Flue size vs intake size

Anyone able to assist with a tech question regarding rocket stoves?
Here's my dilemma. This will be a small heater for my shop. Hopefully the first of many for my home and others. My heater is an old 100# propane cylinder. The intake and horizontal burn chamber is 4x6 heavy gauge steel tube, the vertical insulated chimney (if you will) is 5" square tube stock. The issue is that I want to use 3" insulated pellet stove pipe for the flue or exhaust.
Do you feel the size difference, other than the CFM being lower on the smaller pipe, would somehow inhibit the functionality of the stove? I understand the air velocity will not be as great but this particular unit will be for periodic and power outage use.
Or am I over thinking this whole thing; just build the damn stove?
Help?

Robert, great question, I've wondered that myself. I have read several times in this forum that the burn chamber and heat riser should be the same size and that in order to avoid back-drafting the exhaust ducting needs to be at least the same size. I've also seen some pretty interesting tips from HVAC guys about substituting a big box in place of using 90 degree elbows in the mass ducting, reasoning being the turns slow down the flow. I also read a post about using flexible pipe and the interior texture of the pipe can slow down the exhaust as well. Smooth and straight if possible, same size in and out as well as oversized elbows or boxes for turns is what I've gotten from other people's posts so far.

I have yet to build one of these things, I'm still in the planning stage. So far I've made some alcohol stoves, paint-can wood gasifiers and a brick rocket stove for cooking and am constantly amazed at the sheer amount of knowledge and understanding one can get from playing with something so primitive as fire, yet continue to hose up the technology I'm trying to apply!

Good luck with your heater, please post your findings.

Hi Robert! Please don't "build the damn stove" yet! I'm glad you posted your question.

The stove you have in mind may have several problems which will be better rectified before you start your build.

The first issue is the one you bring up. If you intend to build a "rocket stove" with these components, then you absolutely need to adhere to the parameters of flue size equal to or exceeding the heat riser. The dimensions you stated for your combustion chamber and heat riser are ok. you have 6"x4" (cross sectional area of 24 sq inches) feeding 5"x5" (25 sq inches) but you want to exhaust the system to 3" pipe (3.1415 x 1.5 x 1.5= 7 sq inches) yikes! This absolutely will not fire as intended. This stove will require 6" diameter flue pipe in my opinion. (28 sq inches)

The other problem you may encounter is that the heavy guage steel will absorb much of the heat of combustion slowing down your rocket until it gets good and hot. You indicated that you intend to insulate the heat riser, but what guage steel are the tubing? how heavy is it?

Is the propane cylinder going to act as the radiator and you would attach a flue pipe at the bottom? I'm not sure exactly what you had in mind for your design.

Hope you find all the help you need! The folks here are very helpful and friendly! Best of luck!

Thanks for everyone's help. The flue will be 6" black stove pipe. Otherwise the remainder will be 1/4 inch thick 5" square tube. I'm now slightly concerned about the insulation on the riser.
I originally was going to wrap it in a high-temp batting with a stainless wire wrap to hold it in place.
So now I'm thinking about a sheet metal enclosure outside the riser in place of the batting then filled with
Refractory cement. Any thoughts on that? The feed tube and flue will both have clean outs.

So now after even more reading I'm thinking hybrid. Steel 5" square feed and burn chamber with a riser of only 2" going up inside a refractory brick riser of the same size, 26-28 more inches. Would insulate better affording greater rocketing inside and then falling of gasses and heat outside. Any thoughts? Instead of building it "on the tank", I'd build it on the bottom of the tank then slide the tank over the top of everything and weld the two back together.