Colin Saengdara

Martin, Any updates on your stove? We are entering a new heating season. I've been wondering how your system performed for the balance of last season. Hope all is well!

Colin

Hi Tom,
What type of furnace do you have that died? I had a working oil burning furnace that i hadn't used for a couple years since I've been heating primarily with wood. I decided to clear out all of my ducting and the furnace to make space in my ancient basement for other projects like a rmh. After removing the ducting, cold air returns and the furnace shell i was left with a very substantial steel heat exchanger. Removal of the injector revealed an intact refractory core! Seeing that, i just had to convert it to a wood fired heat exchanger!

I used fire brick to build a J tube feed and burn chamber feeding directly into the refractory core and fired it up. It works great! The design isn't perfect but the surface area is truly substantial and because it is attached to the chimney, there is always draw through the system. I extended the exhaust down to the floor and buried it in sand (temporarily) to steal as much heat as I could from the flue gases.

As far as the heat exchanger design goes, it consists of what is essentially a barrel with a vertical 3" wide opening to a "box-in-box" surround. It is designed for air flow and creates good convective current on it's own. Built into a shell with a proper air gap it has massive surface area and could again be used as a heat exchanger for forced air. Since it is still in the middle of my space, I intend to move it to a new location. Next, I will build a masonry chamber around it essentially mimicking the old steel shell's inner dimensions, but with masonry, and finally using a cob mass bench to extract whatever heat is left in the gases.

I plan to document the build when i relocate it, but I have a feeling there are many of these old furnaces going to the scrap yard.

Hi Tom,
What type of furnace do you have that died? I had a working oil burning furnace that i hadn't used for a couple years since I've been heating primarily with wood. I decided to clear out all of my ducting and the furnace to make space in my ancient basement for other projects like a rmh. After removing the ducting, cold air returns and the furnace shell i was left with a very substantial steel heat exchanger. Removal of the injector revealed an intact refractory core! Seeing that, i just had to convert it to a wood fired heat exchanger!

I used fire brick to build a J tube feed and burn chamber feeding directly into the refractory core and fired it up. It works great! The design isn't perfect but the surface area is truly substantial and because it is attached to the chimney, there is always draw through the system. I extended the exhaust down to the floor and buried it in sand (temporarily) to steal as much heat as I could from the flue gases.

As far as the heat exchanger design goes, it consists of what is essentially a barrel with a vertical 3" wide opening to box surround. It is designed for air flow and creates good convective current on it's own. Built into a shell with a proper air gap it has massive surface area and could again be used as a heat exchanger for forced air. Since it is still in the middle of my space, I intend to move it to a new location. Next, I will build a masonry chamber around it essentially mimicking the old steel shell's inner dimensions, but with masonry, and finally using a cob mass bench to extract whatever heat is left in the gases.

I plan to document the build when i relocate it, but I have a feeling there are many of these old furnaces going to the scrap yard.

Matt, Could you provide a link to this "Dragon Heat's" shipable cast core? I couldn't find anything.

Also, Thank you for your inspirational designs!

-Colin

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!

Hi Martin, I was looking over the whole project again and wasn't quite sure about something. Did you end up deciding to leave one brick removed as an ash and ember catcher? if so, is this essentially the first brick you would see looking above from the feeder side? My thought is that maybe you should try replacing the brick and insuring that your highest concentration of heat is not directly below the feed opening. Is it possible that you have a large accumulation of oxygenated fuel causing an updraft there?

Also, I like the suggestions you are getting for troubleshooting! It's nice to see so much support! Good luck again.

- Colin

P.S. the good news is while you try all these things you are drying out your system, so it may start performing better anyway.

Hi Jon, First thing's first. Don't start changing anything too quickly. There are two places we need to look first. Allen asked you about the CSA of your heat riser as it opens to your barrel. If you didn't make any errors in measurement and your barrel is nearly 2 inches above your combustion tube/heat riser this will give you sufficient csa (cross sectional area) at this zone. Here you would calculate the circumference of the riser at it's tallest point x the gap height. Circumference = 2x pi x radius or 2 x 3.14 x 3 = 18.84 cxheight 1.5" =28.26 (your csa for a 6" diameter pipe) so a 6 inch diameter pipe will require AT LEAST 1.5 inches gap between the heat riser and the bottom surface of the barrel lid. It is highly recommended that you design it with about 2" to insure that the gap is maintained under operation. The high heat can warp the bottom of the barrel and this could close the gap somewhat. Use a straight edge of sufficient length and measure the gap between the lip of the barrel and the center of the top. Compare this gap with the gap near the outside of the barrel to determine if your barrel is warped downward which could impinge the air flow. Also, it's a good time to double check that you were indeed measureing the height of the center of the inside surface of the barrel (not the outside lip which can be a difference of up to half an inch in my experience) when calculating your gap.

The next area of concern is the opening directing the combustion gases into your flue pipe. You described a brick circle with an opening into your flue. We need to know the cross sectional area of the tightest spot (think 3 dimensionally here since the gases are essentially turning 90 degrees from downward to horizontal). By any chance, in constructing this, is there a spot where the corners of the bricks are closer together than the opening to the duct? It could be inner corners of the bricks. This would act the same as a kink in a copper tubing, the openings are the same csa, but the csa at the kink drops to nearly zero.

It should be one of these two locations because the drafting issue appeared and disappeared when the barrel was added. Can you supply any photos or measurements of these two spots?

Hope this was helpful, it's sometimes difficult to describe these aspects in written language.

Good luck! - Colin

Hi Richard! Regarding the 30" barrel heated by flue gases to preheat and dehumidify the fuel. The issue with this type of arrangement will be that you have a seperate air intake from your fuel intake, and therefore nothing to stop your fuel intake from becoming an inadvertent combustion chamber in a hurry. The same principle that makes your designed combustion chamber work so well will be at work in your fuel hopper (ie. draw). This arrangement can work if you have an airtight lid on your hopper, but then you don't get the desired benefit of removing the humidity first, and you have a serious risk of burns and/or smoke and volatile organic compounds entering your living space any time you open the lid to check how much fuel you have left or want to add fuel.

However, since you live in New Mexico, I think it would be easy for you to dry your kindling prior to use. If not, a passive solar kiln would be super effective and another fun project eh?

Hope this helps! Keep on thinkin'!

- Colin

Hi Martin! Great project and great documentation! I feel pretty certain that what you are dealing with currently is only temporary. I think your system is behaving this way because the flue gases are directly exposed to massive quantities of moisture evaporating off the considerable mass in your system. The heat generated in your combustion riser makes the incoming air less dense than the surrounding atmosphere and creates the draft. But as the combustion gases hit your barrel and begin to cool they are forced down and out. Because you have built this as a masonry flue with lots and lots of surface area, all the moisture will take quite some time to be purged from your system. The difference in density that you need to drive your system in the burn chamber is currently not sufficient to overcome the supersaturated flue gases through all the chambers, and then push that dense air up and out of your basement.

To test this hypothesis, you might want to check a few things. 1 What is the temperature of the duct at the exit of the masonry? If it is cool to the touch or cooler than the surrounding air, that's probably a good indication that my hypothesis is correct. 2 Do you have enough duct to be able to temporarily reroute the flue gases out the basement door (assuming it's a walkout basement)? If you can keep the flue exhaust lower than the heat riser, your system should behave normally immediately. If not, you will need to use the fan until you have driven off enough moisture that the evaporation inside the system does not produce an exhaust that is more dense relative to the atmosphere than your combustion gases are light (relative to the atmosphere).

I hope this helps! I wouldn't pore over your numbers anymore. You were meticulous and I trust that those were correct. I think its just a light air vs heavy air thing. Good luck!

- Colin