Riser size compared to chimney size question

I have an 8 inch piece of stainless pipe 4 foot long that I want to cut and join together to make my burn chamber and heat riser. I'm thinking my burn chamber(horizontal) will be 16-18 inches long and the rest will be the riser so about 2.5 feet. One question that I have is, the existing chimney in my house is 5 inches, so will I run into a problem with the core of my rocket being 8 inches? If so, I thought maybe I could constrict the core where the burn chamber meets the riser. Another option might be to cut the riser pipe lengthwise and then squeeze it back together so it's 5 inches in diameter. If I could get away with using the 8 inch that would be great, but if not maybe an angled throttle at the back of the burn chamber to allow the same volume of air as the exaust at the chimney. I'm thinking the angled throttle would create a vortex kind of like ppotty's design. I'd like to keep the burn chamber 8 inches. Another question is, when joining the pipe at it's right angle would it be better to make 45 degree cut to join them or 90 degree and shape to join?

The last question is the most simple to answer, cut the pipe at 45 degrees.

The first question is a bit more difficult to answer. I have no expirience what so ever on the relation between rocket stove core cross section area and the chimney cross section area. Certainly, chimney length as some role to play. Common sense tells me that I should use a chimney with the same cross section area as the rocket stove riser. It would be a waste not to use that 8 inch pipe you have. But reduce it to 5 inch chimney seems to be a bit too much. Again, if the chimney is very tall, it will have stronger draft and it may compensate for the big difference in diameter. Can't you get a bigger diameter chimney or get a 6 inch stainless pipe to build the stove core?

Thanks for the help. The 8 inch pipe cost me 100 dollars at the scrap yard but I really wanted stainless and thats what we dug up. My chimney is very tall since it's a brick one on a two story house that has a newer 5 inch flue in it that used to have a gas heater hooked to it.

trying2hard explains something via his video



from my knowledge you are fine upto a certain size via the burn chamber where the burn flue used is 3,4" but its your exit flue that can cause problems as well

the ratio is better with 4x4 4" burn flue and 4" exit flue

the ratio of 4x3 causes a stall out on your burner harder to start up the exit flue changed to 5" also has a stall out effect but not by much

any backwash of fumes is caused by a few things

burn flue opening too small for vortex
exit flue too small
wind blowing down chimney ( cap used ) it can also have the opposite effect and cause the system going into overdrive



sorry guys for using your vids but you do mention your problems that youve had and i did remember the problems you had with backwash of flue gasses

I'll probably use the 8 inch but make a removeable throttle that I can put between the firebox and riser to match the volume of air from front to exit. I can do some test burns in the shop to see if the throttle is needed or if it works. I would like to make the throttle in the shape to promote a vortex instead of just a flat wall.

John, my thinking is that if you have a 5inch diameter opening into your riser from the firebox this should match the flue you have, you could probably go a little more say 6inch seeing as you have a long flue with a strong draw. In effect you have 5-6 inch going in and 5-6 inch pulling out. With that big riser on a 5 inch feed you should be able to create a superb vortex. I would go with the shape and fit rather than welding at 45 degree.