Review of air purifier Prem-i-Air Invierno and air analyzer AirVisual Node

Living in Warsaw, I got tired of worrying about the air quality – which some days during winter got reported as worse than that of infamously polluted Beijing. So some months ago I bought an air purifier.

I had a number of doubts about how to use it so it was effective: does it have to work all day? The purifier's power level depends on the room's size, how do I know that it's working as it should? Air quality sensors included in these purifiers are said to be pretty unreliable, are they any useful at all? How does air quality indoors correlate to air quality reported by an official station about 1 km away from home – which daily recommended wearing a mask during winter!? What happens if I just leave the windows closed, or if I open them for a while? ...

So I bit the bullet and bought an air quality sensor, which cost almost as much as the purifier itself.
And these are my results and tips for anyone thinking about doing the same.

My kit

My purifier is a pretty cheap one: Prem-i-Air Invierno. I assume that one reason it's cheap is because it lacks bells and whistles, like an air humidifier; but actually this was a plus for me, because I didn't want one integrated (I had been monitoring my flat's humidity level and it was always good enough). And when purifiers do have an integrated humidifier, it's hard to know by looking at online reviews whether you can actually opt out of using the humidifier at the same time. (Looks like in some models you can't simply leave the water container empty because then the whole purifier stops cold...)

My air quality sensor is an AirVisual Node. I first wanted the more famous / cheaper OriginsTech/Kaiterra LaserEgg; but there was a waiting list (not only in Poland but in Amazon!), and for not much more I could get this one much faster, which additionally measures CO2 concentrations.

How does the indoors air quality measurement correlate with the official outdoors measurement? (with the purifier off, of course)

Rather well. Sometimes (rarely) indoors is reported to be worse than outdoors, but usually it's similar or a bit better. But in general, when outdoors it goes bad, indoors it does the same.

How good is the sensor in the purifier?

You can't really know. The sensor's output isn't shown anywhere, apart from how it affects the power level of the fan. Mind you, there is a color LED that gives you 4 colors (levels), but those levels are rather about how hard the Auto mode thinks it has to work; and this can be adjusted. I even wrote to the manufacturer an email asking for details; I never got an answer.

However, the sensor is good enough for the purifier to do its job! You can just start the purifier in Auto mode, without adjusting anything, and it will (in minutes) draw down the impurities count in the external analyzer to about 0. If the air gets dirty because of anything (like you burnt a match or opened the window), you might notice that the purifier starts working harder/louder for a while, and eventually settles down again.

So what can be adjusted?

The purifier lets you adjust how the power levels relate to the air quality measurements: there are 3 steps, from "very sensitive" to "standard". It looks like it (just?) changes how fast the power levels gets pushed up when the measurements get worse. In the "very sensitive" level, you can burn a match some meters from the purifier and within seconds the air purifier turns to the highest level for a minute. In the "standard" level, it might just work a bit harder for a bit longer.
After some experimenting I settled in the standard level, but I guess that for people with breathing problems the more sensitive modes might be interesting.

Is the purifier noisy?

There are four power levels. In the fourth level, it IS noisy, louder than a fan working at high speed but not as much as a vacuum. In the lowest level, it is barely noticeable. The 2 lowest levels are good enough for sleeping in the same room.

Anything I don't like?

The Auto mode takes care of switching power modes. However, here's the main flaw I have found with the purifier: the Auto mode turns to Silent mode if the room is dark for a couple of minutes. This means that the power will stay in the lowest level, and the color LEDs get turned off. All very nice for sleeping, right? The problem is that the Silent mode gets disabled when there is light... so, unless your room is kept well darkened, as soon as the sun is up the Silent mode turns to Auto, and if the air happens to be dirty enough, power can jump to the loudest level. This can end up waking you up earlier than you would like to.

The solution is easy: just set the purifier to Silent mode manually when going to bed. And optionally turn off the LEDs, if you mind the light (it IS a lot of light!, comparable to a sleeping light for children). It will then stay that way until you again change it manually.

To be concrete, the thing that I would like and can't be done is that I would like to make the color LED turn off when it's dark, but make the power level stay in any manual setting all the time, independently of the LED. But no, the LED will only turn off automatically if the purifier is in Auto mode.

So what about opening or closing the windows?

Here's where I am glad I got the more expensive, CO2-capable air analyzer. Turns out that if you close your windows, PM concentrations stay low (though still if the air quality gets bad outside it will also get worse inside eventually), and it's even easy to have 0 PM concentrations inside with a while of purifier work.

However, CO2 builds up surprisingly (and alarmingly) fast. The health effects are less clear than with PM, but it correlates with "stiffness", and mental performance gets progressively worse with higher CO2 concentrations (which is particularly important if you work at home). 1000 ppm is already impairing. In the open air (in Warsaw at least) you can expect about 500 ppm. 2 people in a 35 m2 room after 3-4 hours reach >2200 ppm easily, even sleeping. Awoken or doing any activity, it's of course faster.

So you're supposed to open your windows periodically to reduce CO2. But then the dirty outside air comes in; so, how to reach some air quality balance?

Turns out that you can keep all day the window cracked open and the purifier working at the lowest setting and then you get both low CO2 (about 700 ppm) and low PM readings. In fact, if the air outside is not very bad, the indoor PM stays low enough that you can keep the purifier off most of the time.

So could I just pass on the air analyzer?

I guess so. You could just keep your window cracked open (at least), and start the purifier in the Auto mode whenever the outdoor air quality gets non-good. That should be enough.
However, for anything more fine grained, you do need the analyzer. For example, today's outdoors air quality has been bad all day, but indoors it has stayed relatively good, so I didn't have to turn on the purifier at all. Conversely, yesterday's outdoors air quality was about the same, but I had to keep the purifier running all day long to have good readings.

So at the end of the day having an analyzer is a higher expense upfront, but you might end up saving enough from not using the purifier when not needed.

Personally, I like to know how things are working. For example, I am now sure that 1) air quality is bad, 2) the purifier does have the expected effect, 3) the window should stay open. Without the analyzer I would just be guessing at things.

So what about the analyzer?

There are some things I don't like in it. The screen is huge but less useful than could be; the analyzer actually captures a lot more detailed data than it shows. AirVisual has a phone app that you can use to get slightly more informative graphs than those shown in the screen, but still is awkward/uncomfortable. So, to get all the detail available, you have to access a CVS file that is published by the analyzer inside a SMB share (and then you can use any spreadsheet program to do your own analysis).

I have written a couple of feature requests to the developers, and they answered enthusiastically, but that was months ago and I never saw anything implemented ;P.

Also, the analyzer has a battery, but it barely lasts 6 hours, even with the screen off. It's kinda useful to be able to take quick measures around the place without having to carry the USB power adapter around, but that's about it.

(UPDATED to fix the maker of the air purifier. Thanks Patryk!)
(UPDATED with a bit more info on the analyzer)

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