2-1/2" vs. 4" Dust Collection

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Blog entry by nobuckle posted 12-27-2011 06:35 PM 10085 reads 1 time favorited 11 comments Add to Favorites Watch

Okay, so I was blessed with some money for CHRISTmas. I’ve needed to obtain a better dust collection system for quite some time. I am currently using an old 1-1/2” system which is okay but something has to change.

There are several options that I am considering. The first consists of a complete 4” system. The second option would be to go with a complete 2-1/2” system. Finally there is the option to use a combination of 4” and 2-1/2” systems.

No matter which route I take I will have to do some serious purging in the shop in order to have space for the new DC system.

I’m thinking that for machines such as the power miter saw, bandsaw, drill press, and router table, a 2-1/2” system would suffice. The TS, jointer, and planer would utilize a 4” system.

What are your thoughts concerning this matter? Feel free to visit my home page to view my shop. This will give you an idea of what I’m up aganst.

Your thoughts and suggestions are greatly appreciated.

-- Doug - Make an effort to live by the slogan "We try harder"

11 comments so far

View Paul Stoops's profile

Paul Stoops

348 posts in 2555 days

#1 posted 12-27-2011 07:26 PM

As I recall, the industry standard for wood working dust collection recommends at least 400CFM airflow in order to collect and suspend dust and debris in a ducted system. This requirement can barely be met with a 4” duct system—and it is not possible to attain that airflow in a 2 1/2” system with a typical woodshop DC. It is astounding how even a larger system like the 3HP unit I have, that can move as much as 1600CFM airflow with 6” ducting, will choke down to very little suction when restricted by a 2 1/2” hose or port.

Dust collectors move a high volume of air at low static pressures, as compared to shop vacs that move a little air at high static pressures. Therefore, I would recommend that you seriously consider a minimum duct size of 4”. In addition, for a ducted system, keep your runs as short and straight as possible, and minimize the use of flex duct.

Many folks have used the Harbor Freight 1 1/2HP DC very successfully, especially when equipped with a Thien Baffle and a Wynn cartridge filter, to assemble a workable DC system with 4” ducting at a reasonable cost. I would also recommend plumbing your system with thin wall PVC ducting. There is a lot of information on such a system here on the LJ forum and others. Also there is a forum for the Thien baffle with lots of build information.

Think your system through carefully before you buy so that you can get the most bang for your buck, and beware of inflated airflow numbers quoted by the DC manufacturers. At best, most of their quoted airflows were measured with a straight, smooth duct not more than 10 ft. long directly into the fan inlet. My 3HP system measures about 1600CFM under those conditions and about half of that when hooked up to my actual 6” shop ducting. There is a lot of information available. Tho very extensive, check out the Bill Pentz website for an exhaustive, authoritative discussion of wood working dust collection. Bill is highly regarded for his expertise in this area.

You may find information in this article informative:

-- Paul, Auburn, WA

View bigpops0259's profile


302 posts in 4143 days

#2 posted 12-27-2011 10:39 PM

Ive been useing a grizzly 3hp Dual Bag system for a few years now. It’s outside my shop in it’s own enclosure. I ran a 6” Pvc trunk line in to the shop, Then connected it to all my machines via smaller branch line down to 4”, 3” to 2” all with blast gates at use point, even a 4” 25” Flex hose to wand to clean the shop, One problem the PVC had a lot of static would arch, till I wraped a copper ground around the PVC and grounded all the machine to the system. Added remote start unit and my shop is almost alway clean. I go outside and empty and change bag with no mess or dust in the shop. Vaccume was about 400.00 plus PVC added over the years never cloggs estimate 3400 CFM with the 6” – 8’ trunk with T’ to 4” and on to smaller.

-- Marty Ohio

View wolflrv's profile


85 posts in 2365 days

#3 posted 12-28-2011 01:42 AM

I just upgraded my dust collection system from shopvac to full 4” system and I can tell you for sure it’s like night and day! I’m using the 2hp HF DC along with a 4-station blast gate box and a trash can separator. I have yet to make the Thien baffle for it, but it’s on my to-do list. You can see pics of my setup on my workshop page. I definitely agree if you’re gonna make the change, go for the 4” system, you won’t regret it!

-- Handcrafted toys, models & gifts at --

View ScotttheSurveyor's profile


31 posts in 2345 days

#4 posted 01-03-2012 12:59 PM

Nobuckle, I am in the same boat. I just got a dewalt 735 planer for Christmas and…..what a mess! I’ve read the reviews about the dust and chips it made. I was in for it. So I tried to rig it up to my 1989 16gal shopvac. Needless to say that didn’t work. So having read the review ealier about it, I set it up at the garage door. before I new it I had a handsome pile of sawdust.The old vac has seen its better days, I think. We got our money’s worth. I’m currently making the folding table that is shown here on LJ. One of the steps is trimming the MDF to size. When the wife saw what I had done to the garage…a nice thin coating of dust on EVERYTHING. And in everything. I was promptly instructed to “fix it”. Well thats good and bad. I have marching orders but have a budget also.I am all ears as to getting advice on this subject. I have a R4511 tablesaw, planer, miter saw, router that will need to be hooked up, and am planning on getting a jointer sometime this year.
BTW, the planer does a fine job. I took everyones advice about taking only a little off at the time. Verry smooth and no problems as of yet…

-- Angular error is proportionate to the distance run

View HamS's profile


1829 posts in 2383 days

#5 posted 01-03-2012 01:22 PM

IMO there are two aspects to dust collection and one is easy the other is much more difficult. The first is the mess of chips and sawdust. Sweep or suck them up and that problem is dealt with. It is only a mess and is relatively easy to bring under control. The other problem is the fine suspended wood dust in the air and that is much more difficult. Most of our operation throw sawdust and wood chips around, however, dust collection usually puts fine dust (measured in microns) INTO the air, suspended and floating until it settles out on everything. My dust collector does that and my shop vac does that and most everyone’s does until you go the next step. Unfortunately, the next step costs more $$$.

As everyone has stated dust collection is all about air movement. Air going into a bag also has to come out. If the stuff riding on the air is going to stay there has to be some filtration. The smaller the mesh size, the better filtration and it is better for the filter to filter the dust out than to let your lungs do it. Unfortunately the smaller the filter mesh, the more power you need to get an equal airflow. That is where the cyclones etc come into play. The cyclone uses gravity and centrifugal (or is it centripetal force, I never can remember) to separate out the fines from the air. It works great, but it is another cost and consideration.

My recommendation, get the 4” bag system, build a thien separator, then build a box around the dust collector. put two or three good furnace filters around the air return to the shop. If you don’t let the air back into the shop you won’t get good flow. The air return is the biggest problem to deal with because it is hard to get that air through those filters. I had my system enclosed and neglected the return and the suction dropped off drastically until I opened an air return, and then the point of the enclosure was negated. I am now trying to find a fan to exhaust my dc enclosure effectively. I am looking for a furnace blower cheap.

Good luck.

-- Haming it up in the 'bash.

View Howie's profile


2656 posts in 2917 days

#6 posted 01-03-2012 02:53 PM

HamS..make sure you get a fan with an explosion proof motor. Fine dust will flash.

-- Life is good.

View 8iowa's profile


1580 posts in 3755 days

#7 posted 01-03-2012 03:55 PM

There is the totally ingrained assumption that “the bigger the pipe the better the flow”.

However, in order to suspend the dust & chips in the airstream, the flow must be kept at a minimum velocity of 3500 ft/min., otherwise the dust & chips will settle in the pipe before it gets to the dust collector. Keep in mind the hard fact that when you go from a 2 1/2” diameter to 4”, the velocity will fall by a factor of the difference in cross sectional area. If you have a dust collection system with 2 1/2” hoses that provides the minimum 3500 ft/min velocity, and change to 4” hoses, the velocity will fall to 1360 ft/min and the system will fail.

Then the only way to get the velocity back up to 3500 is to increase the size and HP of the dust collector. Thus, it’s a shame that many small and very small shop owners are losing valuable floor and wall space with systems that are greatly oversized.

If you have a shop with 1000 or more square feet, and tools fixed in place, a higher HP system with 40 to 60 running feet of pipes and a 4” diameter makes a lot of sense. This even makes it possible for more than one person to work at a time. Many of these shop owners elect to have the dust colletor in it’s own enclosure outside to reduce the noise level.

Here in Gainesville, in my 194 sq. ft. half garage shop, I have a small 3/4 HP dust collector with two 2 1/2” hoses, 10’ and 16’ in length. I use the same system in my 620 sq ft “Workshop in the Woods” in the Upper Peninsula. The only tool that I run into a capacity problem with (cu ft/min) is a 15 inch 3 HP planer. Even then, if I keep my cuts to around 1/32” my dust collector works just fine.

Frankly, I doubt that I’ve even made a small “dent” in the “bigger the pipe the better the flow” mindset.

-- "Heaven is North of the Bridge"

View Howie's profile


2656 posts in 2917 days

#8 posted 01-03-2012 04:43 PM

Why is it that most tool manufacturers put 2-1/2” connections on the tools if they need 4”? My TS,BS,CMS,Planner and router fence all have 2-1/2” connections. My jointer(that I don’t use) has 4” and my d/c’s have 4”

-- Life is good.

View nobuckle's profile


1120 posts in 2755 days

#9 posted 01-03-2012 09:43 PM

It has become painfully clear that I don’t know squat about dust collection. It’s going to take quite a while for me to get through all of what Bill Pentz has written on the matter.

I know that my decision to purchase the HF 2HP system may come as a disappointment to some but in the end I felt it was the most reasonable decision. I feel confident in this decision due to the facet that I can upgrade with a Wynn cartridge filter and a Thein seperator. I will eventually place the unit outside the shop. Right now it’s freezing cold and I hate to work in the cold. So, in the shop it goes.

I know that this issue is not settled but I would like to thank you all for the support and information that you have provided. I will keep you updated as my dust collection eduaction continues.

Thanks and take care.

-- Doug - Make an effort to live by the slogan "We try harder"

View Grandpa's profile


3259 posts in 2669 days

#10 posted 01-03-2012 10:38 PM

I am still looking at dust collection. I will say that I went to several sites and got some information. Without looking again I can’t tell you whether it was Laguna, Pendtz or Clear Vue. I did find the formulas for doing the calculations. It is easy if you can work arithmetic. They help you size the pipe and work with the velocities.

View nobuckle's profile


1120 posts in 2755 days

#11 posted 01-03-2012 11:18 PM

Thank you sir. I’ll have to check each one to gleen what I can.

-- Doug - Make an effort to live by the slogan "We try harder"

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