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A DIY air air purifier that is higher than a box-fan

A DIY air air purifier that is higher than a box-fan

2023-06-07 16:08:59

I like box-fan primarily based air purifiers. They’re low-cost, trivial to construct, and other people all over the world have accomplished experiments to indicate they actually work.

Nonetheless, box-fan purifiers have two downsides: They create plenty of noise, and so they devour a good quantity of electrical energy.

I wished a brand new design that preserves the perfect points of utilizing a box-fan:

  1. Low cost.
  2. Simply constructed with no instruments.
  3. No proprietary elements.
  4. Good at purifying the air.

The design also needs to repair the worst elements of utilizing a field fan:

  1. Make much less noise.
  2. Use much less electrical energy.

I feel I’ve discovered such a design. Behold, the Cuboid:

Cuboid DIY purifier

Efficiency abstract

I’ve accomplished some experiments evaluating this to a box-fan based purifier, utilizing the identical filters. Right here’s a abstract.

Air purifier Field fan Cuboid
Fan velocity low med excessive low med excessive
Price to construct $71.50 $100
Time to construct 30 s 5 min
PM 2.5 half-life (minutes) 18.4 13.2 9.6 15.1 10.3 6.9
PM 2.5 CADR (m³/min) 1.15 1.58 2.17 1.40 2.02 2.95
Sound stage (dB) 43 48 55 16 39 51
Energy utilization (W) 65.4 76.4 96.0 22.6 40.3 51.3
Energy price/12 months ($) 74.4 87.0 109.3 25.73 45.9 58.4
Resembles IED ★☆☆☆ ★★★☆

Much less is healthier for the whole lot besides the clear air supply fee (CADR). The half-life is calculated in a 31 m³ room. Electrical energy prices assume operation 24 hours/day at 1 kWh = $0.13.

As you’ll be able to see, the cuboid offers main enhancements in noise and electrical energy consumption, with small regressions in price, issue of development, and IED resemblance. Significantly on low, the cuboid may be very quiet and energy-efficient. Considerably by accident, it’s additionally higher at eradicating particles.

The fan/filter tradeoff

Air purifiers push air by way of filters. For those who’ve ever constructed a box-fan-based air air purifier, you observed that after you connect HEPA filters, throughput decreases dramatically. For those who use a weak fan, it’s going to barely push any air in any respect.

Essentially, if you wish to enhance purification, you’ve got two choices:

  • Huge fan: Hold the identical quantity of filter, however run a much bigger/quicker motor to push air by way of it quicker.
  • Huge filter: Hold the identical fan, however set up extra filters in parallel. This may lower the stress on every of them, that means extra complete airflow.

If we need to be quiet and energy-efficient, the selection is obvious: Use a comparatively weak fan, however with a big filter floor space.

Utilizing extra filters has a better upfront price — filters aren’t free. Nevertheless, that further price disappears over time. If every filter can take away a set quantity of particulates earlier than it must be changed, then doubling the variety of filters means additionally doubling the substitute interval.

Tips on how to construct it

(Contact me if you need the precise merchandise I used.)

Supplies used

  • An 8inch (20.32cm) diameter inline duct booster fan ($30).
  • 4 HEPA air filters, every roughly 32 cm x 22 cm and 5cm thick ($70 for all).
  • Two bungee cords (free).
  • Tape (free).
  • Two items of packaging foam (free).

Inline duct booster followers are pretty weak followers usually used to assist with develop rooms or vary hoods. An inline duct fan would in all probability carry out even higher (see the discussion) however at a better price. The one I purchased is rated to push round 12 m³ of air per minute.

You should utilize filters and followers of various sizes if you need. All that issues is that the fan has a smaller diameter than the filters (20.32 cm < 22 cm). The items of froth additionally have to be at the least as giant because the matching dimension of the filter. I simply used the froth that my fan was shipped in.

Step 1: Type a column out of the filters

Take the 4 filters, and assemble them right into a vertical construction. Watch out to align the perimeters as you see within the center, with two sides barely “inside” of the opposite sides.

Step 1 of construction

This column is considerably unstable, however we’ll cope with that.

Step 2: Lower out a base for the column

Lower one piece of froth to the dimensions of the sq. on the backside of the filter column (I taped over a gap within the foam I used.) Place it on the backside.

Step 2 of construction

I did this by setting the filters on prime of the froth, tracing out the form with a pencil, after which reducing the froth with scissors. You in all probability need to err on the aspect of constructing it bigger and trim if needed. It ought to match firmly in order that it’s held in place by the filters.

Strictly talking, you possibly can additionally in all probability skip this step. I did that in an early model and it was OK. Nevertheless, this provides plenty of stability permits the air purifier to work even when it isn’t on prime of a flat floor. You might additionally substitute another materials for the froth.

Step 3: Lower out a base for the fan

Lower one other piece of froth in order that it helps the fan from beneath whereas holding the fan in place. This ought to be giant sufficient in order that that the piece will sit on prime of the filters.

Step 3 of construction

As you’ll be able to see, the froth was lacking a nook, which I solved inelegantly by gluing on a chunk of cardboard.

Step 4: Set the fan on the filter column

Set the fan+foam on prime of the filters, oriented so it’s going to blow air upward, and pull air by way of the filters.

Step 4 of construction

That’s it, you’re accomplished. The highest is barely held on by gravity, plus stress if it’s on. You might clearly make this extra secure or lovely, however I wished to concentrate on a design that’s actually simple to construct.


The one vital price is the fan ($30) and the filters ($70 for 4). That is dearer than a field fan design, the place the fan prices $19 and also you solely want 2 or 3 filters. Nevertheless, as talked about above, the price of further filters evens out in the long term since they have to be changed half as usually.

Measuring filtering efficiency

Experimental setup

I did experiments in a room with a quantity of round 31 m³. To generate smoke, I reduce 5 credit-card size sticks of incense and positioned them on one finish of a desk. On the opposite finish of the desk, I positioned a pill that took a time-lapse video of a particle counter and a stopwatch. I then manually transcribed the measurements from this video.

The air purifier (field fan or cuboid) was on the bottom round two meters from the particle counter.

setup for measuring air purification performance


As a comparability, I connected three of the identical filters to a field fan (Actually the similar filters).

DIY box fan compared to

Listed here are the outcomes with the cuboid. Observe that the y-axis is logarithmic. The straight traces point out that particulates are reducing at an exponential fee.

Listed here are the outcomes of the field fan.

performance of the box fan on different fan speeds

You’ll discover two issues. First and most significantly, the slopes for the cuboid graphs are steeper. This means higher efficiency.

Second, the particles peak at increased values with the cuboid. I consider it’s because the incense was close to the particle counter on a desk, whereas the purifiers have been sitting on the ground 2m away. If the air isn’t disturbed, it takes some time earlier than smoke drifts over the air purifier and it truly begins doing something. Nevertheless, the field fan creates a lot wind that the smoke instantly diffuses across the room and the box-fan air purifier primarily will get a “head begin”.

Whereas wind appears to assist right here, it could possibly be dangerous in different circumstances. I focus on this extra below.

Becoming exponential curves

The y-axes within the plots above are logarithmic. As you’ll be able to see from the dotted traces, these are well-fit by straight traces. That is what we’d anticipate if a set fraction of the air have been being cleaned per minute.

The dotted traces for every curve present a match of the shape y=a×bᵐ, the place y is the extent of particulates, m is the variety of minutes, and a and b are constants.

We are able to convert a match of this kind to a half-life: The variety of minutes the air purifier must remove half the particles from the air when operating in a 31 m³ room. That may be the variety of minutes m such that bᵐ = .5. We are able to resolve this equation as m = log(.5) / log(b).

Air purifier Field fan
Fan velocity low med excessive
PM 2.5 match 90×.963ᵐ 90×.949ᵐ 65×.93ᵐ
PM 2.5 half-life (minutes) 18.4 13.2 9.6
Air purifier Cuboid
Fan velocity low med excessive
PM 2.5 match 300×.955ᵐ 280×.935ᵐ 180×.905ᵐ
PM 2.5 half-life (minutes) 15.1 10.3 6.9

Clear air supply fee calculations

One frequent manner of estimating the efficiency of purifiers is the clear air supply fee (CADR). If the air that got here out of the air purifier had zero particulates, this is able to simply be the amount of air per unit of time.

The matches above have been of the shape y=a×bᵐ. Which means the variety of particulates drops by an element of b every minute.

Right here’s how I calculated the CADR. If the air purifier delivered (cleaned air) m³ of unpolluted air in a single minute in a room with a complete quantity of (all air) m³, then the particulates in a room would drop by an element of

b=(uncleaned air)/(all air)= 1 – (cleaned air) / (all air)

per minute. We are able to resolve this to seek out that equation to get that

(cleaned air) = (all air)×(1-b).

I measured the scale of the room the place I did these measurements, arriving at an estimated quantity of 31m³. From this, I computed the CADR for every air purifier and velocity as CADR = 31×(1-b). This provides the next desk of CADR charges.

Air purifier Field fan Cuboid
Fan velocity low med excessive low med excessive
PM 2.5 CADR (m³/min) 1.15 1.58 2.17 1.40 2.02 2.95
PM 2.5 CADR (ft³/min) 40.5 55.8 76.6 49.2 71.1 104

A latest study from the College of Toronto additionally measured the CADR of an analogous box-fan air purifier. They measure round 92 ft³/min. (They don’t give numbers, however there’s a graph the place a CADR of 100 ft³/min could be 130 pixels excessive and the field fan is 100 pixels excessive.) That is reassuringly near my estimate of 76.6. I’d put a confidence band of round 20% on my numbers as a result of crude measurement of room quantity. Additionally, I used 3 HEPA filters slightly than a single MERV 13 filter, so there’s no cause to assume true efficiency is strictly the identical.

Measuring noise

I measured noise utilizing the wonderful phyphox app on my cellphone from round 2m away. I didn’t calibrate the measurements, so absolutely the values are suspect, however the relative values ought to be positive. I’m estimating absolutely the values off the producer’s claimed noise stage for the field fan.

Air purifier Field fan Cuboid
Fan velocity low med excessive low med excessive
Sound stage (dB) 43 48 55 16 39 51

It’s quieter throughout the board (4 dB is noticeable) however the actual benefit comes when operating on low. The Cuboid is simply above the edge the place it’s noticeable throughout the room, just like a quiet fridge. A Field fan even on low is sufficient to make dialog or listening to music considerably much less nice.

Measuring power utilization

I measured the electrical energy consumption of every air purifier on every fan velocity utilizing a typical wattmeter. I estimated the yearly electrical energy price through the use of the common US electrical energy price of $0.13 / kWh, and assuming the air purifier is run across the clock. Prices around the world range from round ⅓ as a lot (Egypt) to round 3× as a lot (Germany).

By the way, I by no means observed this earlier than: If a tool consumes X watts then operating it consistently for a 12 months prices a bit greater than $X at common US electrical energy costs. This coincidence occurs as a result of

See Also

24(h/d) × 365(d/y) × 0.13($/kWh) × .0001(kWh/Wh) = 1.135($/W).


The higher restrict

The fan I used is rated to push 12 m³/min of air. Nevertheless, the CADR is barely round ¼ that. In precept this implies that including extra filters (maybe a lot extra filters) might enhance the filtering efficiency by as much as an element of 4, with no further electrical energy price. (I checked: The ability utilization doesn’t change considerably when the filters are there to decelerate the fan.)

Must you hassle constructing a Cuboid?

Field followers have 4 benefits over the Cuboid. They’re even simpler to construct. They work with a bigger vary of filters. Their upfront price is decrease. Lastly, you may have already got a field fan mendacity round, or discover one on the road or one thing. It’s unlikely that an additional inline duct booster fan will simply fall into your lap.

So when is a Cuboid price it? I feel a field fan might be positive for those who’re simply going to run it for brief intervals of time and don’t thoughts the noise. However a Cuboid is a lot better if you’ll run it for lengthy intervals.

One option to measure the standard of a air purifier is the quantity of air it cleans per unit of energy consumed. We are able to do that by dividing the CADR by the facility utilization. This provides the next outcomes:

Air purifier Field fan Cuboid
Fan velocity low med excessive low med excessive
CADR/energy (m³/min/kW) 17.6 20.7 22.6 61.9 50.1 57.5

The Cuboid does at the least 2.4× higher all over the place however is 3.5× higher when operating on low. This is identical setting the place the cuboid produces 27 dB (~500 occasions) much less noise.

When is airflow useful?

A field fan generates way more airflow than the cuboid whereas having marginally worse filtering efficiency. That’s as a result of plenty of the air it pushes doesn’t undergo the filters. Is that is good factor?

We’d take a look at this query extra typically. Suppose you’ve got some air air purifier. Is or not it’s useful so as to add a high-velocity fan in the identical room?

To reply that, we have now to contemplate two questions: (1) how shut the air purifier is to the supply of smoke and (2) how shut you might be to the supply of smoke.

  • If the smoke is close to the air purifier however removed from you, then you definately need to reduce further airflow. That makes the air purifier simplest, and retains the particles away from you so long as potential.

  • If the smoke is close to you however removed from the air purifier, you need plenty of airflow. That reduces the focus of smoke close to you, and will get the air purifier doing one thing as quickly as potential. (That is the scenario I by accident created in these experiments.)

Within the different conditions, it’s actually not clear. (Let me know you probably have a convincing argument.)

  • If the smoke is close to each you and the air purifier, the profit is airflow is unclear. My intuition is that airflow is sweet as a result of it reduces the height focus of particles you breathe. Nevertheless, it additionally makes the air air purifier much less efficient. Maybe it doesn’t actually matter?

  • If the smoke is removed from each you and the air purifier, however you’re near the air purifier, then lowering airflow means the air purifier can create a type of protecting bubble round you. This appears good?

  • If the smoke is removed from you however you’re removed from the air purifier, I suppose it doesn’t matter? Including a fan makes the air purifier begin working quicker, but additionally means you get uncovered to the particles quicker. On this case the airflow appears to simply “push time ahead” a bit.

Lengthy-story brief, airflow is typically useful and generally dangerous. One of the best factor is to have a separate fan that you may activate or off when the scenario dictates.

The competitors

There are many industrial purifiers on the market. A few of these have an analogous “can” or “cuboid” design. A few of them declare CADR and energy utilization numbers which might be related. For those who don’t thoughts the additional price, in all probability a few of these work positive.

Nevertheless, the assessments I’ve seen don’t encourage confidence. Each time some group runs a bunch of assessments, they discover that among the purifiers work nicely, some work badly, and virtually none of them meet the specs they’re claimed to fulfill. For those who take the highest decide from the Wirecutter and skim person evaluations rigorously, you’ll see that roughly one individual every week reporting that their unit exploded.

Why don’t producers publish assessments to indicate their merchandise work? It’s not that tough – I, random web individual, did it right here as a pastime. I feel the fact is that persons are suckers, and assume (incorrectly) that if they offer Dyson $600, they’ll get one thing in return that works higher than a janky DIY gadget.) I say: Till producers present proof, give no quarter.


If the out of doors air is clear the place you reside, I counsel you set a air purifier in your kitchen and run it on excessive whereas cooking (or another actions that create smoke). For those who’re solely operating it for brief intervals, electrical energy consumption is a minor concern. If random noises annoy you, it’s in all probability not price operating a loud air purifier on a regular basis. If in case you have a air purifier that’s quiet and environment friendly (did I point out I’ve a design for that?) it’s in all probability nonetheless a web profit to run it on a regular basis.

If the out of doors air is soiled the place you reside, I counsel you run a air purifier consistently year-round. In truth, you may have to run a number of purifiers in numerous elements of your house. The well being advantages are actually fairly giant and you need to take no matter measures are needed. Right here, electrical energy prices will add up rapidly, and noise shall be a major quality-of-life subject. Get purifiers which might be quiet and environment friendly.

Both manner, lowering indoor air air pollution is likely to be the simplest well being intervention you can also make. Making up numbers, running may present 5× the profit, nevertheless it’s at the least 25× the trouble. All of the stuff you set in your physique issues, however clear air will be had at low price and with near-zero willpower. Don’t miss out.

You may ask

Haven’t you spent far more money and time operating these experiments than you’ll ever save?

Yeah, in all probability by an element of 10.

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