Replies

1. 
   jagwah | Mar 04, 2004 10:44pm | #1

   Be very careful about what you use for the ducts. While I'm not a H&A guy some of the issues over the years that my clients have run into is rotting or rusting out duct work.

   The worst here in Tulsa is in an addition in a flood zone. Seems when it rains a little the ducts fill up with water. Some of the galvanized ducts have rust outs and the moisture also invites critters in the homes.

   Nowadays I would think they would require an aggregate fill and weep cloth to keep the area around the ducts dry.

    

1. 
   NormVila | Mar 04, 2004 10:54pm | #2

   I run into a similar challenge when it comes to heating and cooling split-level home designs.  I have had success in simply using 4" corrogated solid (no slits) drain tubing--the same material used to extend the reach of downspouts in guter systems--under the slab-on-grade portion of the home.  Sock-type insulation can be used in conjunction with this method.  Although a greater number of runs must be made, typically one to each vent, the corrogated drain piping is inexpensive, does not collapse under the weight of soil and/or a concrete slab and does not rust. 

2. 
   Ruby | Mar 04, 2004 10:59pm | #3

   The house we live in has such, put in there in 1960 and it seems trouble free.

   So we asked about using the same for the new house and were told that they don't do that anymore in slab houses.

   All ducts and the furnace are in the attic, with ceiling fans to recirculate the air in the house, all computer drawn as of location and size and that it has been shown to be as efficient as any other way to condition air and easier to build and maintain, for on slab houses.

1. 
   CloudHidden | Mar 04, 2004 11:47pm | #6

   >All ducts and the furnace are in the attic

   Domes don't usually have attics. :)

1. 
   jagwah | Mar 04, 2004 11:56pm | #7

   Being a dome and I assume a wall thickness of over 6"? what about the high efficiency 3" dia ductwork they bragged on This Old House a while back. It aloud for going through some narrow walls. Just spider out the duct over the shell before you spray. Sorry if I miss the mark on your particulars. I'm flying by the seat of my pants,(wide wing span but heavy load). 

1. 
   CloudHidden | Mar 05, 2004 10:02am | #13

   >I assume a wall thickness of over 6"

   The structural part of the wall is maybe 3". Anything larger than 3/4" elec conduit is typically not appropriate in the shell walls.

2. 
   Ruby | Mar 05, 2004 12:19am | #9

   ---"Domes don't usually have attics. :)"---

   Some day I will remember to read to whom I am posting.:-(

   You know, maybe to have some kind of attic, or a relatively small upper space for such as mechanical and ducts, on part of the structure, dome or not, centrally and connecting different rooms, over a hallway, maybe, may not be such a bad idea, for the locations where underground services are not appropriated?

   Then, what do I know about building?

3. 
   BillHartmann | Mar 04, 2004 11:41pm | #4

   First you MUST have good drainage.

   I have such a system, installed in 79. There are systems that are used to warm the floor, just like hot water systems.

   But mine is just designed for air distribution. The slab is insulated, minimully, but high for that era. There is enough heat transfer to the concrete so that you can walk, barefoot in an area that I have unfinished concrete. You just want to be quick about it.

   Now with one exception I have not had any moisture problems with it.

   There was construction defect where the drain tile from a higher level was not exteneded out, but was terminated right at the footings for this level.

   Normally there was enough drainage that I did not even know about this for several years. But one spring we had about 3 days of constant rain, followed by a down pour. The ducts would fill about 1/3 with water. But the next day it was all gone and no residual moisture.

   But several cycles of this caused the ducts to rust and a plastic liner to flake off. But it has not caused any long term problems.

   I have been looking at remodeling for the last couple of years and part of that is what would I do differently if I was building new.

   I would do the same thing, unless I went with radiant.

   
   

   ---

   Edited 3/4/2004 3:43:36 PM ET by Bill Hartmann

4. 
   csnow | Mar 04, 2004 11:47pm | #5

   There is an oufit that makes superinsulated underground ductwork.  Comes in rolls.

   Now if I could only find that link...

5. 
   DanH | Mar 05, 2004 12:02am | #8

   Well, our home (split entry built in 1976) has under-slab ducts for the lower level. It is apparently a circular run around the perimeter, with the furnace (downdraft unit) at one long side, though I suppose there may be a "spoke" down the middle to make it a figure 8.

   This works out pretty well, with a couple of caveats:

   1) Transite pipe (roughly 8") was apparently used, and this represents a theoretical asbestos hazard that might affect resale value (though it doesn't worry me living there).

   2) The builder failed to put in an accessible sump, and we have a minor groundwater problem (when it rains hard for several days straight). So I have a pump rig that I stick down one of the register openings when needed.

   3) This setup makes any sort of radon control pretty much impossible. Again, our radon is slightly elevated -- not enough to worry me, but possibly enough to affect resale value.

   4) When it's damp, you can get a musty smell when the furnace comes on after a long time off.

6. 
   jagwah | Mar 05, 2004 12:30am | #10

   http://www.toolbase.org/tertiaryT.asp?DocumentID=2088&CategoryID=962

   Here's a site that talks about the small duct system.

    

7. 
   DavidThomas | Mar 05, 2004 01:13am | #11

   Jim, I've done tens of thousands of feet of underground vent piping but for a different reason - carrying toxic compounds in air from wells and soils off to be treated.  A couple of things I've encountered:

   Water droplets can build up in horizontal runs and slosh around if there are low spots preventing it from draining one way or the other.  Careful sloping (like DWV plumbing) or high air velocity (1000 fpm keeps it moving) solves that.

   Solvent-welded pipe is nice for keeping out the groundwater, but how hot is your air?  Don't take PVC too high.

   Only had gophers gnaw into one pipe run (PVC).  Couldn't understand what plugged it up till we dug it up.  A truly nasty smell - they were long dead, probably from the CO2.

   This is a big one that does transfer from toxics to homebuilding - don't bury the balancing valves!  Strongly consider doing home-run piping so you can 1) adjust, 2) balance, 3) snake any line in the future.  It is cheaper to tee them off a main trunk but that could come back to haunt you.

   David Thomas   Overlooking Cook Inlet in Kenai, Alaska
8. 
   TLRice | Mar 05, 2004 09:26am | #12

   Cloud,

   I have two jobs currently in the installation stage in which I used underfloor ducts. In commercial applications and large open spaces, like church sanctuaries and the like, very common.

   The product to use is called PCD (polyvinyl coated duct) in the "4x4" configuration which has 4 mils inside and out. This is NOT a place to go cheap.

   Code requires the duct(s) to be "pitched to an acessible low point" for drainage in the event of water seepage or flood.

   Have to brace the openings to prevent collapse during the slab pour and typically (always by my specifications) ufd is placed above the building slab moisture barrier, bedded in sand.

   I don't have the specs infront of me now, but if your interested in doing this and doing it right (using plumbing pvc or drain tile is not advised), I can get you the right info.

   Tim