Stud wall cavity for heating ducts

[greg273]

 Has anyone tried this? I was thinking about putting half-inch drywall, then metal, in a 2x6 stud cavity to get some wood furnace air to the crawlspace.  Any thoughts on this?  I'm concerned about the temp, but where I'm contemplating is a good 15ft from the furnace.
  

[n74tg]

Is it your chimney that is going to be routed thru the wall cavity?  If so, is the rest of your chimney double walled or triple walled metal?  Even 15' away from the firebox I think I would still be worried about temps high enough to start fires.

[greg273]

 No, its not the chimney.  I don't have a desire to burn my house down!;D  The furnace has a hot-air plenum surrounding the firebox, for distribution to the ductwork. A 1/2 hp blower connected to a thermostat controls the airflow.
   I guess what really matters is the required air gap between furnace ductwork and exposed wood.   With only 14.5" between studs, thats not going to leave a lot of room to work with....

[jraabe]

Greg:

I did a Google search and found such clearance requirements for ductwork to be inconclusive. It may depend on the temperature of the air you have coming off the heater core. Most modern furnaces and stoves only need a few inches or less of clearance from the heating unit itself.

The first rule with ductwork is to have as much of it as possible inside the heated and sealed envelope of the house. In other words, don't run these chases in outside walls, crawlspaces or attics where insulation will be compromised.

Also, many installers are now making ductwork from a foil-faced insulated board material (I don't know what this is called). It seems that could work in your wall chases.

Sorry to say, but that is about all I know on the subject.

[bil2054]

Hey, Greg.
Just curious about why you want to send heat into the crawlspace.  If you just need a little to help keep pipes from freezing for instance, maybe you don't need a very large duct anyway.  The round flex duct is available starting at. I believe, four inch, and I imagine the six inch could be "squished" a little if needed to fit the cavity.
I watched some guys fabricating with duct board; pretty cool! They measured, then cut 1/2 way through with straight edge and razor knife, and used some kind of foil tape on the edges after bending to shape.  It's like drywall tapers, I'm sure; they can do in a day what it would take the uninitiated a week to accomplish.  I'm sure a short run like you are thinking would'nt take long, though.  Johns Manville is one manufacturer, I think they call it Superduct, or Superboard.

[Amanda_931]

I was hoping that they used the 2" mini-ducts for heat as well as AC.  didn't see anything quickly.  But here's the AC stuff.

http://www.spacepak.com/

[hunter63]

They make flat duct work in two sections, you snap them togeather that are desined to be used in studded walls.
You would be advised to use an interior wall , so as to not compromise the insulation, as was brought up.
Return air can be ducted in the empty space, w/o duct work, but need to keep elecrtrcal and plumbing stuff out to the space.

I put in a return duct inthe floor joists, by boxing the joist in W/ sheet metal sections nailed to the underside of the joists. W/ a grate just inside the front door, It keeps drafts down when door in opened.

Why don't you heat-tape what ever you want to keep warm?