Re: Build Your Own Homemade HEPA Air Purifier?

A Reader writes:

I’m sorry but when I see the IQair units go for $1000.00 I just can’t justify the purchase when looking for some relief from a Cat allergy.

I’ve always had allergy to cats, dogs, barns, hay, chickens, flowers and the like.

Right now the “load” is causing me breathing problems.

(Claritin D, a really hot shower, and introducing moisture into the bedroom by way of a humidifier help)

The cat isn’t going anywhere.

It will be taken to be cleaned, the room bedding will be washed in extra hot water and everything will be vacuumed as best as possible.

So that leaves an air cleaner.

So I’m a I bet I can do this better myself kind of person anyway.

The IQaire claim .003 capture but every HEPA “filter” I find for sale claims only .03 microns.

So how are these IQ people doing .003?

Why can’t I just make my own?

I have access to people that fabricate things for a living.

I give em’ specs or at least desires and they could make anything.

The problem is the filter itself.

Where to buy the best filter?

I see this as a very simple device… take dirty air and pass it through a filter.

Run it long enough and you should clean the air.

With proper placement and a study of thermal air patterns of the room you could plan accordingly.

I’ve read some folks stacking some over the counter air conditioner filters together and using them as pre-filters for the HEPA filter.

Makes sense to me, the less stuff that the HEPA filter captures the longer the life of the filer.

I’ve read webpages talking about making filters for hoods to filter out spores and the such, they used “squirrel” type blowers to pull the air though the filter setup. (over 425cfm to start)

So I think one could make a box with a few cheap filters on the outside and 1 or even 2 good quality HEPA filters in front of a powerful fan and clean the air just as good as the IQ device..

(It may be possible to buy a large sheet HEPA filter and then cut it down to a smaller size and stack the pieces behind each other making a “better” filter more cost effectively)

Your site doesn’t talk about making your own I may add.

Understand I’m only worried about my Cat allergy and not a chemical sensitivity that may well warrant a more expensive system.

Though I’m told Cat dander and its dry saliva are among some of the smallest in size to deal with.

I still think you should cover building your own.

Though one could get some UV bulbs mount them inside the box; make a deeper box with clear tubing to force the air to stay longer in the box for the UV light to kill whatever it could kill.

You could add stages for special chemical removal filters.

The sky’s really the limit of what one could do.

Next would be to augment a forced air system to include a HEPA filter.

I’ve heard arguments from HVAC guys that they hate HEPA filters because it screws with their flow numbers and it requires them to oversize the blowers on account of the airflow loss the HEPA filter introduces.

2nd question: What about whole house systems like the Trane CleanEffects system?

Ed's Reply

Hey Reader;

Build your own homemade air purifier?

Hey, I'm also a do-it-yourselfer.

My home was built of 90% recycled materials entirely by yours truly.

I do all my own auto maintenance and as a young man fantasized building a car from scratch.

There is nothing stopping you from constructing your own air cleaner, but I doubt it will be more effective or efficient than what is available in mass produced $200 air cleaners at your nearest big box retail store.

You can buy many of these used from ads on craigslist.com for $50-100.

When you see IQAir advertising the .003 micron efficiency, it's about the 99.5% capture rate.

Any HEPA will capture some .003 micron particulate, just a lower percentage.

.3 micron is the most difficult particle to capture, so the HEPA engineering standard uses it as the benchmark.

There is no way any build-your-own purifier will come anywhere close to an IQAir.

By the time you assemble the parts for the homemade air cleaner, you will have spent enough cash to just buy a decent air purifier, though not an IQAir.

But for do-it-yourself guys like us, it ain't always 'bout the money, I know.

So where to start?

I recommend beginning with some simpler homemade air cleaners and working up to the experimental HEPA.

I use two "po boy" dust collectors made from $15-dollar 20-inch box fans from WalMart with 1/4 inch polyurethane foam AC filter sealed with tape, on the front or back.

I have tried attaching MERV-rated furnace filters with zip-ties, but the additional pressure causes air to circulate within the housing rather than through the filter.

My poboy works better with triangular pieces of cardboard taped to the front corners to reduce counterflow air escape.

It catches large amounts of visible dust in the 20 micron and up range.

See blog.makezine.com for a quicky box fan build.

And see Ed's prefilter experiments for some of the prefilter materials cheaply available and a few ideas.

A simple dust collector made of a plywood box, squirrel cage blower, a MERV-7 to 13 furnace filter, and polyurethane prefilter would be pretty easy.

These are routinely present in do-it-yourself woodworking and other shops.

For a nicely constructed homemade air cleaner example see;

lumberjocks.com.

I would add a couple layers of polyurethane air conditioner filter as a prefilter, otherwise a pretty sharp build-your-own project, hanging from his shop ceiling.

But these efforts are nowhere close to HEPA standards.

Realize that these shops are trying to filter sawdust with particle sizes mostly larger than 100 microns.

A piece of no-se-um fiberglass screen will catch most of this macro-dust.

I think these shop dust catchers represent what can reasonably be expected from build your own homemades. Congrats to the builders.

You could try home-building a UV photocatalytic setup, with less backpressure than HEPA.

Homemade HEPA?

However, when you start to build your own true-HEPA air cleaner things will get more complicated rather quickly.

HEPA is an efficiency standard, not necessarily a type of filter.

It just turns out that only pleated paper type filters perform to spec, confusing the public into believing HEPA means paper-type filter.

This opens the door to marketers who sell "HEPA-type" air cleaners - they are NOT HEPA.

Over the years, gigantic engineering resources have been invested in true-HEPA air purifier design. vMore recently, Chinese government financed teams have carefully reverse engineered every top selling product in the world.

The result is a glut of cheap but serviceable air cleaners.

We homebuilders will NOT out-perform these guys, not to mention the world class engineering talent at IQAir.

HEPA filters require significant backpressure, which means tight sealing to prevent bypassing.

True-HEPA filters also require powerful fans - the squirrel cage won't cut it.

Some Chinese-made air cleaners, the cheapies, still use squirrel cages, with their inherent balance issues.

Out of round fans are the chief source of returns or dumpster deaths of these "bargain" air cleaners.

Fan design is an engineering specialty in itself, I doubt if your machine shop can produce the balanced impeller found in the IQAirs.

We can scratch-build our own go-kart, even a dirt-track racer, but not a twin-turbo BMW 335i.

It is very difficult to force air through the box, some air always tries to reverse course.

This is why "HVAC guys hate HEPA filters" - the furnace fan is not designed to support the backpressure.

HVAC HEPA requires its own side circuit ducting and never achieves the efficiency of a room air purifier.

See HVAC air purifier for more on the shortcomings of whole house HVAC air cleaners, which include the Trane Clean Air System you mention.

Whole house/HVAC systems are fine for the affluent, but not cost effective for the majority.

I think the best approach there is to focus on keeping the interior - ducts, pans, coils, fans... - of the HVAC system clean.

This pressure will burn up inferior fans.

Engineering teams struggle to build mass produced air cleaners - the Hunters, Holmes, and other department store brands - with just the right mean time between failure at marketing price points and warranty claim budget targets.

The best thing to do is to choose a widely available existing rectangular HEPA filter with strong edging, and build your project around it.

Many filters are built with floppy, weak edges, too-thin cardboard.

Note the sturdy polyurethane edges on the IQAir filters.

But be aware that filter availability is declining during the great recession - choose a widely sold purifier for the filter donor.

It will be difficult to seal the gap between the filter and the box - another chronic builder issue. Maybe caulk the gap semi-permanently, and use a knife to remove expired filters and caulk.

Good prefilters will allow long enough HEPA life that build-your-own guys will not mind the labor at filter change time.

The motor-fan assembly is the heart of HEPA.

I think you would do best not trying to design your own.

Just find an air purifier deal on craigslist or ebay, where many folks sell running air cleaners when filter costs loom, and salvage the parts.

Let me know how any of your experiments turn out.

But don't forget that cat allergy is not primarily airborne - protein allergens are transferred by touching the cat or surfaces it contacts.

Best wishes,

Ed


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