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



## jcalberto (Jul 14, 2011)

Hi Guys,

I am building my house and in a few days concrete will be poured in. My plans is to install 7" duct in the slab where my workshop is going to be. The benefits doing this that I have come up is that I will hide all the duct work with the exception where it sticks out of the slab for each of my stationary tools. The other benefit is that I will save ceiling and wall space. Now the questions that I have is this a good idea and has anybody here done this. Also I prefer using metal duct due to its low resistance coefficient but if I do will it corrode over time due to moisture in the slab or should I use pvc? Last, I am thinking that for the main run underneath the slab I should use 7" diameter pipe to allow maximum cfm at the tool end. Would this size be fine?

Thank You,
Albert


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## ssnvet (Jan 10, 2012)

You'll need a pretty hefty blower to get the velocities required to keep chips entrained in the air stream with 7" pipe. And build up of deposited debris in under the slab piping doesn't sound like a good idea at all.

I personally think its a great idea to put it under the slab… but I would only use PVC.

Let's just say that I don't think you'll need to worry about grounding the exterior….

You'll do well to put a couple cleanouts in…

And you'll want to have your shop layout planned out in pretty good detail, inorder to make sure the risers are located in the right spots.

If you don't back fill with good material (stone) and don't compact well enough, or don't have sufficient depth of fill over the pipes, you'll likely get a nice crack line in the poured floor running right above your pipes.

I put in the 4" black flex hose around the inside perimeter under my basement slab, just in case I needed to mitigate for radon later on…. and can tell you exactly where the pipe is…. right under the cracks :^(


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## wapakfred (Jul 29, 2011)

I've reworked my ductwork probably 8-9 times over 10 or so years, once was for a move to a different house and shop. That's what I think may be the biggest downside to putting it in the slab…it's permanent. You will add tools, upgrade tools, or just change the shop layout. But if I did put it in the slab, it would be PVC….


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## HorizontalMike (Jun 3, 2010)

Since you have the fortunate ability to build from the ground up, I would suggest adding a ~6'x6' slab on one side of the to-be-shop slab and then build an attached room for your DC system. Then duct the shop from there. I would also consider having/building the shop with at least 10ft ceiling (I have 12ft) for head space and/or ducts.

I think Fred is correct in suggesting that shop layout is an evolutionary process and likely to change in the future. Keep your options open…


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## MonteCristo (May 29, 2012)

7" ? That's pretty darn big and an odd size to boot. Better have one honkin' big collector to keep the air moving fast enough. As someone else mentioned, being able to clear clogs is essential. I don't think rust will be an issue. Even if it were, you'd just end up with a nice concrete lined hole anyhow !


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## LukieB (Jan 8, 2012)

I personally think it's a fantastic idea, and dream of one day doing it myself. I agree that PVC is the better way to go, and that backfilling properly is important. And I think 6" would be plenty big for a main line.

Hope it works out good for you, keep us updated along your journey!


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## 1yeldud1 (Jan 26, 2010)

I see you are from Texas. In some parts of the country moisture under the slab could cause problems if it was to gain access to inside the pipe. If I was to install duct work for a dust collector under the slab here in Missouri I would have to us a lot of fore thought to keep moisture out of the pipe (ground water as well as the pipe sweating). Possibly having the pipe insulated or having the pipe "sloped" to a area of the pipe that i could pull put a drain plug in. Something to think about.


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## johnstoneb (Jun 14, 2012)

While putting the ductwork in the slab sounds like a good idea. It's permanent and can't be worked on without tearing a floor up.in the 50's a lot of houses were built on concrete slabs and the plumbing was in or below the slab.in the 60's those pipes had reacted with the concrete in some areas and in others the water was slightly acidic and had eaten holes in the pipes repairs were almost impossible.
You may have your shop layout plan now but that is evolutionary. I am building a 16×24 shop right now and I have changed my shop layout 3 times in sketchup and still not sure that is how it will be when I actually put the tools in the shop. What about adding new tools.


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## longgone (May 5, 2009)

I have no idea how many people put their tools in one spot in their shop and kept them there forever…My shop is 5 years old now and I have added tools and rearranged it several times. My ceiling is 10ft and the ductwork is no problem at that height. I even have some ductwork running along a wall. I sectioned off a 5×8 section of my shop as a sanding room a couple of years ago and extended ductwork into it. Easily done with it running at the ceiling.

Personally, I would never want my ductwork in the slab…


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## jcalberto (Jul 14, 2011)

Hi Guys,

Thank you all for your inputs. I originally got the idea after seeing a workshop setup where the duct work was done underneath the workshop floor. The only difference is that in that workshop the floor is suspended off the ground but in Central Texas all the floors are made of a solid concrete slab with no crawl space. If I were to go with PVC I am not worried at all it reacting with the concrete or the road base back fill dirt that they are pouring to build up my foundation. The PVC I was planning on using is the same PVC that is used in all the slabs here in texas for the waste water which I hope is made to not decay due to bad reaction with its surroundings. Now, I was thinking of 7" pipe after reading an article from Oneida where it recommends not using 4" pipe until you get closer to the tool otherwise you end up cutting the cfm by half. I believe in their example they said it was better to use 6" but I thought maybe upping it by 1" more would be better but maybe this is too much.

Now the thing that has me worried after reading your feedback is the potential loss of flexibility to moving my tools later on. I already have most all the major big stationary tools so after sketching them and placing them on sketchup I felt really confident where I want to permanently place them but I guess by nature one always gets tired of one setup.  Regarding leaving holes in the slab if I sell well I am not too worried either. This is a attached workshop that I am building separate to my attached two car garage. I don't plan to move ever but if I do, new buyer can always cover with temp plate or if he/she likes pour concrete.

Here is a image of my workshop sketchup.










Thanks,
Albert


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## jcalberto (Jul 14, 2011)

So I was going to run the pvc underneath the slab starting from the two state dust cyclone across diagonal towards the band saw. Ass it passed across, it will have branches that will come out of the slab next to each power tool.


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## woodbutcherbynight (Oct 21, 2011)

I would rec not believing you "know" where everything will go forever. As others have suggested you change as you go. Case in point. I bought a mini lathe 10 years ago, made a nice workbench with lots of room to work. Who would know that my Father would give me a second minilathe, with a second set of all the same chisels I already had? So much for plenty of dedicated space with lots of room to grow I spent alot of time working this out to have two lathes side by side and still have not added the extra port for dust collection.

*HorizontalMike +1* He suggested modifying your building to have a 6×6 extra room. I have such a setup and getting the compressor and dust collection out of the shop is a big plus. Having a system you can expand and repair is a big consideration. Heck who knows you could be sitting on the next discovered tectonic plate??? (laughing)


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## NGK (Mar 2, 2013)

Some good ideas above: However, here are some more points to consider. It it's not too late, put your dust collector motor and impeller OUTSIDE the shop, even if it's just covered with a roof. That gets the NOISE out of the shop. Have the impeller outlet return to the shop so that you don't lose "heated" or "air-conditioned" air. Regarding noise, the greatest source of noise is not the motor itself, but the "exhaust" from the impeller. And that is the precise location for a "muffler". Many companies sell them, including PENN STATE and the reduce the noise by 50 percent.

No big difference between metal piping or PVC below the floor in the concrete. As one responder said, even if the metal were to rust and decintegrate, you'd essentially just have a concrete tube. Cracking can be eliminated or minimized by (1) digging the "hole" 6-7 inches deeper where the tube will run, and (2) by running rebar across it on top about every 2 feet.

As another suggested, leave access to the underground portion for clean-out in case slow cfm ever contributes to a plug-up. You could even leave a cable between two different clean-out ports and drag a chimney-sweep or other device through that area as needed. Most likely to happen if air flow is reduced with (1) too many ports open or (2) the chip barrel fills up and reduces air-flow in the filter area.

Don't worry about the diameter being too big, especially ear the motor. I'd start with whatever diameter is by the impeller. Then as you get farther and farther from the power unit, reduce the diameter until you have the typical 4 inches at each machine. Avoid sharp bends. Use Y-fittings instead of T-fittings. Use gradual reducers instead of sharp-cornered reduces. Use long sweeping elbows.

Yes, no initial plan will be perfect as your shop evolves. Plan the lay-out so that one wall has machines that do NOT need dust collection. A jointer, for instance, really doesn't create dust-it makes chips-therefore you don't need dust collection there-just a box. I don't feel a drill-press needs dust collection. And maybe not a bandsaw.

And while you're thinking of underground stuff-run some electrical wiring through plastic piping in the concrete. I have one such line centrally located where I located my table saw-a four-way box with both 220 and 110. And I have another one with multiple outlets near my assembly table. You might want a floor-sweep, too.

My shop is 32×32 and I have never had a plug-up. Ceiling is 8 ft, 8 inches with most piping on the perimeter of two sides, where ceiling and wall meet. One branch angles off from near the cyclone over to the table saw where I have a down-draft sanding center as part of the outfeed of the table saw. Air is also connected to my radial arm saw, two edge sanders, a router table, a 15-inch planer, a 12-inch drum sander, and a 37-inch double-drum sander.


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## sprucegum (Dec 6, 2012)

I have a friend who has his shop in part of a old Dairy barn. He put his ducts in the gutters (concrete trench that the cow s&*t drops into he then covered the gutters with some plywood panels. It makes a nice system and it is very easy to change and repair. I see no reason you cannot use the same system. The easiest way to make the trench bottoms is pour them before the slab goes in, just make the bottoms a few inches wider than you need them then you can set the slab forms rite on top of them. Some blocking between the form boards will keep the concrete from pushing your form around provided you fill both sides equally


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## joeyinsouthaustin (Sep 22, 2012)

+10 to the gutters Idea…..Also add an 8" or bigger line connecting the gutters to a central point it the middle of the room. Think of this as a chase you can run flexible DC hose and electrical through if your shop changes. Also it is much easier to pull flex line out of a chase to clear a clog, than pulling pvc out of the slab  
Carefully mapped it would also be easier to drill down into the chase in the case of a big future change.


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## MrRon (Jul 9, 2009)

I think 6" is large enough. If space is a concern, you can run the duct thru the wall and run it along the outside. I would use PVC. I don't like ductwork, wiring or pipes that can't be accessed. Ducts in the ground, should they break, would allow ground water, if any to get into the DC system.


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## teejk (Jan 19, 2011)

I'll chime in…newer cyclone systems use 7" or 8" at the intake but they probably get reduced pretty quickly. And I would question whether the light weight soil/drain PVC would be strong enough to hold the concrete in the middle of the floor (it works fine for the perimeter drainage because nobody ever walks that close to a wall).

A typical house will use sch 40 (commercial sewer plants require sch 80). Both are $$$$ in 6" or 7" sizes.


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## shipwright (Sep 27, 2010)

Not sure about in the slab but I love my underfloor system. You will have to have access to clean it out though. Mine cleans from the crawl space but could be done from above.
I used a 30' piece of 8" PVC for a main.


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## AandCstyle (Mar 21, 2012)

You have your dust collector in a corner. If you can move it closer to the center of your shop, the runs will be shorter and the DC will be more effective. FWIW


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## MarkwithaK (Sep 12, 2009)

Ductwork in a slap is a freaking nightmare and In all my years in the HVAC industry I have never found one viable reason for it. It will eventually rust and when it does you are screwed. Now of course I am referring to duct intended to move air but I would say that the same detriments would ring true for DC ducting.


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## jonwright (Feb 4, 2013)

Personally, I wouldn't run duct in my slab.

In all the planning I've always done, no matter what about 1/2 the time I'd say "I wish I woulda done this" I fretted for a YEAR on my house addition to get everything just right in the floor plans and even now that it's built "I should done this differently"

I'd want the flexibility of having the ducts along the ceiling myself. What if there's a new tool you want and you need to run additional duct to it?

Electrical outlets, on the the other hand, I can see that in the floor. Just be careful of spills and I'm sure code would require GFCI unless having eletrical in the floor is code violation.

I DO like the idea of the ducts in the floor, and for that reason alone if I do get to build my shop I may put in pier and beam foundation.


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## NGK (Mar 2, 2013)

Art-AandCstyle-NO, you don't want a dust collector in the the shop at all. And especially in the CENTER of the shop. (1) Who wants all that noise centrally located? (2) The center of a shop is prime location. Think human movement AND material movement. Who wants a dust collector in the way every time you turn? Spend 50 bucks more for a more powerful motor if that's cfpm is your biggest fear.

As I said elsewhere, I have 8-inch running down one 32-foot wall and 7-inch running across the adjacent 32-foot wall. Plenty of suction at even the furthest 4-inch drop-down, with a 2 HP unit.

I cant find it now, but somebody said 6 inches is as big as they would go for inlet piping. Well…..if you dust collector has 7- or 8-inch at the inlet, why CHOKE it?


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## AandCstyle (Mar 21, 2012)

NGK, sorry that I wasn't clear. It should be centered to minimize the length of the runs, BUT it can be centered on a wall and still be outside of the shop. I really just think it is better than having it stuck in a corner. Is this clearer now?


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## doyoulikegumwood (Jun 21, 2007)

i had in floor in my old shop and loved it.now keep in mind I have a clear vue 1800 which has a 7" inlet and a 5hp motor. i set it up so there was a clean out / hook up basically every foot in the floor it was not cheep to do but worth every penny. i was able to move tools without any problems. but CV1800 will set you back 1600 and to set up the way i did will requer a very big dust collector.


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## NGK (Mar 2, 2013)

Art at AandCstyle-Yes, clearer now. However it is difficult to have your unit in the center of a wall because it's extremely difficult to have incoming lines from TWO directions at the power unit. The extra elbows will restrict flow as much as extra length of run, and the STRAIGHT run down one entire wall and just one sweeping elbow at another corner for another entire run along that wall is what I prefer.


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## Hawaiilad (Feb 16, 2010)

Because you just might move those tools around several times in the years to come, I would build a small attached unit for your set up, and either run the PVC pipe up the wall and across the ceiling, or along the base board area. This way you can always splice into the pipe at any time you need to when adding more hook ups.


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## roman (Sep 28, 2007)

Bad Idea


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## shipwright (Sep 27, 2010)

From a practical standpoint:


If you put it in and don't like it after a while… cover the holes and don't use it anymore…. easy
If you don't and later wish you had …. jackhammer up the slab. .... really hard

I have underfloor dust collection (and wiring) and absolutely love it. I've had enough shops that I know how I want my tools organized so I won't be remodeling every couple of years. Ultimately, you have to make the decision yourself but it's easier to leave the options open than to retrofit…. and PVC doesn't rust or corrode.


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## rustfever (May 3, 2009)

I am a General Building Contractor, a Concrete Contractor, and a Woodworker. I would like to give you MY THOUGHTS on what you are proposing.
a] If placing duct work under concrete floors, be prepared to later jackhammer up the floor or abandon the system.
b] If using 7" ducking [I do in my wood-shop], you will find PVC/ABS pipe does not come in that size.
c] A dust collection system will need 'long sweep' fitting, not found in any plastic/PVC/ABS pipe. You will need to make them yourself. [Good Luck!]
d] If placing pipe in a formed concrete 'Gutter' or 'Trench', you will need to have a much wider trench to be able to service the dust collection system pipe and fittings. 'Much Wider!'
e] Your trenches will need to run straight and must be a consistent width.
f] When your trench changes direction, you will need a much larger area to accommodate the 'sweep' fittings
g] You will need to cast embed angles on the sides of the trench to accommodate you trench plates or covers. these embed angle will need to be set accurately to prevent tripping hazards.
h] I would suggest nothing less than 1.125" thick ply. be used as a 'cover' or 'trench plate'. the 1.125" thick ply will allow you to span 2'-0" width with no significant flexing. If you have greater flexing, you will have trouble moving tools/equipment/supplies around.
i] direct burial of any dust collection system piping in concrete/earth is going to haunt you. As in 'Bring $$$$$'

You mention building this shop from the ground up. Then, I have an idea. 
Make the ceiling higher and suspending the ducting system from it. Mine is suspended from a 10'-0" height ceiling. All of my ducts have 8'-9" clearance under them. I have never had trouble, except once when I tried to use the large forklift [10K#]to set a piece of equipment. I changed to a 4K# fork that would fit.


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## dhazelton (Feb 11, 2012)

How would you adjust blast gates and such? You'd have to be pulling a huge volume of air to get good suction at all points. Also you are talking about pouring an incredibly thick slab - $$$.


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## sras (Oct 31, 2009)

I have ducting under my slab floor. Here are some thoughts.

I used 4 inch but would use 6 inch near the dust collection unit if I did it again.

My shop is roughly 15×24. I put the collector in one corner, ran an input to the center of each wall and put 3 inputs spaced down the center of the floor. So far I have been able to move things around and make connections work. The built in locations do require a bit more thought. I would probably consider more input points, but so far I am content.

I ran a ground wire inside the PVC. Good for safety, but a major pain. Shavings get caught on the wire. I would try to come up with an alternate grounding strategy - maybe metal ducts or conductive tape.

So far, I have been able to clean out any clogs, but I REALLY wish I had added clean out accesses in a few places.

I do enjoy the ability to move around my shop with no ducting in the way - I would definitely do it again.


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## oldnovice (Mar 7, 2009)

If I had the money and/or inclination to build a shop I would build it like a computer room. A gridded panel floor over the slab with access to the sub floor where plumbing, electrical, Ethernet, and dust collection can be installed and moved as needed. This makes for a very flexible workshop and all the infrastructure is out of the way but still accessible/expandable for changes in the shop.

Really over the top I would make a laminar air flow system, besides the normal DC, to pick up the minute air borne particles. This is really high tech and need a lot of filtration power but would also allow use as a finishing room (barring any ignition sources)!

*Enough dreaming!*


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## danr (Sep 5, 2009)

I live in central Texas.

I have about 2K sq foot shop.

I used sprial metal pipe / fittings (look it up online) in the concrete slab (along with 220 and 110 electrical). Works great, no issues. My slab guys dug the trenches for the duct work after the beam trenches were completed. I put down about 6" of pea gravel in the duct trenches, and then we set the duct on top of the pea gravel.

This pipe/duct company was great to deal with:

http://www.spiralmfg.com/spiralf.htm

I had a few discussion with the owner (he is very knowledgable and he comes to TX every year to hog hunt). He loves Texans.

This is a great way to run duct in your shop. Hope all goes well for you.


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## derosa (Aug 21, 2010)

I think it sounds like a great idea to me, just add some extra locations that you can seal over till the day they may be needed if you decide to reorganize. Hard to tell from scale but I would want to set the jointer to be able to run a slightly longer then 8' board through it without hitting the wall and through the planer without hitting the bench unless the bench is low enough to serve as an out feed table. If that were the case I would figure them to be in a good, probably never need to be moved spot, and so good fixed dustcollection points. To me it looks like the tablesaw is far enough from the jointer for that standard. I chose that because some projects will need just over 8' boards like decorative doors, a new workbench top can be 6-7', maybe a window bench in a study, or whatever. Not much tends to be longer, though not impossible, but I'd want to be able to do bigger projects without moving the big tools around and away from their dust hookup.


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## NGK (Mar 2, 2013)

I don't feel most folks even need a jointer. As I stated earlier, cupping and twisted boards can easily be 'fixed' with a sled thru the planer and edges can easily be done with a 6- or 8-ft straight-edge (level) fastened to the rip fence of the table saw.

If you insist on a jointer, put it on non-swivel wheels to pull straight out from a wall and push back against the wall when not in use. Jointers don't need dust collection-just a box attached at gravity fed chute. Jointers don't make dust-they make chips.

I have half of the support table for my radial arm about (and the wall behind it) positioned so I can rip a 10-1/2 board. With a heavy and unwieldy piece that long, the radial arm table is a useful support for setting up and starting a rip on the table saw.


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