DIY air filtration unit for <$200

[Zach Browning]

I just finished building a filter system with an attached baffle box for noise control. It turned out even quieter than I had expected. With the door of the laser open, the sound of the air rushing into the exhaust channels at the back of the bed seems louder than the 1.5HP DC motor sitting right next to the laser. After running it for a bit engraving wood the HEPA filter has turned a slight yellowish color, does this mean I need to add more charcoal? I could only detect a slight hint of burnt wood smell.

[Glen Monaghan]

No idea what exactly you have in front of your HEPA filter, but seems reasonable that the yellow color could be sap/smoke from the wood, in which case you need a better prefilter and/or more charcoal.

[Zach Browning]

My setup has a house furnace filter as a pre-filter, then a bed of charcoal, then HEPA. I went from 2" of charcoal to 3" and added a second furnace filter, which seems to have helped. Right now I'm just using a cheap filter that I cut to fit, as 18x18 filters aren't sold anywhere locally that I can find. What kind of MERV rating should I be looking for in a pre-filter?

[Glen Monaghan]

I use a whole-house filter that is nominally 16x19x4 inches and rated MERV 13 to serve as a relatively inexpensive sacrificial prefilter. Likewise, I use a 16x19x4 HEPA-class filter and so decided to go with 16x19 for the activated charcoal (AC). Because of the amount of AC in the bag I bought, I opted to go with two 16x19x5 trays of AC (which is actually more like 4.5" each of AC because I used an adjustable-sized foam furnace filter on the bottom of the trays to retain the AC).

The 16x19 whole-house and HEPA filters are each 4 inch thick and deep pleated to provide a large surface area for lower restriction and longer life.

[Raphael Weil]

I use a whole-house filter that is nominally 16x19x4 inches and rated MERV 13 to serve as a relatively inexpensive sacrificial prefilter. Likewise, I use a 16x19x4 HEPA-class filter and so decided to go with 16x19 for the activated charcoal (AC). Because of the amount of AC in the bag I bought, I opted to go with two 16x19x5 trays of AC (which is actually more like 4.5" each of AC because I used an adjustable-sized foam furnace filter on the bottom of the trays to retain the AC).

The 16x19 whole-house and HEPA filters are each 4 inch thick and deep pleated to provide a large surface area for lower restriction and longer life.

Hi Glen, happy holidays!

Can you share the link of where you're finding 16*19 Hepa's? I can't find them to save my life. Thanks!

[Glen Monaghan]

I'm using the Bionaire Hepa Filter model A3501H. Most places list it for around $100 but some vendors such as FiltersUSA have it for around $58.

Just found it at Sears for $46...
http://www.sears.com/bionaire-hepa-f...adCell=pvt_1_4

[Raphael Weil]

I'm using the Bionaire Hepa Filter model A3501H. Most places list it for around $100 but some vendors such as FiltersUSA have it for around $58.

Just found it at Sears for $46...
http://www.sears.com/bionaire-hepa-f...adCell=pvt_1_4

Thanks Glen!

[Bryan Spiegel]

I used this as a basis for our first filter. We clogged so many standard, hepa filters and wire screens, that we couldn't make it work. Tried lower fan settings, more pre-filters, everything.

Just ended up being a waste of 60lbs of charcoal and a move to a location where we could vent the thing.

[Dan Hintz]

I used this as a basis for our first filter. We clogged so many standard, hepa filters and wire screens, that we couldn't make it work. Tried lower fan settings, more pre-filters, everything.

Just ended up being a waste of 60lbs of charcoal and a move to a location where we could vent the thing.

What were you trying to filter, plywood fumes? Rubber? Certain items will completely clog ANY solidmaterial filter in short order.

[Bryan Spiegel]

Acrylic and birch engraving. Found that most wood smoke gets on everything.

[Zach Browning]

I'm having issues as well. My system is running in this order: Laser>MERV 12 Furnace Filter>18"x18"x4" Bed of carbon> HEPA Filter>1.5HP dust collector motor. When running wood it gets most of the smell, I ran a 2 hour engrave job on some birch ply yesterday and only had a very slight burnt wood smell. I ran a piece of plexi today and had to leave the room shortly after starting the small job. My HEPA is also turning yellowish/brown, and I'm not sure if that's normal, or means I need more carbon, or a better pre-filter.

[Dan Hintz]

I'm having issues as well. My system is running in this order: Laser>MERV 12 Furnace Filter>18"x18"x4" Bed of carbon> HEPA Filter>1.5HP dust collector motor. When running wood it gets most of the smell, I ran a 2 hour engrave job on some birch ply yesterday and only had a very slight burnt wood smell. I ran a piece of plexi today and had to leave the room shortly after starting the small job. My HEPA is also turning yellowish/brown, and I'm not sure if that's normal, or means I need more carbon, or a better pre-filter.

If your HEPA is turning brown/yellow, it's the acrylic vapors getting through to it.

How often do you change your carbon bed? You need to weigh the charge, and once it doubles in weight, it's time to replace. My bet is your bed is way overdo for a change, so the HEPA is the last line of defense and is getting hammered.

[Zach Browning]

If your HEPA is turning brown/yellow, it's the acrylic vapors getting through to it.

How often do you change your carbon bed? You need to weigh the charge, and once it doubles in weight, it's time to replace. My bet is your bed is way overdo for a change, so the HEPA is the last line of defense and is getting hammered.

The filter has only been in use for about 2 weeks, and this was the first time I've run plexi. It probably only has about 18 hours of wood smoke and ~15 minutes of plexi fumes through it. I'm thinking the 1.5hp motor may be pulling the fumes through the charcoal too fast for the charcoal to pull the contaminants out.

[Glen Monaghan]

I couldn't stand cutting acrylic with only 4" of AC. I use 8" now, which is good enough for most of my needs, but I find it insufficient for longer acrylic cutting jobs (20 minutes or more). The coloration of your HEPA filter suggests wood smoke/resin is getting trapped there and will clog it in fairly short order.

First, try a MERV 13 for the prefilter to catch more there, then add another 4" of AC if you have space, and finally, try using a speed controller on your dust collector to dial down the flow rate to the minimum needed to properly clear your machine of smoke (giving the AC more of a chance/time to clean the air).

[Zach Browning]

Unfortunately, my DC can't be speed controlled, so I may have to double or tripple up on the pre-filter to add some in-line resistance. I'll see about getting a higher MERV pre-filter as well.