Underground HVAC duct

[J Geez]

I am building a 14x30 attached addition to my home. I am almost ready to pour the slab, and my wife has a new idea and now wants a cathedral ceiling. Now I don't have anywhere overhead to run my HVAC duct from my current house system to the new room. I need to run a main trunk HVAC duct the long way (30') underground beneath the slab to the other side of the new room and then two branches off that the short way (14') that will come up through the floor near the walls. They need to come up through the floor or else I would have to cut the foundation to get the ducts into the wall. So How is this typically done with the ducts under a concrete slab? PVC pipe?

[hallerb]

Your better off installing a seperate furnace and AC unit. You might be better off to use tubing in the slab for heat.

have you checked to see your existing furnace and AC has the capacity for your addition? Espically with the catherdral cieling.

[J Geez]

I have been through all that-- I know I can use my current system. I just need to know about the under slab ductwork.

[Frank Boettcher]

When I lived in the Tulsa area, they used to use downdraft furnaces and put the ducts underground. As I recall, they trenched areas for the ducts, layed in regular sheet metal duct work and then filled the trenches with concrete, then poured the slab over the top bringing the transition ell to a retangle that would get a floor register attached to it. It was quite common.

Your challenge would be getting it from the original house to the start point.

[Stretch]

Foremost makes ducts for underground use. Fiberglass or plastic. With a long duct run, especially with part of it underground, it is not likely the heating air would be very warm when it got to the far end. Smae problem with cooling air, it would not be very cool.

Also, it is not likely you would get enough air. Sounds like you would have a lot of pressure drop.

I would try a high sidewall register on the inside vertical wall blowing towards the outside wall(s). With proper register sizing and proper duct sizing, this would be much more doable with likely much better results for comfort. Your contractor should measure air flow with a flow hood when he is done. Also be sure to put at least one return in the addition.

Stretch

[m Ransley]

Dont you risk permanent mold covering the inside of the ducts from condensation. The dirts temp at certain times will make it happen.

[Colbyt]

I am just going to answer your question.Let you decide what is right.

In a similar situation 2-6" round PVC pipes were run under the slab and insulated. Two may not be enough for the size of your addition but some quantity of 6" PVC ducts may be.