The fan I had lying around is rated at 75 watts however I measured it at 60 watts on high. 75-100 watts would be ideal, although a little loud. I have also tested the air flow from a 55 watt fan which was similar. For reference my $500 Phillips 2000 Air Purifier runs at 40 watts.If buying a fan I imagine a box fan where you can mount the filter on the back would be better, as its important to get that clean air around the room.

I have read 90 days. The way to tell is not by how dirty it gets but by the reduced air flow, as explained by How long can you run an air purifier before changing the filter? One idea I have is to put a ribbon onto the fan when on a low fan speed so it doesn't lift (likely around the middle). Then when the ribbon drops on full fan speed you know the air flow has dropped by 10-30%.This is a tricky one as it depends on a number of factors, being:

Making a few assumptions I have calculated room size for different PM2.5 readings . Based on that I am guessing 28m2 if outside PM2.5 is under 50, then reduce room size as per the chart.I'm still trying to figure this out. I cannot find much evidence either way. My testing showed putting the filter in front increases the current by 1%, so I assume its not too bad. Even so until I can be confident I'm turning it off when I leave the house.This Reddit post is the most informative. In a nut shell it's unlikely.These can also be added. They are relatively cheap and capture gasses and smells. If your aim is only small particles then its not required. A thin filter I have dropped the airflow by 34% so it does come at an airflow cost. This is only important if you are pushing the purifier to its limits.I explore this topic in another post How does a MERV 13 filter compare to a HEPA filter?

A reading of 5 PM 2.5 after 15 minutes

Test 2: In Out

Test 3: Room 20m2

I got up in the morning and the levels were 65-87 PM2.5 inside. Civic was reporting 35. I put the fan to use in the rumpus room.

The test setup

Test 3: Repeat of Test 2 but with PM2.5 counter on the other side of the room

Test 4: DIY $11 MERV 13 vs $500 Phillips 2000 Air Purifier

Test 4: DIY $11 MERV 13 Repeated Test

Conclusion





I still have concerns around how a MERV 13 performs on small particles below 0.3, which is where HEPA sounds better.

Updates

Update 1: V2 - Improved Box

Update 2: V3 - Round Shroud instead of Square

I ran this in a 3m x 3m room for 15 minutes and the room went fromPM2.5 down toPM2.5 after 15 minutes. I estimate it did 23 air changes per hour.To measure the PM 2.5 I did need to run my actual purifier for about 30 seconds for the reading to come up. I do feel a bit silly using a purifier to test a purifier!For this test I simply measured behind and in front of the fan.Going from 8 PM2.5 to 2 PM2.5 is a 75% reduction. Interestingly we can derive from this that on the output there is 1 ug/m3 for < 1.0 micon, 1 ug/m3 for 1.0-2.5 micon, and 0 ug/m3 for 2.5-10 micons. This aligns with the expected filtering rates being better at for the larger particles.It took 31 minutes to get down to 10, the safe yearly limit, and it settled around 5-6 PM 2.5.This test interests me the most.Currently I do not have a fair comparison, so please take this with a grain of salt. The Phillips was tested after cooking (hence high reading) and also with the particle sensor placed further away.Interesting they both finished the test on the same reading of 4 PM2.5. The Phillips was slightly quicker, and if the test continued I suspect it would have maintained a 1-2 PM2.5 lower reading.I will repeat this test if the smoke returns, fingers crossed I don't get the chance.I have repeated the Test 3 and the results are very similar. Observed 1 is Test 3, while Observed 2 is the latest test.I now run this along side my Phillips 2000 air purifier and the readings are lower than the air purifier can do alone for where my son spends his day (living room + rumpus). I suspect that its due to the fact it does double the air flow, which I explain further in Why Air Flow Matters More Than Filter Rating . As an added bonus I expect this will extend the life of the $80 filter in the Phillips 2000.I have measured the air flow using a wind meter over the fan to be an average of 1.144m/s, which based on Air Flow Conversion Calculator is 531m3/hr. I measured my Phillips 2000 air purifier to be 416 m3/hr.Ideally I would have a larger filter, as 40cm x 40cm is smaller than the 50cm x 50cm fan. However for the price I am not complaining.Update: Does the DIY purifier capture small particles? The filter fell off, so I have used the box it came in to make a box instead. My aim is to make it look like a box fan.I'm yet to test this one properly, however it does seem to boost the flow vs no shroud by 20%. I am now using that in the Building It section.