Can PC Fans alone really mix large rooms?

Can PC Fans alone really mix large rooms?

Let's find out!

Some people doubt that little PC fans can mix and clean a large room uniformly -- maybe PC fan CR Boxes only clean locally, leaving distant corners at higher contamination?  You can feel air moving a couple meters in front of our SickleFlow fans, and see it move 3-4 meters redirected from a fog machine.  But then does the circulation just stop past that?  

Intertek's official measurements of Clean Air Delivery Rate are done in an AHAM AC-1 spec 10'x10' room w/ mixing fan.  Does this procedure really translate to a 25'x25' open family/dining room or a small classroom?  We wanted to find out once and for all!

Since we advertise the CDC minimum 5ACH (air changes per hour) room coverage for our purifiers, ideally they should clear the air uniformly over that advertised square footage, right? We got access to an apartment near a freeway with 600 sq ft open living area to do a real life test at full scale!  Since the XXL uses MERV13 filters, its expected performance in this space depends a bit on particle size:

 AC-1 Particle Distribution Luggable XXL CADR XXL ACH for 600 sq ft
Smoke (.1-1um) 320 cfm 4 ACH
Dust (.5-3um) 400 cfm 5 ACH
Pollen (5-11um) 480 cfm 6 ACH

ASHRAE 241 spec reduces these to a single infectious aerosol CADR via weighted average:

VirusCADR = .3*SMOKE + .3*DUST + .4*POLLEN 

For Filtrete MPR1900 filters, VirusCADR tends to come out exactly the same as DUST CADR.


All measurements were manually recorded by kid helpers with a new, factory-calibrated TemTop PMD331 particle counter known for low noise, especially in .3-.7 um range.  You can repeat this experiment yourself with a cheap PM1.0 or PM2.5 sensor, though a bad air day and multiple readings might be necessary to mask their noise.

To equilibrate the apartment to freeway-dusty outside air, sliding doors and windows were opened and a box fan turned on for half an hour, then closed and box fan turned off.  Particle counts appeared stable in this equilibrated state e.g. around 12,000 counts/L for 0.3um.  Then the XXL was turned on for 64 minutes and we walked around the room recording each particle size reading and minute stamps at the 7 tagged locations, TemTop at waist height.  The cycle through all 7 locations took 8-10 minutes.

Note this apartment was built in the 1960s so it's not sealed or insulated particularly well even after closing windows and doors, which adds some realism to the experiment.  We hung a curtain in the hallway and put foam under the hallway door, but did not seal everything with tape.  The furniture and cabinetry displace some air volume, but most of the ceiling is 9', so we'll assume standard 8' ceiling to cancel the cabinetry displacement.

For result scatterplots, we've superimposed the expected 4-6 ACH exponential decay curves for each particle size for reference, starting from the average pre-purification room particle count.  The formula for these curves is:

         RefCount(elapsedMin) = EquilibratCount * exp( - elapsedMin / (60 / refACH))

David Elfstrom also teaches how to extract a numerical ACH from the slope of a log-scale scatterplot.  For this apartment, the log scale scatterplot won't have a perfectly straight line for fit due to the room leakage.


For particle sizes .3 um (SMOKE) through 5um (POLLEN), the drawdowns closely adhere to expected rates of exponential decay given rated CADRs and room size.  Moreover, the drawdowns are well synchronized at all 7 locations across the room, hugging the predicted decay lines.  The sofa location 3.5 meters directly in front of the XXL side fans tends to lead by a couple minutes, and the foyer lags by a minute (near likely leaks to inner hallway and exterior door), but this is surprisingly little temporal dispersion!  Look carefully at how the🔺Sofa location at room center leads with cleanest air first, as this will be important to compare with the HEPA in part II.

Luggable XXL drawdown of .3um particles in 600 sq ft spaceLuggable XXL drawdown of .5um particles in 600 sq ft spaceLuggable XXL drawdown of .7um particles in 600 sq ft spaceLuggable XXL drawdown of 1um particles in 600 sq ft spaceLuggable XXL drawdown of 2.5um particles in 600 sq ft spaceLuggable XXL drawdown of 5.0um particles in 600 sq ft space

After dropping 80-90%, the particle counts stabilize without continuing to drop toward zero as they would in a well-sealed room. The hallway curtain and door leaks are a source of contaminants coming back into the room that the purifier must keep fighting.  Once the XXL is turned off, these leaks cause particle counts to double or triple again within 25 minutes.  

Purifiers so quiet and energy efficient that they run continuously at full strength are the best protection against recurring infectious aerosols and outdoor air particulates.  


Since most classrooms also range in size from 600-900 sq ft, these results are particularly assuring, especially with two Luggable XLs to reach ASHRAE 241 targets as suggested here.  Repeating this experiment using a particle counter in your kids classroom makes a great science project.  Nothing like hands on testing to make visible the benefits of clean air!

These results are also consistent with those of an EPA test of bio-aerosol drawdown by a 234 CADR CR box in a 3000 cubic foot chamber (about half size of this apartment):

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