I just installed a system and used 6" PVC. Stuart is correct, the fittings will eat your money. I made most of mine, though. Go to
http://harderwoods.com/tools.html and look at the Pipe Joint Template Software link. There is also a link for directions of gluing PVC pipe for connections. The only PVC fittings I used were 2 Laterals (Wyes) and 1 45 degree bend (quarter bend). Avoid using 90 degrgee (half) bends if you have the space.
I actually used 5" metal HVAC flex fittings from one of the big box stores, with a 6x5 metal HVAC reducer to connect the DC to the piping. I heated the piece of PVC to soften it, and shoved the 6" end of the reducer on to expand the openeing the little I needed to. I used the foil tape on these parts once set to stop any leaks in the metal duct.
My shop is a 1 car garage. (See attached image) Currently I have a bandsaw, table saw, and router table hooked up to the DC with a extra 4" connection for the jointer or planer. The saws and router are all on the same wall (All the table heights are the same, so they serve as outfeed tables for each other. This is the darker grey shaded areas on the image.) When I need to work on something too big for this layout, I wheel the tool to the middle of the floor. I made sure to connect the pvc to the tools with enough flex hose to reach the middle of the garage. The DC piping system runs across the floor at the wall and has 2 6x6 Laterals (wyes), as shown in blue on the image, shaded light grey. I made blast gates based on the directions from Bill Pentz's site. The 2 laterals are connected to each other with a gate on the upstream side of each branch, and one at the upstream end of the last lateral. (3 blastgates, heavy blue lines) The first branch goes to the bandsaw. I just used 4" hose connectors and the software from the link above to give me a template to cut the 6" pipe and connectors so I could glue them together. I put a 6" cap on the end of the pipe. I also bought a 6x4 reducer for the end of that pipe. After heating it to soften the plastic, I was able to get a 4" DC hose to 4" PVC adapter to fit inside. I had this and didn't try any other fittings. With blast gates on the 2 bandsaw connections, I can close them down, replace the cap with this reducer on the end of the pipe, and I have a hook-up for the jointer or planer. The tablesaw is off the second branch. Right now it is just a 4" connection. I am getting a Shark Guard, though, so there will be a connection for that as well. Both to the same 6" branch. The straight thru run goes to the router table. For that, I took a 6" end cap and drilled 2 holes. 1 at 2.5" for the fence, and a second at 4" for under the table. Glued standard hose connections in the holes and pushed the cap on the end of the pipe. The flex hoses are not shown connected in the attached image.
I originally set up with 1 lateral between the bandsaw and tablesaw. That run actually has 3 4" connections placed using the template, each with blastgates. A couple weeks later, when I added the router, I also added the second lateral.
I have not sealed the joints, and, since I only have about 15' of pipe to any one location, I have not experienced any problems. I have been planning on cauking the joints, but they are pretty tight. I lit a match, then blew it out to get some smoke off it and it went straight up when put next to the joints. If there were any leaks, the DC would have sucked that smoke in. Not sealing the joints have made it nice because I have been able to use and test the layout without worries of a long project to re-arrange things.
I do not anticipate a need for DC on the other side of the shop, but it has not been built out yet, so that could change.
Regards,
Karl