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?

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.

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

wrote in message
oups.com...
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.

On Sat, 25 Mar 2006 19:09:30 -0600, "J Geez"
wrote:

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?

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.

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

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

"J Geez" wrote in message
. ..
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?



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.


--
Colbyt
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