Ducts in slab?

[ac4me]

Does it make sense to blow air thru slab on grade to help cool the house?
Would I use metal ducts or will pvc work? I have a HVAC contractor designing the geo system, but just wanted to ask this. All the main ducts will be below ceiling in living space. Thanks for the input.
I live in Nashville TN. 3000 sq. ft.

[fat eddy]

Never seen that done before, If you want to stick with typical designs I would say not to.

[woowoo]

I have in slab ducts
Your better off planting a few trees

[md master]

I have never seen in slab/underground ducts that don't have water that seeps in. Unless your slab is 8-10" thick the ducts will be buried under the slab. Don't bother unless you like high humidity and mold.

[marauderx]

Originally posted by md master
I have never seen in slab/underground ducts that don't have water that seeps in. Unless your slab is 8-10" thick the ducts will be buried under the slab. Don't bother unless you like high humidity and mold.

Exactly. Ducts under an underground slab is a VERY BAD IDEA. Even the tightest of ducts will gain moisture and from that comes a whole new host of issues.

Is the contractor thinking about pouring the concrete once the ducts are in place? Another nutty idea that you should avoid. If you want something like that, go with a water system and use PEX in the slab.

[millerman]

Check out some 70's 2-story buildings with 1st floor slabs. Many of them had ducting in the slab, churches also. When you pull the registers and inspect them you will see water. Bad idea.

[bornriding]

Why would u put air through slab on grade. Concrete slab has no effect on cooling btuh, so what would cooling the slab do for you ??
If you are talking the supply & return ducts running through slab. I agree with most everyone. Have had to vacuum out duct system below grade due to water in ducts.
Owner had complained of gurgling sound when unit on.

[cool-blew]

Its surprising how many diggers we have here in So. Utah, I dont normally bid them, but have done a few.

I dont like the fact that it increases the overall load of the home, the ground acts as one big heat sink. Try running 120° air through an underground system then check that leaving air temp in the back bedroom, lucky to get 90°!

[duct dr]

I've actually seen a few here in AL but they were return ducts, not supply. During the change out, the worst of worst happened, a kitten got in the ducts and went exploring. took an hour to get the little sh#$ out.

[berad]

Originally posted by ac4me
Does it make sense to blow air thru slab on grade to help cool the house?
Would I use metal ducts or will pvc work? I have a HVAC contractor designing the geo system, but just wanted to ask this. All the main ducts will be below ceiling in living space. Thanks for the input.
I live in Nashville TN. 3000 sq. ft.

It was used all of the time in my area until the water table rose due to city growth causing poor watershed. They used metal. I would rather see something that won't rust away. Many of these did, but as I siad, the local climate changed and the ducts spent a lot of time wet. Most of these slab houses have now been converted to horizontal units in the attic a long with all of the ductwork.

[cem-bsee]

use pvc ducts in lots of #69 gravel with insulation under ducts, over gravel -- slab must be elevated --

but, all depends -- where are you?
someone can design a workable system & hse

I would just have crawl, or bsmt --

[metal magician]

I think what he's getting at is this-
They are designing the system with main trunks that are going to be in his living space taking up head room and he wants to know if it would work just as well to put the mains in the slab via transite heating and save the room by not having a soffit. Am I correct?

[41gasman]

I have Auncle in noth Carolina.
When he built his home about
5 years ago he did something
very similar to what you are
describeing.He did NOT use
metal ducting.What he did was to
lay down 10" to 12" of gravel
(for drainage)than installed
concrete building blocks.
the ones with three holes
in them.They were laid down
diagonally to the width of
the floor plan.This was done
to get the maximum length
possible.Then the foundation
was laid doen on top of that.
a couple of dams were installed
so that the air flow went down
one side than back though the
the other.Keep in mind that
he built on top of a mountain
so water saturation was not
a problem.He also has a chiller
as a back up but it rarely
comes on.Keeps a three story
house at 72 to 74 degrees
with no problems.If I understand
what you are trying to do
is using the ground as a great
big heat sink.Than if you spend
alot of time thinking on how
you can do it in your area.
than I'm sure you can come
up with a way.I have to tell
you that I think that a good
geo system is the way to go.
from some of the home owners
and techs that have posted here
they are very very economical
to operate
41Gasman

[classicrock4you]

In my area there were many houses built in the 50's and 60's with 4 inch slab ductwork with heat only. Then someone
came up with the bright idea of installing central air conditioning in homes, and people tried to retrofit. Talk about static pressure!