My experience with rocket stove one winter in.

as you may recall, I built the core of my rocket stove with diagonally sawn laid up courses of soft file brick. It formed a, octagon 5.5 inches across the height of a 55 gal steel drum. I tried venting it out the side wall, but the wind kept causing blow back. I tried putting a whirley bird vent on the exhaust. That helped slightly, but still at all reliable. I have a 10" exhaust pip that comes out the side of the barrel at the bottom with a 10 " pipe. This serves as a plenum, and then I reduced it in stages down to 5" and ran the pipe through a hole for a hot air vent down into the basement to the brick chimney that used to be for the oil furnace. (Never mix fuels in the same chimney) I got all the draft that I ever needed, and in fact had to put both a barometric damper and a conventional damper to control the draft. This worked perfect, except for a few relatively warm days with low barometric pressure and little or no wind. On sever occasions I was able to remedy the problem by going down into the basement and putting some lit news parer into the chimney to jump start the draft. On A few days, even that wouldn't work, but as they came on relatively warm days, it wasn't a major problem. Some days, when the weather conditioner were just right I could easily exceed 900 degrees F at the top of the barrel.

I have decided to change a couple of things.

#1 I hadn't realized that it is a really bad idea to be sending warm moist exhaust up a clay brick chimney. That gentle white condensation at the top of the chimney is very nice - except, that there is not enough heat going up the chimney to dry out the moisture. It wouldn't take too many seasons of this to completely soak the clay bricks and induce a mechanical failure because of the weight of the bricks stacked on top of one another. So I must get a stainless steel liner.

#2. the 5.5 dia. heat riser is OK as a parlor heater, but not to provide the main heat for the whole house in Rutland Vermont. I'm going to take out the 5.5 inch dia. set up and rebuild it into about an 8" setup, 3 or 4 courses higher, again with soft fire bricks cut on an angle. With this set up I will be able to burn much larger pieces of wood. I about wore out my shoulders spitting wood down to kindling wood size to be able to fit it in the burn chamber. I'm thinking with a larger diameter heat riser it will also give it enough umph that it seems to need sometimes to overcome the conditions above that on some days caused problems. Likely I will have to increase the run the the chimney with 6" pipe, but we will see .



#3 I am gong to try an experiment with placing a 55 gallon drum filled with water directly on top of the upside down barrel that covers the heat riser. This will weigh about 400 pounds and will give as much thermal mass as 800 pounds of stone, and will be taking heat from the hottest part of the unit (the top of the barrel) (Of couse this will have an open vent at the top to prevent pressure buildup, and I will add more bracing underneath the stove down in the basement.)

Thanks for posting about your experience. This is the kind of information that accumulates to become the conventional wisdom, which for mature technologies is wise. You reinforce a bunch of things that are already known, but a concrete example may be more meaningful to someone new than just "this is how it's done".

I think you would better use an 8" duct from heater to chimney, though if the draft is good enough a 6" might be sufficient. It would certainly be easier to start if you don't go down through the floor to get to the chimney. Is there an obstacle to getting to the chimney on the main floor?

Your water barrel idea sounds good, and I will be interested to see how effective it is (it will certainly work, but how well?) If the original barrel is whole it will definitely be able to support the load, but if there is a hole in the side that might be a weak point where buckling can start. How is the barrel supported? Does it have masonry below it all around? Again, uneven support could provide a stress point where failure could begin.

Re your apparently unlined brick chimney: I think the worst failure mechanism related to condensing moisture would be at the top or wherever the chimney is exposed to external temperatures. While the fire is not going, the chimney will get cold and absorbed water could pretty quickly disintegrate the brick as it freezes.

except in this case the exhaust going into the the chimney while the fire in the unit is blazing, is about 70 degrees f of really moist air. Over not too much time that warm moist air will disintegrate the clay in the chimney brick from the bottom up. This is the same reason it is a terrible idea to vent a really efficient gas fired water heater into a brick chimney without a stainless steel, or at least an aluminum liner. The exhaust simply isn't hot enough to heat the bricks enough for them to dry themselves out from the stored heat between firings. As a precaution for this year I am disconnecting the flu pipe going to the chimney and allowing the chimney to free vent through the summer. That will dry the bricks out somewhat from being exposed to the moist air from the stove all winter long.

I had thought that 5" pipe was going to be too small, but as I said, draft was never really a problem. I might have been able to go down to 4, but there wasn't any reason to try to go smaller as I had plenty of clearance for 5". I am thinking that by getting a liner that it will increase the draft (but we'll see) It's a pretty open space, and I get a pretty good discount at the hardware store so I can afford to experiment with different sizes of flu pipes. I love to tinker. I might go crazy and build a cylindrical masonry support wall to support the upside barrel upside on without any cuts in the sides of the barrel for the entrance tunnel or the exhaust. I thought I would incorporate openings for them into the cylindrical wall, which would now allow me to go three of 4 courses higher with the heat riser. But that said - I think that the present barrel with the hole for the exhaust and the rectangle cut out for the input tunnel would be plenty strong enough to support another 500 pounds. But if I think that there is a problem I could fill it only half way or so and still get an incredible amount of thermal mass until I bolted together a bracket to hold the 2nd barre up independently. Thank you for your concern though.

David, two points.

70f° is too low to go into the chimney. I think the general consensus is to be about 150f° a good practice.


And i think you are ready for a batch rocket

why would I want to send 150 degrees up the chimney when I can send 70 degrees up? 150 degrees would hold a heck of a lot more moisture and that combined with the warmer air would rot the clay brick in the 4 story tall chimney even faster. By cooling the exhaust down to 70 it precipitates out some of the moisture, which drips out of the exhaust pipe into a bucket placed in the basement to catch it.

Simply because it warms up the bricks more, and avoids a little better the stalling smokeback problems.

This thread brings up something that's been on my mind for awhile.
given the incredibly low temperatures of exhaust there are common dear we use a PVC chimney?
Or if not PVC due to the gick, ABS?
Cheaper and easier,you could insulate the outside of the pipe if you wanted to or paint it black for a solar chimney effect. Tees and cleanouts offer access for preheating, though a hair dryer is as hot as you would want to get. A barrel at the base could collect any condensation.
Plastic pipe is perhaps not very permie,but is it practical?

Besides needing glue ,6" & 8" pvc or abs is going to cost much more than using hvac pipe ($11.00 for a 5' stick) I used more expensive black stove pipe to stick up out of my mass then switched to hvac to go up & then back to black to go out through the roof. For me, this thread made me know of the clay chimney holding in moisture. This had never occurred to me. I don't have a clay chimney but I may want to help somebody who does in the future. I'm glad to be made aware of this !