Sizing Ductwork

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Forum topic by ThosP posted 05-27-2014 10:35 PM 1295 views 1 time favorited 13 replies Add to Favorites Watch
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6 posts in 1486 days

05-27-2014 10:35 PM

Topic tags/keywords: harbor freight hf 2hp dust collector harbor freight 2 hp dc

Although portions of this topic have been clubbed to death (IN PART) on many web-sights, I still have a question regarding the Harbor Freight 2HP Dust Collector. Has anyone on the “LumberJocks” blog been running 6” dia. ductwork with this unit? I was told that w/6” ducting, the “inexpensive” HF motor may burn up and to stick with the standard 5” dia. . I did some testing on my own, comparing assorted diameter “Inlets” on the blower and found conflicting data to that 6” duct recommendation. Any comments are appreciated.

13 replies so far

View ducky911's profile


237 posts in 2815 days

#1 posted 05-27-2014 11:25 PM

I have the grizzley 2 hp 220v single phase sucks 1700cfm. I have it hooked to a bunch of 6” pvc. Take a look at my workshop pictures(on my home page) to get an idea. DC is in a shed next to the shop. I think my motor is similar to a HF, but I am not sure.

View Jim Finn's profile

Jim Finn

2658 posts in 2948 days

#2 posted 05-28-2014 12:49 AM

I have a 2 HP dust collector but it is a Grizzly. I ran 6” steel duct and it works fine.

-- Website is No PHD just a DD214 and a GED

View ThosP's profile


6 posts in 1486 days

#3 posted 05-28-2014 01:56 AM

Thank you both for responding so quickly! If you are each using a 2HP, 220 volt Grizzly, their is a good chance those motors run much closer to 2HP than the Harbor Freight units also listed @ 2 HP. The HF DC’s comes with 5” hose fittings . Based on some experimenting I have done with this blower, amperage #’s with 6” ducts drew very close to 1.7 HP. Countless bloggers across the internet quote ~ 1.6HP as being the actual #. With my entire system researched, designed and 85% built, I am now having second thoughts based on recent blog comments found elsewhere. If I go with 5” ducts, will I be starving the system and be left with less efficient dust collection or do I go with 6” and possibly get good performance but maybe frying the motor if those web bloggers are correct. Strangely enough using a 5” duct I did measured 2.0HP! With so much conflicting details, I would hate to have to re-purchase/design a bunch of new parts at this point! If their will be more air-born dust with the 5”, what good is the system?

View Rick's profile


9728 posts in 3059 days

#4 posted 05-28-2014 01:58 AM

You might get more responses if your Topic was in a more Appropriate Forum. Understandable as you’ve only been on here for 7 Hours.

Why not review the list of “Forums” and choose the one that Relates Best to your Topic.

This Forum deals with any Members who would like to Report Problems or Suggestion related to the Site.

IE: “Site Feedback”.

-- LIFE is what happens when you're planning on doing Other Things!

View bbc557ci's profile


595 posts in 2100 days

#5 posted 05-29-2014 02:10 AM

Always a contest between velocity and volume….

That said, us use a cyclone and 4 inch hose on my 2 hp HF and it works quite nicely :o)

-- Bill, central where near the "big apple"

View TheFridge's profile


9608 posts in 1512 days

#6 posted 05-29-2014 02:14 AM

Never heard of frying a fan motor because of less resistance.

Or any motor for that matter.

You measured close to 2 hp in terms of amperage because the motor is working harder. Not because it’s working less. If it draws the same cfm through a larger hose with a lower amperage that is the way to go.

Electrician for 14 years talking.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View WoodES's profile


96 posts in 1717 days

#7 posted 05-29-2014 04:44 AM

I have the 2hp grizzly and run 6” pvc with a main trunk line about 40’ in length, 3 side branch lines of the same size that feed the main. Main feeds through a Thein separator and only a little fine dust passes through the DC. Bottom line is all sucks, but that is the point.

View Matt Rogers's profile

Matt Rogers

110 posts in 1996 days

#8 posted 05-29-2014 12:36 PM

Dust Collector motor current increases as the volume of air pulled through the system increases. So if you close off all blast gates or put a tiny 1” hose on the collector, the impeller will be spinning in a sort of vacuum and not have to push very hard against the little air around it. If you have it spinning with no resistance to the air coming in and out (huge hoses), it will pull lots of CFM, but the work that the impeller has to do increases and it can overload the motor. Put an amp meter on your collector with all the gates closed and then with the inlet hose completely off and you will see the difference. You should check to see how many amps it draws witht he inlet hose off to see if it is higher than the motor nameplate rating.

-- Matt Rogers, and

View ThosP's profile


6 posts in 1486 days

#9 posted 05-29-2014 03:09 PM

Thank you all for your reply’s! I have used a digital meter to measure a variety of hose diameters with this blower. That data indicated that using a 5” dia inlet created an equiv’t of a 2.0 HP draw (manufacturers claim). Using a 6” dia inlet, I measured 1.69 HP (says that is the w2ay to go). My dilemma is that this data suggests that I can run a 6” system while one of the “true” DC gurus specifically told me “DON’T DO IT!!!. I have tested inlet sizes from 4.5” dia to a full open blower W/O the “Inlet Cover” in place (2.49 HP). I fully know the higher the amperage (HP) draw, the harder the motor works, in this application because it is passing too much air. It is that measured data that I have compiled which reflects a different story than what the expert says. I would like to hear from more who have set up their HF “2HP” DC with 6” ductwork, with success, before I complete my build (just saw WoodES’s reply after hitting the submit button then went back to edit)). I do intend to locate the proper forum on the LumberJack’s web later today to get more exposure on this item. Thank you-all again for your input.

View English's profile


658 posts in 1503 days

#10 posted 05-29-2014 11:55 PM

You need more data!

If the power went down when you increased the inlet size, you were moving less air. Power has a direct relationship to work done or in this case air (cfm) moved.

To property evaluate the fans performance you will need to add a length of duct to the blower, keep it straight. Then at some point along this length of duct measure the static with the duct open fully and completely closed. From this static measurement we can determine your real air flow and how much work the fan can perform.

You can make a simple utube with water in it for your instrumentation.

Do this test with both 6” and 5” duct with your sample point at the same distance in both cases.

-- John, Suffolk Virgina

View WoodES's profile


96 posts in 1717 days

#11 posted 05-30-2014 04:28 AM

One detail I forgot to mention was the reduction from 6” pipe or hose (6” hose for the center table saw drop) to 4” pvc or hose in the last few feet of the run. All tools are feed with the 4” hose from outlets.

View ThosP's profile


6 posts in 1486 days

#12 posted 05-30-2014 05:30 PM

Thanks John. It was the Forest for the Trees scenario, I guess. I was concentrating only on the #’s and not recognizing what those #’s were actually telling me. I ran five different intake “sizes” from a fully closed off blower intakes to fully open (Inlet Cover removed). I will now go back and add the U-Gage to the two duct dia’s (5” vs. 6”) and that test. I used a u-gage permanently connected to my old 1HP DC system just to monitor when I needed to do a good cleaning job of the filter-bag. Reflecting back on my original #’s, it is looking like the 5” ducts MAY be the better choice, but running the tests you suggested will have to be done to confirm.

View English's profile


658 posts in 1503 days

#13 posted 05-30-2014 05:49 PM

You may want to read this thread. It covers the HF very well. About half way down the first page is Bill Pentz’s HF review.

I started with a HF in my shop, it is a great chip collector, but found that even without the losses of a separator, just using a Wynn filter, the HF could not collect the fine dust created by most of my tools. I was coughing allot when I worked in my shop and for hours after. I researched, ran a few tests on the HF and decided I needed a bigger DC.

-- John, Suffolk Virgina

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