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Even though this thread is years old, it probably bears some updating since there is a subforum here for the dust collection discussion that is a cornucopia of helpful information:

Having read through the information and links posted there (specifically, and Dust Collection Research - Home) [WARNING: There is a LOT of reading involved in those links...], I changed my mind completely about what a dust collection system is and needs to do.

At first, for one tool in a hobbyist's shop, the Harbor Freight DC that I had was more-or-less adequate for the occasional bit of wood destruction. What I noticed, however, was that it could not keep up over the longer haul with my tablesaw/jointer/planer or other higher-volume dust creators if I wanted to run a line for any distance between multiple tools, even with blast gates. Thus I began my pursuit on how to "build a dust collection system" out of the HF unit. The multiple suggested mods turn out to create a Frankenstein's Monster of a system that really, in the long run, doesn't do the job particularly well. This is my opinion, based on a number of factors:

1) Inefficient system design (the dust collection setup -- ducting, joints, etc.., not the HF unit)
2) Inaccurate specifications of the DC (the HF unit, not the DC setup) -- it will simply NOT do what the supplied specifications state it will do.
3) Poor understanding of the expected results of a dust collection system (including the setup and the unit itself)

Let me summarize my newly-found understanding of the issues above (after reading extensively the links noted above):

1) Inefficient system design:

A DC system needs to breathe. This means that any reduction of the diameter of the main line of the DC will dramatically decrease the performance of the entire system. You can reduce at the end machine (e.g. tablesaw, router), but if you reduce the main line by using either an inline reducer or poor choice in routing the main line (using multiple 90s, Ts, flex pipe or simply too long of a run, etc), you are shooting your dust collector in the foot. If your DC unit comes with a 5" or 6" inlet, then do NOT reduce to 4" for convenience of available materials. There is NO advantage (and, in fact, there is GREAT disadvantage) to "immediately reducing" to allow you to use materials that can be found at the local box store. The internet is your friend -- you can find cost-effective purpose-built ducting in the right size. In my case, my craigslist-found Grizzly G0440 has a 7" inlet. By doing some math (found in the tables within the links above) I realized that the capacity of the machine could be easily enhanced by actually increasing the size of the main trunk from 7" to 8", giving a sizable gain in machine efficiency. The long and the short of it is that you can only pull so much air through a given diameter of line at any amount of "power". There is a point of diminishing (and negative) returns, but there are charts and calculations that will allow you to arrive at the right amount of CFM for a given machine/setup.

If a machine inhales, it must also exhale. The exhalation can be vented outside or inside. However, if you limit the machine's ability to exhale (exhaust) with a poor filter, then you are, once again, shooting your DC in the foot. The pleated filter that came with my G0440 was replaced by a Wynn Environmental 13F230NANO. This means that I am using a true HEPA filter on the output, eliminating a large portion of the ultra-fine dust that the system will produce. This change went from 90 feet of pleated filter to 230 feet and from MERV 11 to MERV 15 in terms of filtering efficiency. [I had originally bought this filter for the now-discarded HF unit.] Why not exhaust outside? Well, my neighbors don't like dust and the fact that I like to keep my cooled air inside my garage. Here in Texas, that's a thing. :) Why not use the cloth bag that comes with the HF unit? For one, it is a horrible filter for outgoing air, allowing significant dust to escape into the air. For another, it is very restrictive and doesn't allow the machine to exhaust effectively. As more fine dust coats the inside of the bag, your machine's efficiency goes down, down down into that burning ring of fire... Sorry, my inner Johnny Cash is showing.

Take a LONG look at the WHOLE system, and don't design the dust collection system into an existing shop layout -- design the layout of your tools around an efficient DC setup. You might say this isn't possible -- trust me that it is. This means reducing and eliminating "hard 90" turns (utilize a minimum number of 90 degree "sweeps" instead -- the fewer the better!) and instead choose 45 degree turns wherever possible. Design your shop with the shortest run of pipe possible: I went from an initial design that used 60', multiple 90s and lots of gates to a design that uses 40' of main line, one 90 degree sweep and two 45 degree angles. Use 45 degree angle Y's (or wyes) instead of Ts. You can make saddles to do this yourself (which I will outline later). By carefully selecting materials and constantly evaluating the incremental changes in my system design, I also saved around $600 in materials.

Use properly specified materials for your main line: heavier-gauge snap lock pipe, spiral pipe, etc. The less flexible duct you use, the better: You want smooth runs of line which will decrease turbulence in the air stream. Try not to use the adjustable fittings found in the HVAC section of Home Depot or the like. Use of PVC pipe with no grounding is asking for it. You won't find many people who have actually SEEN a problem with it, but some of us (including me) have and the results can be unpleasant. It is NOT an internet rumor or old wive's tale. I couldn't care less if you (or someone you know) says they've been doing XYZ for 20 years with no problems -- to them I say "you only die once".

For "big reductions" (e.g. 2.5" from a main trunk from something like a router table) you might have to actually do multiple ports on the tool. You've got to MOVE AIR to make the dust collection happen. On my system, for each drop I am using 6" (from the 8" trunk), and reducing to 4" only where necessary. In one case I have to use multiple ports -- my router table.

2) Inaccurate Specifications

The HF DC unit advertises 1500 (If I recall correctly) CFM at 2hp. This is a laughable lie. My G0440, advertised at 1350 CFM will inspire awe when compared to the blazing "1500 CFM" from the HF unit. [In truth, HF is not the only company doing this... I'm an equal opportunity basher when it comes to this!] Come to my house and see -- I have the old HF system still. My Griz unit is powered by 220v and will, in fact, produce the HP and CFM as advertised. When you compare the two machines, you will shake your head at the difference. There is no flipping way that the HF unit can do a higher CFM than mine. My Griz's 2hp class F motor is substantially larger than the HF motor, which makes one wonder how, indeed, the HF unit can make the claims that it makes.

I do not fault someone for buying something based on a recommendation of someone they trust or from reading (and expecting truth from) the specs. But the reality is that you're lucky to get half of the advertised performance out of the HF unit. As long as you're fine with that, then all is well. But don't buy into the line that spending the $500-$600 (or more) on "upgrading" the unit is going to get you a great performing unit -- it won't. You're money may increase the performance somewhat, but can't overcome the problems inherent in a poorly designed setup. Also, don't buy into the fallacy of "sunk cost" -- spending good money after bad to improve on something which cannot be improved upon to the extent that one desires will not solve the problem. You may have to cut your losses and actually find a better solution. Or, as I call it, "Craigslist to the rescue"! Wait for the right deal on a purpose-designed piece of equipment to replace something that is no longer meeting the need.

3) Poor understanding.

A DC system is, at its heart, a glorified vacuum. But, much like a Ferrari is just a glorified car, when you do everything that is supposed to be done to allow the Ferrari to do what it is supposed to do, what a car it is! You want the 'big dust' to get sucked up. That's easy. You also want the finer dust sucked up. Ok, done. But there is ALWAYS super fine dust (swarth) that the DC cannot get from the machines for a number of reasons [see above, and in links]. Using a face mask (like the RZ Mask) may help, or even a positive-pressure ventilator when working with hazardous materials. Also, using an overhead "shop air filter" will help. You will likely never get ALL of the superfine dust, but you can certainly come close with multiple steps in your overall solution.

I'll ramble on a little more when I've got my shop put back together...

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