dirty 3M lying about.  Wait a month.  :)

And just a tiny bit of data on the used 3M filter:

The thermostat, by default, comes configured to alert me at 90 days, or 300 hours of FAN runtime. It tracks the hours of runtime of the fan itself, between times that I click "changed filter".  So I know this filter is about 50% "dirty" by it's standards.

Now lets look at the data, because this was both fun, and surprising.

Here we have a graph of the pressure differentials between the different stages of the experiment.  I've noted with the blue lines, each phase.  The phases in order are:

  1. No filter at all, AC off, FAN On.
  2. Brand new filter
  3. Medium filter
  4. Dirty Filter
  5. System returned to normal
  6. Resume normal AC operation

And now drumroll... the data (averages over each time period):

StatePressure difference (mb)
Power Draw (W)
No filter
New Filter
-0.479 (0.377 adjusted)
Medium Filter-0.429 (0.427 adjusted)
Dirty Filter0.009 (-0.438 adjusted)

So I still need to pore over this a bit, and do some thinking.. but I can see a few things:

What does all this mean?  I think I need to keep this experiment going, for a long period of time.  I need to collect data over the next 150-250 hours of runtime on the current filter.  How does it change over time?  The super dirty filter was an abomination, so it was expected to show an incredible difference (a 0.8 mb difference!).

Initial thoughts:

Yes.  I can see the difference from the various filter states.  There isn't a massive difference between clean and somewhat dirty, but in theory, if we went the whole 300 hours runtime, it might be as high as 0.1mb.

I do not yet know the answer, when should I change.  I feel this will require longer term study of filter as it dirties.

The power draw also seems to be a really decent indication.  The difference between clean and medium was only 3 watts, but the dirty one was a whopping 46 watts.  That means it moves as dirt gets in there.  It might even be an easier way to track this, than the pressure sensor setup.

None of these, not even the super dirty filter, moved the gas sensor on the BME680 at all.  It is definitely not picking up "dirt".


Tim Rightnour wrote 08/20/2019 at 01:27 point

Makes sense like that.  I think my assumption was it would attempt to do a constant RPM, and therefore need more juice to create the higher suction.

  Are you sure? yes | no

Patrick Allison wrote 08/21/2019 at 18:32 point

It's not that the fan spins slower - it could actually spin faster. Exactly what it does depends on the torque/speed curve of the motor itself and how much you actually change the load by. The thing most people are used to is the case where you block the inlet on a vacuum, and the motor spins much faster. No air to move = no load = fan spins at no-load speed, and consumes very little power.

But at a minimum, the power the fan consumes is related to the power it delivers, which is torque times RPM. And if it's delivering less power (less load) it's probably consuming less as well.

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Tim Rightnour wrote 08/22/2019 at 01:13 point

Thanks, I appreciate the explanation. I think I understand that much better now.

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Patrick Allison wrote 08/19/2019 at 18:37 point

"The power draw figures completely amazed me.  I thought it would be the other way around.  The dirty filter uses LESS power.  Wow.  No idea."

Windmills generate less power with less wind, right? Fans use less power when they push less air.

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