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 Post subject: DIY air purifier?
PostPosted: Sat Feb 12, 2011 6:09 pm 
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Joined: Wed Jan 18, 2006 3:41 pm
Posts: 177
Location: Buffalo, NY
My house is dusty, I have allergies, and commercially available air purifiers probably don't meet my needs. Obviously, I need something inaudible. I'd like to circulate and filter air in the bedroom without hearing it. I also can't have a unit that burns through a lot of electricity, and ionizers are out.

I was thinking I could make a simple, effective dust filter based on forcing air through a furnace filter. The obvious approach is to duct tape a 20" furnace filter to a 20" box fan and call it a day. Hence my question: how can I reduce the speed of a box fan to the point where it's silent? Has anyone modified a box fan for really low RPM? Would the backpressure due to a furnace filter drop flow to an unacceptable level?

An alternative idea is to make a box full of pressurized (or more likely depressurized) air, with three or four sides of the box covered in a furnace filter. Furnace filters are flexible when you cut their paperboard frames. It would need to be air tight, and I'd need a fan or blower capable of dealing with the backpressure. But silent blowers are unheard of. :wink:

Or I could buy an inexpensive, commercially available air purifier, tear out the fan, and caulk a silent 120mm fan in its place. Again, backpressure would be the issue.

I like the box fan idea best, but mine is way too noisy on its lowest setting. Thoughts?


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 Post subject: Re: DIY air purifier?
PostPosted: Sat Feb 12, 2011 11:30 pm 
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Joined: Sun Jul 12, 2009 10:30 am
Posts: 372
Location: Canada
You can use 4 or more large computer fans I have two 220mm fans that I use as a window fan in the sumer during the night and they are extremely quiet.
But for an air filter (not purifier) you will need a large filter area. Lager the air filter the less pressure you need to get the air through.
I do not know exactly what type of motors are on the box fans but if they are AC brushes motors an easy way to reduce the speed is to use two connected in series so only half of the voltage will be available to each so the power on each fan will be one quarter of the spec and it will be much more quiet.

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 Post subject: Re: DIY air purifier?
PostPosted: Sun Feb 13, 2011 12:25 am 
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Joined: Thu Oct 21, 2004 4:49 pm
Posts: 336
Location: Upper left hand corner, USA
The SPCR approach to noise is where possible avoid creating heat to start with, so you don't have to use fans/etc. to get rid of it. Have you fully explored applying that approach to your problem? i.e. before dealing with filters & fans - start by removing source(s) of dust.

Somebody I know who has significant asthma has bedroom that is just for sleeping in - keep as many dust sources as possible out. Bedroom has no carpet, no drapes, clothes stored in another room - just the bed and things need for sleeping. Anything in room is covered/enclosed so the room is easy to vacuum (and clean it frequently). Even if don't go to that length - enclose things (using drawers, cabinets, boxes, dust covers) so more of the house can be vacuumed more often.

After that - if the house has central heat, have you investigated possibilities of using the heating/cooling system as air circulator/cleaner. Many recent furnaces have variable speed blower systems, which can be set to just circulate air through the furnace filtering system without heating or cooling it. If you have such a system, it could help reduce dust in whole house. Hopefully the furnace fan is far enough away from bedrooms that noise wouldn't disturb. (For this to work also need a house with ductwork designed for circulating air.)

If furnace is not appropriate for that - could consider running air cleaner in the bedroom while you aren't sleeping (to clear remaining dust from air), then turn it off at night.

Soft-mounting the air cleaner (i.e. put the whole thing on a cushion) might help (We have a HEPA air cleaner that we do this way, it helps - but is still noisy). One might look at ways to reduce internal vibrations as well.

Another consideration with using replacement fan in air purifier would be decreased efficiency at filtering air. The less air you have passing through the filter, the longer it will take to remove a given amount of particulate. (See for instance the consumer's reports review of air cleaners. If memory serves they test them by using a set concentration of dust and seeing how much they remove in a given amount of time.) So as you reduce the speed of the fan (or use slow moving fans) you will reduce the effect of the filter. Thus need more fan/filter pairs to produce the desired effect.

If using smaller fans (e.g. computer type fans) - might want to put the filter first (bearings on computer fans are not too happy when covered with dust).

If you do find a quiet HEPA filter I would be interested.


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 Post subject: Re: DIY air purifier?
PostPosted: Sun Feb 13, 2011 11:43 am 
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Joined: Sun Sep 05, 2004 1:51 pm
Posts: 687
Location: Gefle, Sweden
To my experience as a seasonal allergic and relatively mild asthmatic it's the ionizing that makes the most difference in a typical air purifier. Get a vacuum cleaner with a good hepa filter since in that application the filter is really abused, but make sure whatever air purifier you buy has ionizing built in. For a non-ionizing air purifier to be effective and quiet it will have to be really big in order to mechanically filter a lot of air. (which is why the small home applicances can't be quiet) If you build one yourself, go for oversize. Several layers of fabric will do a good job for a makeshift filter, especially if you build a large intake area with relatively low pressure. Not much need for an expensive hepa classed filter there. (although you could always add one at the exhaust end, which can be smaller)


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 Post subject: Re: DIY air purifier?
PostPosted: Sun Feb 13, 2011 6:00 pm 
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Joined: Thu Oct 21, 2004 4:49 pm
Posts: 336
Location: Upper left hand corner, USA
mkk wrote:
To my experience as a seasonal allergic and relatively mild asthmatic it's the ionizing that makes the most difference in a typical air purifier. Get a vacuum cleaner with a good hepa filter since in that application the filter is really abused, but make sure whatever air purifier you buy has ionizing built in. For a non-ionizing air purifier to be effective and quiet it will have to be really big in order to mechanically filter a lot of air.


This is the reverse of the results that Consumer's Reports found when testing filters. (The ionizing ones they tested did not do as good a job as the filters.)
Whether filter is ionizing or not it is going to have to have a lot of air moving through it to be effective.

One should also be cautious with the ionizing filters since some of them generate ozone, which is a respiratory irritant and may make allergies or asthma worse for some people. http://www.webmd.com/asthma/news/200504 ... lth-hazard

http://www.epa.gov/o3healthtraining/effects.html

Whether ozone generated is a significant health issue or not is unclear, but
since ionizers don't work as well in testing, I would stick with filters.


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 Post subject: Re: DIY air purifier?
PostPosted: Wed Dec 05, 2018 10:15 pm 
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Joined: Thu Nov 29, 2018 7:35 pm
Posts: 2
mkk wrote:
To my experience as a seasonal allergic and relatively mild asthmatic it's the ionizing that makes the most difference in a typical air purifier. Get a vacuum cleaner with a good hepa filter since in that application the filter is really abused, but make sure whatever air purifier you buy has ionizing built in. For a non-ionizing air purifier to be effective and quiet it will have to be really big in order to mechanically filter a lot of air. (which is why the small home applicances can't be quiet) If you build one yourself, go for oversize. Several layers of fabric will do a good job for a makeshift filter, especially if you build a large intake area with relatively low pressure. Not much need for an expensive hepa classed filter there. (although you could always add one at the exhaust end, which can be smaller)

I am totally agree with you. Beside with the built-in ionizer, the air purifier also uses some electrostatic paper filter or UV-C bulb to increase the efficiency.


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