I am brand new to this site but have been really interested in RMHs for several years and have been researching a lot. I finally have my first opportunity to build an RMH!
A little about me really quick... I am pretty darn mechanically inclined, very handy, a terrific problem solver with a great natural sense for "how things work" (physics). I am no engineer, but I am very inventive and love mechanical things. I also have a B.A. in glassblowing (long story) and learned to make all of our glassblowing equipment (furnaces, glory holes, etc.) that we used to heat glass to 2,300 degrees!! So... you could say that I have some experience with this type of stuff and these kinds of materials... just not in this particular application. So I am turning to the experts!!
Now on to my project and related questions. My father has a 50' long by 15' wide greenhouse that is currently empty but he is interested putting a RMH in to extend growing season AND keep chickens in. We are in central Indiana where summers get hot and humid and winters can get cold and snowy. So basically he has the worst of both worlds.
I am going to suggest to him that he first of all make some moveable panels made of blue foam with Mylar/Reflextix glued to one side to help reflect sunlight back into the greenhouse during the winter and then reflect sunlight away from the greenhouse during the summer... kind of a passive heat type deal since it's a greenhouse and he can really take advantage of the sunlight to help heat/cool the greenhouse. The greenhouse runs from east to west making this a perfect potential solution to making the most of the sunlight.
Next he wants to use an RMH to heat the greenhouse. He is interested in putting the main RMH inside the east corner of the greenhouse just inside the door with the combustion opening on the outside the greenhouse to take advantage of wind to feed the air intake and make sure that chickens aren't exposed to smoke or carbon monoxide. He wants to run "benches" along the 50' length of the greenhouse filled with sand with the RMH duct work running through the center of them... all along the entire 50 feet of the greenhouse on one side, then a 90 degree turn around the end with another 90 degree turn and then back along the other side and along the other 50 foot length of the greenhouse... then out to a chimney and up like 6 feet. A total of 115 feet of duct and mass!!
So now my questions for you experts:
1) Is the length of this duct work too long? Too ambitious? Can we realistically expect the heat to travel 115 feet and then up the chimney? I have seen some pretty elaborate duct works for RMHs... but never anything quite this long. But theoretically it seems feasible. Yes? No? Would we maybe need to incorporate a fan at the opening of the air intake? I'd rather not, but I am open to it.
2) Is the diameter of the duct critical? I think I have read that it is... but not sure how or why. We were thinking 6" diameter duct would work, but is this too small? Too large?
3) Which burn chamber concept should I go with? I have seen a number of burn chamber designs and... I am not sure which direction to go. My father is really interested in a "down feed" concept because he has lots of trees that shed limbs and loves the idea of shoving some longer limbs in the opening to the burn chamber and letting gravity sort of "self feed" them for a while, but I have also seen this fancier "front feed" version that sort of "V"s at the bottom and people advocating for its effectiveness. I think it's called a "Batch Box"? Is this something I should be interested in? Why or why not? Advantages? Disadvantages? Bad idea for outside the greenhouse? Ideally we'd love this RMH to keep the greenhouse warm for 24 hours so that it only needs attending to once a day. Does type of burn chamber affect this or is is all just in the thermal mass itself?
4) Do we need to incorporate a way to clean the 115 feet of duct work periodically? I realize that RMHs burn very efficiently and cleanly, but... everything in life has to be cleaned periodically, right? LOL!!!
5) Are there any concepts, terms, mathematic equations, formulas or anything like that I should know about before taking on this project? Anything that you could point me toward in terms of arriving at an effective design, setting realistic expectations, etc. would be incredibly appreciated!! Please... feel free to post links to concepts, formulas, photos (I am SUPER visually oriented, so I LOVE pics!), etc. I am here to learn!!!
6) Any particular materials that you would suggest we use for this project? To be honest... we are probably not interested in using cob. I love the natural look of it and all, but I think for a project this large it would be a CRAZY amount of work and really messy... and it's just for a greenhouse/chicken coop. As I mentioned before, I am very comfortable with refractory materials of all kinds (brick, castable, fiber blanket, etc.) as we built all of our own equipment for glassblowing. We will more than likely use refractory materials for the main unit but were considering just using wood boxes filled with sand to surround the ductwork and use as our thermal mass. Anyone see any problem with this?
7) We want this RMH to REALLY ROCKET so that the heat flies through the entire 115' of duct work and heats it all up from beginning to end as much as possible. Any tips on how to really get this thing rocketing the fully 115 feet? There is power in the greenhouse already but Dad is going to add solar panels too, so maybe an electric fan if necessary? If there's a way to design the RMH system so that it jets on its own, that would be really awesome.
Any other pointers, warnings, tips, suggestions, etc. that you all would recommend I consider? I am not at all committed to any particular designs or concepts yet, so I am wide open in terms of advice and suggestions at this point! We have not committed to anything really yet. It's the perfect time to get wisdom from you experts!!
Alright... PHEW!! Sorry for inundating you all with so many questions. Just trying to do this right and end up with amazing results!
I thank you all in advance for your wisdom and insights! I am SO EXCITED about this project!!!
For the mass you might consider concrete. It isn't as nature friendly as cob but it is a lot easier to install. Just be sure to fasten the duct securely against moving both up and down - we learned the hard way that a duct in wet concrete will try to float Also, you can put a lot of rocks in the form before you pour the concrete to reduce the amount of that you'll need.
And for gravity feeding you want sticks that are straight or you'll have to fidget with the loading a lot. If they are too bent or whatever they can wedge against each other and refuse to fall, allowing the flames to rise up too far.
And I'd caution against putting the feed & burn areas outside, especially if there's wind. If the wind isn't blowing in the right direction it can make it backdraft. Sometimes it backdrafts so strongly and quickly that it can put the fire out, especially when its just getting started. Once the burn chamber is hot it isn't as likely.
Creator of Shire Silver, a precious metals based currency. I work on a permaculture farm. Old nerd. Father.