Running Duct under Slab... Good Idea or Bad Idea???

[Chris Padilla]

For slab construction, if you want duct and power under the floor, use trenches with 1/4"/foot pitch and removable plywood covers sitting in recessed ledges to each side as others have suggested -- A better solution would be to build a crawl space and wood floor high enough for flexibility later. My home is built on a hillside and I have a crawl space under my 800 SF basement shop that ranges from 9' to 4' of head room and a wooden floor. My CV cyclone is on a slab in the 9' lower corner and all of my ductwork is all from below.

That is pretty cool, Robert. Post some pictures of your shop and underneath...would just be neat to see that.

[David C. Roseman]

Albert wrote:

Well, I think you all have raised good concerns. I had been planning this for quite some time but never asked for any feedback until now. My plan was to install the duct at the same time the plumbers are doing the rough plumbing that goes in the slab but I much rather have the flexibility. So now I just do it the traditional way and run my duct from the ceiling. The other good think about doing it this way is that I don't have to pay the plumber any more money =).

Albert, sounds like events may have overtaken this reply, but if not here's my view. Why not use a hybrid approach, as I did? For machines likely to be on mobile bases or located near walls, plan on ceiling and wall runs and drops. I suggest 4" thin-walled Sch. 20 pvc "sewer/drain" pipe to or near the machines, feeding back to larger pvc pipe (also Sch. 20) for the trunk lines, depending of course on the capacity of your central dust collector.

For your table saw, presumably stationary out toward the middle of the shop, run 4" Sch. 40 pvc plumbing pipe under the slab for ducting to the cabinet's dust port, and 1-1/2" Sch. 40 pvc electrical conduit for 220v and 120v power cables for your saw (and router, if you use a router in your extension table). Bring the pipe and conduit up through the floor using 90-degree sweeps, not 90-degree elbows. Use primer and pvc cement on every joint for water-tightness. No need to slope the pipes as you would if carrying drain water, but if you are worried about the possibility of condensation collecting (I wouldn't be), you certainly could slope them toward the sweeps at one end where you could access it if necessary.

If for some reason you eventually want to lose the floor penetrations, no big deal to chip out 2" of concrete around the pipes, cut them off, and plug, fill and fair with ready-mix.

For the ceiling and wall runs, you can use a ground wire inside the pvc ducting if you feel the need, but that's another subject, on which much has been written.

I used this approach when we built our current house nearly 20 yrs ago, and it's worked very well. For the table saw, especially, it is so nice not to worry about tripping over duct and conduit, or to maneuver around vertical drops when handling large panels.

David

[Glenn Kramer]

In my shop I ran 6" PVC vacuum lines directly across the shop beneath the middle of the slab. They are placed in trenches filled with drain rock. The trench is lined with a permeable membrane to prevent the sand from contaminating the gravel. As the line crosses the shop it picks up the dust outlets from the SawStop and Hammer jointer/planer. At each shop wall (perpendicular to the buried line) the pipe rises to wall mounted horizontal line to service machininery along the walls. These wall mounted line offer complete flexibilty for future changes.

This installation as been in operation for 27 years. If I had to build another shop I would design the piping exactly the same way. I also ran galvanized compreses airlines in the same trench. If I sold the home I would fill the floor openings with concrete.

My 2 cents!

[Charles Musick]

Hello, not sure if this applies.
Have you thought about a geothermal heat pump? I wonder if this ideal would work, but run the piping under the slab. The air from under the slab would be around what tempt? maybe 68? it could be pumped in, to help cool or heat? I really don't know much about them. I saw a show about a guy who lived off grid. He used something like this to cool his house. He basically buried pipes, and put a fan in them. The air pulled thru the pipes, then either cooled or heated to ground temperature (68 ?) and then into his house.

[Kyle Kraft]

I'd use the white pvc coated spiral metal duct, the coating will stop corrosion and it will surely outlast you. Plan your shop layout carefully using grid paper to scale. When I built my shop I did just that and then arranged my equipment to the print. Nothing has been moved since and I am completely happy with my layout. I ran only one piece of ductwork "under" my slab about a year after it was poured. I had a concrete cutting outfit come in and sawcut my slab then I built a form and molded the concrete liner in place with a lip so I could make a plywood cover and have it flush with the floor. I ran 4" metal duct and my power cord in the trench and never looked back.

We had a carpenter shop at work years ago and ALL the ductwork and electrical conduit was in or below the slab. When they closed the carpenter shop they sawed the conduit off, ground it flush and grouted the duct holes and conduit...now we have a flat floor.

[Jaromir Svoboda]

I did run 6" PVC pipe under slab. Works great.