As a regular reader of this blog, you know that I try to expose some of the more inappropriate claims and weaknesses of various products in our little air cleaning world. I have seen it all – or at least I thought. But this one was truly mind-blowing (ha) and totally unexpected. Recently we had the opportunity to evaluate a certain competitor’s popular product. I won’t say which one it was, but it was a puller. (Caution – serious air geek material ahead).
HEPA air cleaners can have two general configurations – puller and pusher. Pushers have the fan mounted upstream of the filter. The air sees the fan first, and then gets pushed into the filter. Pullers are the opposite- the air sees the filter first, and is pulled through the filter by a fan mounted downstream of the filter. For many pullers the manufacturer will place the actually fan right next to the air output – the last step of the system.
This puller design has a very important and hidden weakness. Even as a professional in this industry, I didn’t realize how bad it was until I measured it in our own laboratory. You see, if a pusher’s filter becomes clogged or the air intake mechanically blocked, the air output will stop. This is normal and expected – nothing in should equal nothing out. But not so with the puller system we evaluated. You can totally block the air intake, and it will still happily pump out lots of air! So where does the air come from? It comes from the output vent – air which is short-circuiting so you have both air going into and coming out of the same place. This short-circuit air has not been filtered in any way. The fact some companies have placed the fan right next to the output vent increases this phenomenon.
So this only happens rarely when the filter is totally clogged right? That’s where the shocking point comes in. It happens all the time, even with a new filter. In our tests, fully 50% of the output air was not filtered at all, but was room air simply going round and round the fan at the output. This results in a high flow of “mixed air” that has only been half-filtered. It still has plenty of contaminants, but is now being whizzed about the room at considerable velocity. This is misleading, since the air flow at the output appears to be strong, but it’s still quite contaminated.
In our Aerocure systems, we use the pusher design, which is more complex and a little more expensive to make. However, you can be assured that the output air has been 100% filtered, and not in any way mixed with room air.
So if you are evaluating an air purifier for your home, cover up the air intake. If the unit has been designed properly, there should be no air coming out. If there is still air coming out, run away because that represents unfiltered air which is leaking into the unit, or short circuiting at the air exit. Units which just move dirty room air around do more harm than good.
To your health!