Micro RMH

I'm sure this has been proposed before, but I didn't see it. I am wanting to build a smaller RMH to heat my 10x14 planting room. I was thinking a 4" x 6" insulated inner barrel, and an 8" outer barrel. So the wood feed inlet would be 4" into the inside of the riser, an inch of insulation on either side, and then down and out the 8" outer barrel, exiting into a 4" horizontal chimney to run the length of the room (14').

Will this work? Do I need more insulation inside the riser? I was thinking I could do a 4"x8" riser to give 2" of insulation on either side, and then a 10" outer barrel. From what I recall in previous readings (4 months ago, but I don't think much has changed), the key is to have the same flow area throughout the system to ensure you aren't choking anything back. Like I said though, I want to keep this as compact as possible, as I'm only heating a small space, and don't have room for much more.

I intend to run the exhaust through the bottom of a fish raceway for aquaponics in the future. I think the mass of the water would certainly work in place of a massive clay/cob bench. I figure if the water is getting too warm for the fish I'll have a bypass chimney that will just run outside the tank to heat the surrounding air.

I want to get started building this soon, so someone please tell me if this is a no go. Thanks!

According to Ernie and Erica Wisner (www.ernieanderica.info), anything with an exhaust smaller than 6" tends not to work very well. That has to do with the flow of gases.

Adrien Lapointe wrote:According to Ernie and Erica Wisner (www.ernieanderica.info), anything with an exhaust smaller than 6" tends not to work very well. That has to do with the flow of gases.

Well everything can be scaled down right? I understand if you tried to choke a 6" inlet (or larger) down to a 4" that you would have flow issues. I also understand that as a result of the gases heating and wanting to expand that either you have to have a larger diameter exhaust, or a higher velocity in the exhaust. If needed I can certainly step up to a 6" exhaust. Like I said, my goal is just to see how small of a stove I can build and have it still be highly efficient. Heh, I'll probably end up just wasting a bunch of money on different size stove pipes and be left with nothing useful. That's how most of my experiments end up

tom Brue wrote: Heh, I'll probably end up just wasting a bunch of money on different size stove pipes and be left with nothing useful. That's how most of my experiments end up

Yeah! And you can have lots of fun in the process.

tom Brue wrote: Well everything can be scaled down right?

Yes and no. According to what Ernie and Erica said at the workshop I attended last weekend, with everything in proportion to the exhaust pipe, the 6" model seems to be the smallest that works well. It has to do with turbulence and flow. I guess you could make some modifications to the design to make it work, but it could end up not being as efficient or worst, you could create two chimneys and smoke back in the house/greenhouse.

I would suggest, especially if it is your first RMH to stick to a proven plan as these ones :6" RMH and 8" RMH. They are very detailed and I think you get 1 hr consultation (don't quote me on that) with Ernie and Erica. It is probably worth trying the bigger model and see how it works and then experiment with the smaller ones. Just my 2 cents, maybe I am not adventurous enough!

For small spaces, you might also want to consider pocket rockets.

For small spaces, you might also want to consider pocket rockets.

Hehehe...I could take this so many ways (inappropriate). But for real, does that exist, or were you being silly too?

I can think of a couple places scaling down would require some thought.

1) Size of the fuel. This is "easy" in concept, but may turn into a pain, depending. In order to get nice fires, the wood sticks need to fit the opening while leaving space for draft. If it takes too much trouble, it wouldn't be copacetic.

2) This may be the real problem. Heat... Quantity thereof vs. mass vs. insulation. Your small fire makes small heat (relatively). But the materials you use may still require large heat to reach/maintain the temps to pull the draft you need. Reducing the mass reduces the heat (from the fire) and time required to raise it's temps, but lower mass won't hold the temps stable for as long. The balancing act may get hairy.

Hmmm. And,

3) There may be some assumptions made about how much heat (BTU) people need as a basic minimum to actually be useful. Hence the minimum stove size recommended.

Rufus

tom Brue wrote:For small spaces, you might also want to consider pocket rockets.

Hehehe...I could take this so many ways (inappropriate). But for real, does that exist, or were you being silly too?

It does exist for real! It is described in Ianto Evans' book

Below is a picture of one.

I think I'd call the one in the foreground more of a 'barrel rocket'...

The little one in the back on the left that looks it's made out of a stump and some 4" stovepipe looks interesting...

The small one in the back is made with a metal bucket. I think it is an old grease pail. Below is a better picture of it.

NOT everything can be scaled down.

What can't be? The surface-friction in the exhaust pipes.

R Scott wrote:The problem is surface tension and friction are constant so they won't scale with the rest of the system.

You can find that quote within the first few posts at this topic:

https://permies.com/t/17703/stoves/Small-CSAs-less-than-inch

Hey!

So im new to the site but ive been into these cool things for a while. Never built one till today. It worked pretty good. I have 3" piping with a two foot heat riser thats 6" in diameter. Then back to a 3" exhaust pipe that goes out two feet then straight up two feet. No insulation yet as I lit it today to see if it would even flow right. I like to start small so I went with this. Ill post pictures in the day light. The thing got so hot only using twigs it melted all the glue off my metal tape and it the heat riser fell a bit. Tomorrow im getting a 4" section of pipe and insulation the 3" heat riser with dirt. ill try that and see how it goes. I couldn't boil water since my heat riser slipped a bit and got messed up. but before it all shifted I scourched my hand by touching the top of the heat riser 30 seconds after a flame was lit. Why did i touch it you ask? I didnt think it was working. I can guarentee you. it was. Hottest was 417*F when my thermometer would just display "---" for freaking hot!

Why am I posting this? To simply say it can work. will it work as well at the larger ones? I doubt it. Especially after 45 minutes and slapping it together but it still works.

Any pictures?

Unfortunately not yet. I built it about 7:00 tonight. Fired it at 7:30 and roasted it till 8. Pictures are coming tomorrow when there is day light and its easier to see what I did.

On the plus side even with no insulation at all, as in just the metal ducting, I was still able to hit 417*F on the top of the combustion chamber. So I have high hopes that if I add a 4 or 5" pipe on the heat riser pipe filled with either sand or dirt I can get that number even higher on that combustion chamber. Maybe even a more even temp across the top as apposed to a single spot on the top of the combustion chamber.

Much smoke coming out of the exhaust?

Adrien Lapointe wrote:Much smoke coming out of the exhaust?

At first, yes. But then once the tape all burned off and everything stopped smoking there was no "visible" smoke coming from the exhaust. If I put my hand over it, there was plenty coming though. Had the thermometer saying it was 275*F. I ran my hand around all the joints and could not find anywhere to which the gas was escaping.

Im going to also try and burying the bottom of the combustion chamber in dirt so only the feed tube and the combustion chamber are seen. I will also insulate the heat riser on the combustion chamber with either a 4" or 5" tube lined with dirt. Not sure which will work best so Ill start with 4" and see what happens.