Comment by DannyBee | Hacker News Reader

Comment by DannyBee | Hacker News Reader

Errr - this is expensive for what you get. Among other things, the CFM is so low that it takes hours to reduce the particle rate any appreciable amount, as their own tests show.

You'd do a lot better with a standard high velocity fan (also 35 bucks, and you can get them cheaper): http://www.northerntool.com/shop/tools/product_200361823_200...

You want to be changing the air at a much higher rate than they are.

They quote a 13.5 square meter bedroom. This is 145.313 square feet. If they have 8 foot high ceilings, it's 1160 cubic feet.

In order to be changing the air so little as they are, my guess is this box fan is ~40CFM, giving ~30 minutes between air changes.

For 30 bucks, you can get a 2000+ CFM fan (see above), and put the filter on the intake side rather than the output side. This would change the air in the room roughly twice a minute, or roughly 60 times faster. There will be some loss of CFM due to static pressure increase, but ...

They also probably want a cheap, washable, electrostatic pre-filter so you aren't wasting the filtrating on easily caught particles. Otherwise, you are just going to go through this hepa filter really quickly.

This is Thomas--founder of Smart Air. Thanks for the ideas!

I totally agree--we've been doing tests with more powerful fans, and the results are great. We'll be coming out with a more powerful model soon. Of course, it's a bit more expensive to get a more powerful fan.

We've avoided using pre-filters because we don't want to cut down on air flow (and HEPAs aren't all that expensive), but we'll probably use a pre-filter on the more powerful model.

About where to put the filter, is there a reason to put it on the intake side? I've heard several people make that argument, but I've never heard a good justification for it.

There are a couple reasons to put it on the intake side:

1. Otherwise you are pulling dust particles directly into the fan bearings/etc (though it may be that you don't have a good enough seal for this to matter anyway). All this does is lower the life of the fan. This is one of the reasons you see filters on the intake side of an A/C or furnace - to protect the innards.

2. If a non-dust particle gets dropped into the fan, with the filter on the outtake side, it is likely to get propelled through the filter at high speed. With the filter on the intake side, this will never happen (since it would not hit the filter), and even dropping it near the rear of the filter is unlikely to cause damage.

(There are other reasons that would matter more if this was an enclosed ventilation system, but it isn't)

Note that without pre-filtering, i can't imagine these filters are really lasting that long without being cleaned. Are you measuring static pressure increase at all to see how long they last before needing to replaced?

HEPA filters are not usually meant to be sole filters and last a long time. They are just too fine and load too quickly. Some form of coarse separation (IE >5 micron particles) is usually necessary to get any real life out of them.

I haven't tried both, but it does seem like on the intake side, suction from the fan blowing would tend to hold the filter in place. But I would expect you would still need the strap (partly to keep it together when it isn't running).

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