Dust Collection Under Concrete

[Trey Isbell]

Have not posted in a while but I recently started on a new shop and had a question I'm sure a few of you have done this already. I am going to put all my ductwork under my slab just wondering about moisture problems. So my question is for any of you who have done this did you lay your ductwork on top of your polybarrier. I figure that is the best way to go to keep my pvc from condensating and causing problems.

[George Bregar]

I channeled my slab and ran PVC. There was no polybarrier as this was originally a 3 car garage. By "pvc from condensating" I assume you mean keeping condensation from happening in the PVC. Hasn't been a problem for me even in the summer where you are pulling warm moist are through the ductwork which is cool. You're moving a lot of air which should move that condensation out if it's present.
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[George Bregar]

I also ran power under to my table saw and jointer FYI.

[Trey Isbell]

I planned on running power to my tablesaw but I am putting my jointer against a wall. I was just thinking that the pvc would sweat like concrete sweats without a vapor barrier under it. Thanks for the reply

[Bob Wingard]

I believe the PVC in and of itself would qualify as a vapor barrier. Think about it's original intended use ... if it wasn't an effective vapor barrier, I don't think it would survive as plumbing components.

[Jaromir Svoboda]

Use glue to glue pipes together.

[Charlie Ross]

Are you sure you're not going to want to move any of your tools that will be connected to your DC system?

[David Gutierrez]

A vapor barrier will not prevent condensation. Condensation is formed when warm moist air hits a cool surface causing the moisture to drop out of the air as condensation. The PVC is impermeable to water vapor. i think the original question is still valid but i have no answer.

[Thom Porterfield]

In order for there to be condensation, there must be moisture in the air. And, as David said, that moist air must be warmer than the surfaces it contacts.
Unless you live in extreme cold climate (like north of the Arctic circle), the temperature of the bottom of your slab, and therefore the soil it touches (you can forget about the vapor barrier--it's very thin) is probably within a few degrees of the interior temperature of the shop.

Seems to me that if your shop is so humid and warm that the air in your DC system is moist enough to condense on a surface only a few degrees cooler, you have more problems. Think about it. You run the DC--air flows in the piping under the slab. Dust-laden air, too. Within seconds, friction will have warmed the ductwork and cart off any moisture that may have, during idle times, collected there.

Seal the ducts well and I don't think you have anything to worry about...


...except perhaps lack of flexibility in relocating machines later. :)

[Roy Turbett]

I ran 4" PVC under my slab and didn't use a vapor barrier. That was 15 years ago and I haven't had any problems with it. One thing I recommend is that you use tight 90 degree elbows at the point where dust enters, especially from the table saw. This way if there is a long sliver from a cutoff it will hang up before it enters the PVC and is easy to retrieve. This only happens rarely. Where the PVC exits the concrete, use long sweep elbows.

[Larry Edgerton]

Another thing I did to mine is run a champher bit around the inside of the pipe before fitting. If you slip a pipe in a fitting there is a sharp edge where the two come together. The fittings have a champher, so should the pipe. Less chance of a snag and slightly better air flow.

My old shop had PVC overhead, but my new shop will be PVC underground with electrical as well. I am setting mine up with the connections below floor level so if I choose to not use one the floor will be flat with a cover. I am tired of cleaning around cords, and pipes hanging all over the place. I want to be able to roll my benches from machine to machine with nothing in the way.

Larry

[ian maybury]

One issue with going under concrete Larry is that you need to be pretty sure of your duct sizes, machine locations and hook up points.....

ian