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

[Albert Chio]

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

[Peter Kelly]

I'd be concerned about selling the house later with all those holes in the slab...

[Larry Browning]

Albert,
How about UNDER the slab? Or How about a trench that can be covered? That would give you access to the ducting when something goes wrong. Plus, you say they are pouring concrete in a few days?? AND you are just now deciding/planning your in floor ducting???? WOW, you got more guts than me!

[Jamie Buxton]

Putting the ducting under the slab has some drawbacks. You don't say where you are, but if the duct ever gets water in it, you're in trouble. Or if it rusts, again you're in trouble. I'd accomplish the same thing by casting a trench into the concrete. The duct drops in later. You can also run power in the trench. The trench gets covered with wood or steel. I'd use 1 1/8" subfloor plywood. Make your choice now, and cast ledges in the trench to support the cover, so the top of the cover is flush with the floor.

[John Coloccia]

I move my machines about once a year. I think that 5 years from now you'll have a foundation full of unused pipe and a bunch of pipe hanging from the ceiling anyway.

[paul cottingham]

I move my machines about once a year. I think that 5 years from now you'll have a foundation full of unused pipe and a bunch of pipe hanging from the ceiling anyway.

+1. You will lose a lot of flexibility.

[Albert Chio]

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 =).

Thanks,
Albert

[Thomas Bank]

I could see moisture in the ducts leading to clogs. Although you don't specifically state it, I gather that you're saying ductwork for dust collection?

[John Coloccia]

Since you're starting from scratch, might I recommend that you run about twice as many outlets as you think you'll ever need, and have them run with 10 gauge wire. Also, don't just do one big circuit. Run lots of different circuits. First, you will be running multiple tools at the same time...air cleaner, shop vac + a tool...maybe an airconditioner? Dehumidifier/humidifier? Compressor. You'll be surprised just how many different circuits you might need. Second, you'll be able to go to any circuit you wish and turn it into a 240/20A or 30A any time with nothing but an outlet change and a breaker replacement in the box. If you do this, ask the electrician to label the neutrals or you will have a heck of a time figuring out which 10AWG white wire goes to that one circuit you want to change. This has saved my bacon in my shop NUMEROUS times....just countless times. I actually rewired my whole shop with exposed PVC conduit. It's ugly but quite functional and easy to move/remove/replace/whatever as my needs change. That 10AWG wire, though, is one of the smartest things I ever did and it just keeps paying for itself over and over and over again. You may need to get some industrial outlets to handle the larger wire, but it's worth it IMHO.

[Mark Burnette]

My shop is laid out with a garage door in the front and a big rolling door in the back so placing the TS in the middle between these seemed the obvious choice. I ran PVC & electric conduit under the slab over to that area in 2 runs--one for the saw & another 4' behind for "whatever". This has served me well for 20 years. I have 2 regrets: I should have run a 3rd line in front of the saw to give me the option of setting up my planer or jointer there instead of behind where there's less room. And I should have used proper deep electrical floor boxes in the cement instead of cheaping out and using wall boxes. Over time they fill with dust and if I don't use a plastic "safety" cap to keep them covered the recepticals will clog up, too. Minor issues & I've only had to clean them out once so far.

I used rigid foam under the slab (for comfort) and have never had any moisture issues. I do have a hairline crack over each duct but no separation or heaving.

I LOVE not having any hoses or wires hanging down or crossing the floor.

[Mark Burnette]

3rd regret: Today I would probably run a 6" duct under the slab but 20 years ago the consensus on the brand-new internet was 4 & 3" lines were plenty big enough for consumer-grade equipment. I have no complaints & never have had to clear a clog--or leave the building for air--so 3" drops have done the job but I now know that larger is better. OTOH a 6" hole in the floor is a hazard where a 3" hole I can just leave open.

[Albert Chio]

Thanks guys. These are all great inputs. As regards to electrical, I will have some lines running underneath the slab for my table saw, jointer, and other power tools. I was able to sketch my work environment with all my tools using google sketchup to get a pretty good sense where I need to place the conduit for each of my tools. I am also having all my electrical for my house and my workshop done by my uncle who is a license electrician and we have already discussed the ideal of using 10 wiring and isolating the workshop to its own breaker box with room to expand.


Thanks,
Albert

[Jim Andrew]

If I were putting pipe under a slab, would for sure use plastic. My problem is, I would decide to buy some new piece of equipment, and have no place to hook it up. Problem with building a new house, for sure the plumbers will decide to run their sewer line through your shop area. You might check with your inspector, but don't think the plumber would have to run your duct pipe. You are the home owner, and should be able to run that yourself. But your next problem will be the concrete finishers complaining about having to go around all your pipes. Might have to cut them off just below the top of the concrete, and cover with tape so they can pour over the top of them. Then after they are done, just go around and knock the concrete off and go around the edge with a chisel. Better yet, wrap tape around the outside so you can put a coupling on the pipes later.

[Kirk Poore]

When I did my shop, I put in a trench from one wall to the middle. All the machines along the walls get drops down the walls, there is a machine cluster in the middle (planer, TS, shaper) that is hooked up to the 6" pipe in the trench. I used PVC in the trench, and after gluing it up in my back yard I filled it with water to make sure it didn't leak. It was fine. I dumped the water and put the pipe in place. The trench has a stepped edge allowing me to put down 3/4" ply as a cover and be flush with the rest of the floor.

I don't think you need to pipe under the slab everywhere. But one pipe to the middle sure is handy.

Kirk

[Robert Payne]

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.