I'm in the process of building a router table dust collection system, similar to the one shown in the image below.

Multi-port dust collection for router table

On many of the designs I've seen (including the one above), the top port accepts a 2.5 inch hose while the bottom one accepts a 4" hose. I'm wondering if there's anything wrong with using two 4" ports instead of a 2.5/4.

I've got the HF 2HP DC unit (lots of acronyms there).

  • 2
    Oh and try and avoid 90 degree connections when ever possible
    – bowlturner
    Commented Apr 6, 2015 at 15:06

2 Answers 2


bowlturner already mentioned that reducing from 4" to 2.5" does not work very well with a dust collector and suggested some possible reasons why a 2.5" fence hookup is so common for router table dust collection.

I'll try to specifically answer your question about two 4" ports:

I'm wondering if there's anything wrong with using two 4" ports instead of a 2.5/4.

The typical rule of thumb suggests that you want to maintain about 4000 FPM air velocity through your dust collection system. If you drop too far below the recommended air velocity, dust can pile up in the lines.

If you split a 4" hose into two 4" hoses, you've effectively cut the air velocity in half for each branch. (In this particular example, the 90 degree tee and the question mark-shaped flex hose will add some extra resistance. The air will follow the path of least resistance, so the straight branch into the cabinet will still pull more air than the branch that goes to the fence--but for the sake of simplicity, we'll ignore that for now.)

In order to maintain 4000 FPM after splitting into two 4" hoses, you either want to start out with the dust collector pulling the air twice as fast through a single line of the same diameter, or you want to start out with the dust collector pulling air at the same velocity through a line whose cross-sectional area is twice as large before splitting into the two 4" lines.

Now let's plug your specific dust collector into the equation. WOOD Magazine tested the Central Machinery 2hp dust collector from Harbor Freight and found that it only pulls 536 CFM @ 4.25" WC static pressure (a far cry from its advertised 1550 CFM). Coincidentally, 536 CFM is almost exactly the amount of airflow you need in order to draw the air through a 5" line at 4000 FPM. But if you're just using 4" hose all the way, 536 CFM equates to 6142 FPM through a 4" hose according to Bill Pentz's static pressure spreadsheet. So at first glance, it appears this dust collector falls short by quite a bit. But that's not the whole story.

The good news is that 4000 FPM is only necessary to lift dust vertically. If we're talking horizontal and downhill runs, 3000 FPM is adequate. So if you can smooth out the bends in the line (specifically, replace the tee with a wye and straighten out the question mark-shaped loop), you should have no trouble pulling air through each branch at about 3000 FPM. Mathematically speaking, your dust collector should be able to handle a slightly improved configuration just fine, and realistically speaking, the pictured configuration is suboptimal but possibly "good enough."

  • So...just to clarify. If I use two four-inch ports and do a 45 degree bend rather than a 90, it should be good enough? Would it be improved with a 2.5 in on the top?
    – dfife
    Commented Apr 7, 2015 at 16:02
  • Yes, a 45 will be better than a 90. Probably the best you could do is to use a wye to split a 5" line into two 4" lines, or take two 4" lines directly from your dust collector all the way to the two ports on the router table and fence. I'm doubtful that a 2.5" hose connected to the fence would improve performance unless it's hooked up to a separate Shop-Vac, or perhaps if your line to the dust collector is too long and you need to pull most of the air through the lower port.
    – rob
    Commented Apr 7, 2015 at 16:18
  • 1
    Thanks, @rob. I've already got everything running on 4", so I don't think I'll be switching to 5 anytime soon. I do plan on putting blast gates at each connection so i can partially block one and devote more suction to the other. (e.g., mostly block the top when I'm cutting dados or mostly block the bottom when I'm rounding edges).
    – dfife
    Commented Apr 7, 2015 at 16:55

If you are using a dust collector and not a shop vac the 4" in both should work great. I've noticed that the tools I have the neck down to 2.5" do not collect as well. The dust collector works on volume and a 4" hose allows a lot more volume.

The shop vac work the other way (static pressure?) and the smaller openings 'focus' the air movement.

I suspect the 2.5" on top either has to do with economies, (cheaper materials) or usability (flexes easier and is easier to keep the hose out of the way).

Or as TX Turner pointed out, the fence has a 2 or 2.5" connector.

This is from my experience and limited knowledge of airflow.

  • 1
    The other reason they may go with 2.5" is the size of the fence. Hard to fit a 4" connector at 45 degrees with a 2" tall fence.
    – TX Turner
    Commented Apr 6, 2015 at 14:17
  • @TXTurner Ah yes, I missed the most obvious answer of all!
    – bowlturner
    Commented Apr 6, 2015 at 14:18
  • Fortunately, I haven't built my router fence yet, so I'm able to be flexible in the connection.
    – dfife
    Commented Apr 6, 2015 at 14:53
  • Most off-the-shelf fence dust adapters are 2.5", so....
    – keshlam
    Commented Apr 7, 2015 at 12:48

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