Ducting choices

Hi, I'm slowly gathering materials for a rocket stove mass heater and am having trouble sourcing the horizontal ducting at a reasonable price. I've tried local scrap places but they don't seem to have solid flue liner. New prices are seemingly prohibitive for the long lengths required. I have been trying to search for any examples of using flexible steel flue liner as a cheaper new alternative but I haven't had much luck. I'm wondering whether the potential draw back would be in internal surface not being smooth and restricting the flow?

Does anyone have any experience of using flexible flue liner for this purpose? I'd be really interested to hear your views on this.

Many thanks

Beaglebox

Hi thanks for the post but I don't think using big barrels in my particular application is going to work. The whole bench is only 3 metres in length. Thanks anyway though.

The flexible stuff with rough interior surface introduces unwanted drag. The least expensive metal ducting commonly used in RMH construction is galvanized HVAC stuff from you local builders supply. Even that is becoming more expensive these days.

I've been tossing this around for a while now myself. But one option instead of metal ducting is to build the exhaust channels from a suitable clay brick (following traditional masonry heater construction techniques) with the interior of the channels "plastered" smooth with clay mortar, for unrestricted gas flow. This way you get your exhaust flues and some thermal mass all in one stroke, as it were.

The advantage of going the metal ducting route, however, is that the RMH can be fired up before completing the mass / bench fill around the ducting, to insure the system is working and drafting properly with your specific ducting layout, number of turns, and etc.

Ed Burns wrote:Hi thanks for the post but I don't think using big barrels in my particular application is going to work. The whole bench is only 3 metres in length. Thanks anyway though.

Then, you will have to go the bell route! A 3 m bench might house about 18 feet of pipe, may be, if real wide you can fit 27. A 6 incher, for it's peak performance needs about 30' While a 4m² ISA bell will do just fine. That's a low bench of 3m long, 60cm high and 70cm wide made out of bricks and slabs on top. You could double or treble the top layer for more mass. And if on the cheap, you can always make a bell out of two barrel halves. That gives you about 1.8m² per barrel iirc. Double this, and you have about 3.6m². Treat it as a double bell, and you are about spot on for a six incher. (you need to reduce the size of the bells by 20% if you make it a double bell. ) Stuff some cob over this and you're done. You can even do dead end bells. Where the intake and exhaust of the bell are on the same end. Three half barrels end to end could do if you have the chimney at the other end, treating it as triple bell. A half barrel is 29 cm high, 58cm wide and 88cm long So 88x3=264 cm long. You make a perimetric wall around all this with house bricks, using the back wall as the fourth retaining wall. Stuff the three half barrels in there, with their ends cut 7 or( 8 cm above the floor. one intake one exit. Fill on top with whatever you feel like. Mud, clay, cob. You can even use concrete. Any of the four mixed with stones if you like. That's infill. If you use gravell on it's own., make sure you have taped all the joints between barrels and flues and floor. You've raised your perimetric wall about 2 bricks above the top of the half barrel. When all that is filled you have 42cm or thereabouts. That's near ideal sitting height. Then put whatever you like on top. Pavers, planks, concrete slab, bricks, pizza stones, slates. Anything you can think of.

Just to give you an idea.

http://donkey32.proboards.com/attachment/download/345

You have zilions other choices besides flue pipes.

http://donkey32.proboards.com/thread/560

Look at this one, not half barrel, but could be done half barrel style.

https://permies.com/forums/posts/list/40/30289

Here's a range experiment of mine.

http://donkey32.proboards.com/thread/1216/new-range-candidate-rocket-retrofit

Check ye aulde pics!

http://donkey32.proboards.com/thread/833/tube-incher-bell-volume

Dutch ways!

http://technologieforum.forumatic.com/viewtopic.php?f=19&t=27


http://batchrocket.hostoi.com/html/foto.html

Wall type!

http://heatkit.com/research/2009/lopez-rocket.htm

Hi Ed; Where are you located ? As byron said, in the states it is most common to switch from heavy stove pipe to light gauge galvanized hvac pipe shortly after you get into your mass. Home depot sells 5' sticks of 8" for 11.00 $ If you can't find any smooth wall pipe then using clay brick is better than using the corrugated pipe that seems to be common in europe . Just make sure that you keep the same csa as a round pipe all the way to the exit.

Another one i had forgoten about.

http://donkey32.proboards.com/post/12724/thread

Ed : once you are into your Horizontal thermal mass ductwork, you can get away with the lightest pieces of crap you can buy 1 they only act as a form
around which you are going to cob ! The H.V.A.C. grade everyone is suggesting is also called cold air return lines and is the lightest headset grade out there !

Consider going directly to major rehab sites ( like after a Fire ) You find them by looking for houses with windows and doors boarded up with plywood and
a huge dumpster located outside, Firemen use water ( they put the wet stuff on the red stuff ) Basements and furnaces get flooded, furnaces usually with
ALL the duct work gets replaced - yours for a couple of cases of beer or just sneak it out of the dumpster ! Hope this fills the gap For the Craft ! Big AL