2.5" vs. 4" Dust collection hose...

I’ve run a 4 inch hose to a separator trash can lid and out with
a 2.5” hose to a dumpy Craftsman 6 gallon shop vac and the
setup pulled a pretty acceptable amount of chips from a couple
of different planers I used it on.

Try to keep the 4” line as short as possible, I’d say.

That is a good question. I have never thought of it because generally you want to start big and reduce as you need to, and you are going in reverse. I believe you will be better off running the 2.5” all the way to the tool and having the adapter at the tool, but keep the hose as short as possible. This should at least keep your air velocity higher closer to the tool. Either way your air volume is going to be painfully low for a table saw with 2.5” hose, but I know you already know that. My shop vac with 2.5” hose moves about 80 CFM, and you need at least 400 at the table saw to even remove most of the bigger debris, and ideally you would be in the 800-1000 CFM range to remove fine dust.

-- PaulMayer, http://www.vernswoodgoods.com

I have been using a shop vac and pre-separator (trash can and Thien baffle) for a couple of years. I just completed building my second one about a month ago, this one is a “top hat” design, and I used four inch ports so I could upgrade at some point and still use it. Right now I run 2.5 inch hose from the vac to the separator, then from the separator to the tools (table saw, miter saw, router table and planer) one at a time.

My next upgrade, as soon as I can pick some up, is to change the hose between the separator and the shop vac to four inch. This latest separator works so well that in the process of producing about 30 or 40 gallons so far of sawdust, there is NO visible dust in the shop vac or in the shop vac HEPA filter. Since there is nothing but air between the separator and the vac, upgrading this to four inch should reduce friction loss without causing any problems.

I plan to continue to use the 2.5 inch hose from the separator to the tool, because I am concerned the shop vac might not move the air fast enough in a larger hose to keep the saw dust in suspension.