Buying a Large Air Purifier for a Large Room
Air purifier output is usually given in terms of CADR – clean air delivery rate.
CADR involves three numbers – each one corresponding to a different particle type – pollen, dust, and smoke. This is nowhere close to being representative of all the particle types that exist. For this, and many other reasons, we don’t like using CADR to compare the processing power of different air purifiers.
Instead, we recommend you use CADR to estimate CFM. Usually the CFM is only slightly more than the highest CADR rating of the three. So, if the pollen CADR for a unit is 130, the dust CADR is 124, and the smoke CADR is 120, chances are that it’s CFM is somewhere around 140 to 150 – slightly above the highest CADR which in this case was the 130 CADR for pollen.
An air purifier’s CFM is the total number of cubic feet of air it’s able to process every minute. This is much easier to think about on a practical level than something more abstract like CADR. So, use CADR to estimate CFM and then compare the CFM of different units to compare how much air – how much cubic feet of air - they’re able to process per unit time – per minute.
CFM and room size
With certain products, the larger the room, the “bigger” the product you buy for that room. For example, for a small room you generally buy a smaller TV and for a large room you generally buy a larger TV.
The same line of thinking applies when buying an air conditioner for a house. The larger the house, the greater the square footage, the more BTUs are required for the air conditioning unit to cool the house.
You might be inclined to think the same logic applies to air purifiers – the larger the room, the greater the CADR/CFM air purifier required to clean the air in the room. This, is in fact what you see when you look at manufacturer recommendations – units with higher CADR/CFM are generally recommended for larger areas.
And we agree, to an extent. Yes, higher CFM is required for larger areas.
But here’s the thing:
There’s a threshold of square footage where you don’t want just higher CFM but higher CFM from multiple sources – multiple air purifiers.
Our Recommendation
The cut-off (or threshold) is at 300 sq. ft. Below 300 sq. ft. a single unit will work just fine. Above 300 sq. ft. our testing has shown that multiple 250 CFM air purifiers are much more effective than a single higher CFM air purifier.
Why 250 CFM and not less? A 250 CFM air purifier is the perfect combination of value, energy efficiency, low noise output, and versatility. It’s also the perfect size – that is to say has the perfect output - and perfect price for a unit that you can duplicate in extra-large rooms.
For rooms considerably larger than 300 sq. ft. multiple 250 CFM units (instead of a single high CFM unit) are recommended.
We make this recommendation based on multiple factors.
First, higher CFM units are few and far between – selection is limited. You have a much larger pool of units to pick from in the 250 CFM category and from the pool of units you get to choose your pick of the litter – the best of the very best in the 250 CFM category and at its price point.
At higher CFM there aren’t nearly as many units to choose from. You may be forced to pick a unit with poor energy efficiency, high noise output, or expensive filters. In other words, there’s no clearly defined really good CFM unit. All high CFM units have substantial shortcomings. There is, however, great options in the 250 CFM size range with the best options having little to no shortcomings at all.
The second reason we recommend 250 CFM units over higher CFM units has to do with the design of the air purifier. Remember, one air purifier has one inlet and one outlet. It may be placed in one corner of a room and even if it’s placed in the middle of a room it has to “reach” the corners of the room to purify the air in the whole room.
Multiple air purifiers can be placed spread out in different locations in the room. This equates to multiple outlets/inlets in different locations in the room which allows for better intake of unclean air throughout the room and for better distribution of clean air throughout a large room compared to a single high CFM unit in one location.
Finally, we recommend 250 CFM units over higher CFM units because of value. 250 CFM units give you a lot of CFM per dollar spent. High CFM units usually do not.
250 CFM units are so well priced that you can essentially buy two 250 CFM units for close to the same price and often much less than the price of a single high CFM unit. Now you not only have 500 CFM of total processing power (compared to 300 to 400 in a single high CFM unit) but you have that power spread out in multiple locations (and we just went over how this is beneficial also).
Air quality metering
Cleaning the air in a large space can be much more difficult and complex than doing so in a small room.
In a small room you can shut the door which essentially seals the room. You can run a 250 CFM unit on its highest setting to absolutely crush particle concentration in the room and then set it to lower settings to lower noise output and still maintain relatively low particle concentration.
A large room can present several unique challenges:
Ceilings may be higher
higher ceilings equate to more three-dimensional space in the room which equates to more air in the room - more air for the air purifier to have to process.
Open floorplans
it’s difficult if not impossible to seal off “rooms” that are part of an open floorplan. Air is a gas – it moves freely between such rooms. You cannot put an air purifier in your living room and size it according to your living room square footage if your kitchen is attached with no doors and/or walls in between.
More disturbances
larger areas usually have more doors that open, more furniture, more people, etc. – all of which can contribute to more disturbances of surface particles that become airborne particles when disturbed. Not to mention the fact that if you open a door to the outside that outside air with all the particles it contains is going to infiltrate the room extremely quickly. Other sources such as cooking food, doing laundry, etc. also contribute particles to the air very quickly.
The bottom line here is that improving the air quality in a large room is generally more difficult than it is for smaller rooms.
Again, in a small room you can seal off the room, run a single 250 CFM air purifier and be confident particle concentration is going to get very low.
The same isn’t true for a large room. Yes, if you buy multiple units according to our guidelines above you can be reasonably confident but you’ll never be absolutely sure without some way to monitor the air quality in the room.
That’s why we recommend a particle meter for anyone looking to improve air quality in larger rooms. A particle meter will allow you to track particle concentration in the room before, during, and after you run the air purifier.
You can even buy a single air purifier and a particle meter to start. You can then track the air quality in the room putting the air purifier in different locations and running it on different settings. The particle meter will tell you the exact particle concentration in the room at all times. If you find that you’re not able to achieve sufficiently low particle concentration in the room with a single air purifier – then you can buy an additional unit.
The Bubble Myth
You may have heard or it may be intuitive for you to think that an air purifier creates a “clean air bubble” around it. Following this line of thinking you may think that you can put a low CFM unit right next to you and have it lower particle concentration in the air immediately surrounding it, including the air around you. And that because of this – that you can use a low CFM unit in a large room and have it adequately clean the air around you as long as it’s close to you.
Unfortunately, things just don’t work this way.
Particles move from areas of high concentration to areas of low concentration. Right next to the air purifier – by its outlet – particle concentration is very low. But as soon as clean air (low in particle concentration) comes out of the air purifier it distributes, and unclean air (high in particle concentration) swoops in almost instantaneously and throughout the whole room.
That means you have to buy an air purifier with an output that matches the whole room.
Again, if the room is really large – over 300 sq. ft. - we recommend multiple top rated 250 CFM units.
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Comments (3)
I wrote earlier to complain about the idea of replacing one high-CFM purifier with two smaller ones, on the grounds that it takes 8 1-inch cubes to fill a 2-inch cubic volume. (This is easy to visualize.) However, I've since spoken with someone with fluid dynamics experience who says the position taken in this web page is correct. One way for a sceptic like me to be convinced is to play with the CFM calculator. For a specific volume, doubling the air exchange frequency requires merely doubling the CFM, not multiplying it by 8.
Wait, what am I saying? One 500-cubic-ft volume is filled by 8 250-cubic-ft volumes. Just think about it in terms of 1-inch and 2-inch cubes: you need 8 of the former to fill 1 of the latter. So how could two 250-CFM air filters provide the same filtration as one 500-CFM unit?
Great info! But I agree with a comment left by someone on another page of your site: 500 CFM is not 250 CFM x 2; it's 250 CFM x 4. In other words, you can fit four 1-square-inch cubes inside of one 2-square-inch cube. This might invalidate your claim about the value of 250 CFM units vs. 400 CFM units. You should acknowledge this in your post. It may not invalidate the many good points you make here. Furthermore, I'm not that clear how you are deriving CFM from the published CADR specs for a unit, so perhaps if we were comparing the CADR values directly (messy as they are), your point about the value of multiple units would stand. But given how clear you are about everything else, it would be good to address this point.
You are confusing cubic feet with feet cubed. The article is precisely correct. 2 cubic feet is not a cube 2 feet on the side, it is 2 cubes each 1 foot on a side, etc. So, a 2 foot cube contains 8 cubic feet, but it cannot be described as 2 cubic feet.