I'm new to rocket stoves. Idea question.

So I'm new to this whole rocket stove stuff... I've only been looking at it for 3 days. Would this design work? if not where and I wrong in my thinking?

I'm tying to make one for a large greenhouse. I want it to be able to burn long as I don't have the time to feed it every day. So I thought I'd make sure my thinking is right before I finish the designs.

Thanks guys.

Welcome to Permies, Garret!
First, a few background questions. How large is "large"? What is your climate and how warm do you want to keep your greenhouse?

Is your sketch showing 12" diameter logs? How long would they be? Would the "V" hopper be parallel or perpendicular to the burn tunnel?

The hopper might work to deliver logs as long as they stay in the right size range, but I think there would be difficulty keeping the fire burning properly. The logs need to be sufficiently in the coal bed to be hot enough for reliable combustion, but not so much that they smother parts of the coals. The standard J-tube does this automatically, with the ends of logs standing in the coals, leaving enough airspace for oxygen without making a big lump of tight coal. It is possible that it might work reliably, but it is untested and you would need to experiment before depending on it for heat.

What size of system are you contemplating? The largest common size is 8" diameter throughout; larger ones have been built, but are rare. A 12" system would be truly a monster, and might require different treatment... nobody really knows for sure.

By "burning long", are you contemplating several days between tending? That sounds pretty risky. If you have a really big greenhouse, a mishap not caught for a couple of days could be very expensive. Also, there is generally no reason to keep a rocket mass heater burning continuously except perhaps in the very coldest weather. The design gives a big slug of heat in a relatively short, hot fire, and the heat is all trapped and released over hours or days depending on the system configuration. Your hopper would essentially be running flat out no matter the weather, and would either be too hot on milder days or too cold on colder days. The RMH is not a woodstove and does not need to run continuously to heat its space. This is something a lot of new people don't grasp at first.

Thanks for the fast reply!

Apologies, I wasn't very clear on the whole idea.

The greenhouse will be 55' by 18' I live In middle TN. So it can get cold now and then. I need to keep it 60+ It will be used in a 7,000gal aquaponics system.
Yes it is a 12" diameter log. The length would be about 4' but then cut into 2 2' pieces (In hopes of creating a better burn with more surface area close to each other) the hopper will be perpendicular to the burn tunnel, the logs should fall/roll in and next to each other. I've tested 4 logs, 2 logs burning next to each other and 2 on top waiting. It worked pretty well, the last little bit didn't burn off all the way.

As for size I really have no idea how big it needs to be or can be?

By long I mean I can check it once a day and add logs but I don't have time to start it and tend to it all day or night. I can set up a system to pump the excess heat underground a couple feet down to use later. But the 7,000gal fish tanks should be taking most of the heat from 4 logs burning(just my thought).
I most likely Don't fully grasp the mass heating part.

Thank you.

First, I think you would benefit from reading this thread:
Rocket stoves in Greenhouses , our own forum topic
It's very long - you can probably skim to some discussions of large systems.

It would also help you to buy the book Rocket Mass Heaters, third edition or as a digital download from rocketstoves.com. It explains all the basic principles and gives practical advice for building them. Be very careful of youtube videos - the majority of them give bad advice known by experienced practitioners to fail over time. There are some very good videos there. Anything by Ernie and Erica Wisner can be relied on. Rob of Bigelow Brook Farms has some good greenhouse RMH experience to share.

May be worth a check. I would say i don't endorse the technology oif the magazine. As i haven't tested it. But soo far, the idea is neat.

https://permies.com/t/54074/rocket-stoves/Rocket-Mag-heater-Rocket-Mag

I have a feeling the mag heater might not throw heat fast enough for your big system, though if you scaled it up to 8" it might work. It's unknown territory at this point.

"I can set up a system to pump the excess heat underground a couple feet down to use later. But the 7,000gal fish tanks should be taking most of the heat from 4 logs burning(just my thought)."
The RMH automates this system by running its exhaust duct through a large mass which absorbs the heat and releases it slowly. You would locate this mass under the tank (insulated on bottom and sides of mass) to warm the water gently.

An important clarification: do you need the air temperature to stay above 60F, or just the water? Steady air temp is much more difficult.

Thanks for the replies!

The big log idea seems to not really be working (not as I had hoped), so before I keep trying to make it work thought I'd run another idea past you guys.
I'm now thinking about using 4" x 4" x 2' logs going in from the top and going vertical( as high as I fell like making a loader). There will be 6 logs (in a 3 by 2 grid) ends (about 6") in the stove at one time. There won't be much of a air gap between the logs to start with but as they burn I would imagine the air gap could get quite large, guessing up to 3-4". Any one have any thoughts on the idea or if the air gap could be a problem??

I can do 3" x 3" if you guys think that would be much better as I have to build a log splitter wedge for it anyways, but the 4x4 would be easier to make and I could buy some 4x4 to test with.

I'll add a bit more info... my math is saying I need to burn 3.5 4x4x6" logs every hour for about 60,000btu( what I should need to heat the greenhouse).

Thank so much!

Given that it is a greenhouse with that large of an amount of water, it could be greatly beneficial to make it possible for the sun to warm up the water prior to cold weather. Adding insulation to the north side of the greenhouse could also help trap some heat. If you used something like foam or wood panels, these could be moved to block the sun from hitting the water when it is too warm.

Van Powell on YouTube has a number of videos about rocket stoves, builds a greenhouse with one inside, and talks about using thermal mass, such as water, to hold more consistent temperatures. Your larger scale project would have additional considerations, but his videos are rather well thought out and should be able to help you accomplish your goals.

I don't think it's a good idea to try to run a RMH constantly 24/7 - they aren't designed to work that way. I repeat, they are not regular woodstoves, and trying to regulate the burn rate like people do with woodstoves will lead to inefficiency and creosote in milder weather (just like woodstoves).

If you can estimate the burn rate for constant heating of your greenhouse, I would double it and plan your RMH to handle that much wood, so you will only need to burn 12 hours per day in the coldest weather. That would mean as little as a couple of hours per day in milder weather, with the heat going into mass connected to your water for even heating.

We do need to know whether it is important to you to keep the greenhouse air at 60F, or just the water. Also, is this an existing greenhouse, are you building it from scratch, or do you have beds/tanks in place already?

I might suggest building a batch box instead of J-tube combustion core. It is designed to burn a larger quantity of wood at a time and release a huge slug of heat into the mass. An 8" batch box system has been used to heat a 2000 square foot auditorium/shop building in Montana.
http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions