How do you run HVAC ducts in concrete?
Got another question where I need educated. Currently working on an addition where we are putting in a footer and slab foundation that will serve as the floor for the new addition. In this project we are merely extending the LR outward about 8 feet. A review of the HVAC air supplies in the existing LR (along the wall we’re opening into the addition) seems to indicate that the supplies are “molded” or somehow embedded in the existing concrete slab serving as the floor for the existing LR. We need to extend these 8 feet into the new addition.
All of my HVAC work is done the typical way using ducts and I’ve not had to bury any supply ducts in concrete. I’ve been at places such as my church where I’ve seen the air supplies are inside the slab. But I”m not familiar with the construction technique used to accomplish this and I am sure this is a common practice. How does one do this? Do you run metal duct and then do the concrete slab pour around it, or do you just somehow mold a chase inside the slab that serves as the duct? If so should it be lined with foam or ductboard? If someone can educate me on this type of construction technique I’d be most grateful.
Replies
Looks like you have already figured it out yourself, take your choice --eg "PLAN AHEAD"
I see PVC used a lot. Ground sqirrels like it here in CO. Makes nice tunnels and nesting area. Just chew into it.
We use PCD coated spiral ductwork when burying ducts. PCD is a white plastic film that covers the ducts. Actually, PCD can be order with the film inside, outside, or both. It uses a special tape to seal the joints, and is much stronger than normal sheet metal ductwork, so you don't have to worry about it collapsing, although you do have to make sure people don't put too much weight on it (wheelbarrows full of concrete). It has ransition pieces to convert to normal ductwork after it is out of the slab. Dig trench for the duct, put in some gravel, install duct, backfill with more gravel. It is also more expensive, so get pricing before you estimate the job.
I only dealt with this once, about 20 years ago, so I don't have any sage wisdom to pass along, but I do have one caution ...
Bend some stiif wire, or even light re-bar, wicktets to anchor the ductwork down. It can be a real headache if it starts to float out of the slab during your pour.
Formerly BEMW at The High Desert Group LLC
ahhhh.... how do you know this? wanna share? LOL
Thankfully, I can plead ignorance. I was new to concrete work, it was someone else's job, and I was just manning the dumb end of a shovel. Just my observations.Formerly BEMW at The High Desert Group LLC
As Nick ponted out, it should be below the slab, not in it. A layer of gravel and the vp over that should prevent the float out problem from occurring. Add rebar in the slab over the duct run(s). 6x6 10/10 mesh is not sufficient reinforcement over an area of non-compacted fill under the slab. I would probably also place a control joint cut on each side of the run. If the slab does crack over the duct, at least you get to control where it happens.
One of the other guys mentioned one practice that I've heard of and is common practice. I dealt with it a little bit when I used to sell ductwork. Round spiral duct coated with PVC can be buried in concrete. The coating gives the ductwork some protection against rusting. You'd want to check with a ductwork supplier (try McGill Airflow- they have offices throughout the US and excellent technical support) to make sure the gauge of the metal is substantial enough for the pressure of the fresh concrete. Buoyancy is a concern and you'd need to hold the duct down to prevent it from popping up. Also, from the designs I've seen, the duct is typically placed within the slab, not beneath it. The benefit of doing this would be that when the ductwork begins to deteriorate (and it will eventually), there's still a concrete-lined passage way for air to move through. I think the more complicated issue for your situation is how will you tie into an existing line that was encased in concrete? (I'm assuming it wasn't installed beneath the slab.) This need to tie in might require you to rip out all the existing lines in the concrete (or abandon them) and start fresh where the lines tie in in the basement.
Check out this link:
http://www.foremostduct.com/undergnd.htm
This is PCD ductwork. It is intended for under-slab use, and the manufacturer tells you what you need to know to use it. Pretty straightforward.