making ducting

I need some duct that will maximize surface area to increase heat loss from the exhaust without causing too much friction.  I don't believe that it will be available to buy locally, so I'm planning to build some myself.

I'm building an 8" rocket stove, so the cross section of this duct should be at least 50 square inches.  my first thought was to buy some galvanized flashing to bend into shape and bolt together and caulk at the seams.  I've also thought about buying a couple lengths of 6" galvanized stove pipe that come flat and snapping those together and bending them into shape.

at the top of this rectangular duct, I'm planning to run it into the side of a horizontal round 8" stove pipe to exit the room through the wall.  I've got a piece of 8" class A chimney for the part inside the wall.

I haven't worked with metal much, so I really don't know what to expect or if these ideas are complete rubbish.  advice?

the flat pack ducting will do fine. if you are worried about the galvanization at all; go build a pocket rocket and use it for the chimney for ten min. but unless you are getting 700 or 800 F in the exhaust you dont need to worry.

I'm not really understanding what you're wanting for the end result but bending the galvanized stove pipe isn't a bad idea; it's got the snaplocks you need to put them together so you don't have to worry about allowances for connecting the steel to itself. How about trying flat oval instead of a rectangular shape? That way you don't have the corners of the rectangle causing more friction in the duct but you still have a large flat surface like you want. Instead of this: [___] go like this: (___) but with a more rounded radius on the ends.

I would suggest some zip screws at the seams so it doesn't pop apart, not self tappers as they will eat up holes in your material and strip out and possibly not hold.

Lemme know if that helps. I don't know what tools people use online to draw out their ideas to share them.

Chris

thanks for the tips, all.  they do help.

Chris, I guess I was leaning toward rectangular because I thought it would be easier to join to the round stove pipe for the exit.  seemed like a rectangular hole would be easier to cut than an oval hole.

I also figured it would be easier to make a rectangular joint with the stove, since I'm using rectangular bricks.  I'm not too attached to that plan, so if there's a simple way around those issues, an oval shape would probably work fine.

I could probably make the straight sides with round ends work without too much trouble.  I guess that's the front runner at this point.

So I went back and read the thread about your sauna; very nice to have a sauna, by the way, back in Boston, south of the city there was a place called, "Finnish Steam Baths" that was a very nice, clean family establishment I used to go to as often as time and money would allow.

If you want the heat to radiate from the exhaust pipe, as long as you are following the dimensions for the heater, i figure you don't need any kind of round on the pipe. Rectangular should be fine. Your system isn't gonna be horizontal, embedded in anything, so the heat will rise like other fireplaces. If you want the heat to radiate from the pipe and not up the flue, you might want more resistance. Maybe?

It is easier to cut a square into a round pipe. Much.

Do you wanna continue this thread at the thread for your sauna?

Chris

sure.  thanks.

If I needed greater heat transfer I might be tempted to use a strip of flashing a little narrower than the diameter of the pipe, twisted into a helix, rather than trying to monkey with the outside profile of the pipe.

This might be a monumentally bad idea, though: I have no experience with furnace pipes. If the cleanout is a straight shot through the exhaust pipe, the helix might be removable for cleaning. Even that might not be enough to make such a scheme safe.

Sheet metal doesn't like to bend that way, ordinarily, but the edges can be mashed a little thinner using a rolling mill or hammer & anvil, giving them the extra length they need to spiral around the center of the sheet.