Rocket Mass Heater Noob Question

Hello everyone, id like to thank you for taking the time to help. I'm sorry if this isn't being posted in the right spot, this is my very first time posting. I'm putting in my first rocket mass heater after my furnace went out in my small house and it was quoted nearly $3000 to replace so I have been thinking of this RMH for a while now. I feel relatively confident in building it with a few hold ups. I have drywall in my home, so my first thoughts are is the RMH cobb mass is safe up against the wall. I believe so because I know its safe to touch so why not be up against a wall? Or is there something I should cover the wall with first like tile? My second question is I have heard that the RMH doesn't necessarily need a chimney or flue. Is it true that I can just run it horizontally out the side of my house like a drier vent?

Howdy Kyler, welcome to permies!

I moved your post to the rocket stove forum in hopes that our experts will see it. Hope that helps .

Hi Kyler; Placing your mass directly against drywall would be fine except near the core. For safety's sake I would place a piece of concrete board (durarock) over your drywall, anywhere your mass will touch. Space it out a small amount (1"or so) and pour perlite in the gap, this will protect your wall from heat and act as extra insulation to keep that heat in the house rather than passing thru the wall and outdoors. On to question 2 , only in very special locations will a horizontal chimney work well...in most locations you will get blow back when the wind blows from the wrong direction. Much better to have an non insulated indoor chimney rising directly up thru your roof. I know that this means putting a chimney thru your roof and this may seem beyond your comfort level, any carpenter/contractor could install a roof jack for you at a reasonable price. Another option would be to leave horizontally thru a wall , but then you will need an insulated chimney outside the house rising up over the peak,$$$$$$ that insulated pipe is not cheap!!! You didn't mention your floor ... it also needs protection from the heat as well as being able to hold the weight of your mass. Good luck with your build,keep asking questions... you won't be sorry, RMH's are awesome !!!

For my system I use 4" spacing (air gap) between the mass / bench and drywall, as per ASTM 1602-03 clearances for masonry heaters. You may find this document informative:

http://www.stovemaster.com/files/masonry.pdf

Straight out the wall exhaust doesn't work for the majority of us common folk. Unless you live in a magical location it is best to plan on a good chimney system. Mine is a traditional metal chimney pipe with the portion outside of the heated envelope of the structure being insulated. The chimney's exit point should be 2 to 3 feet above the roof's peak or any object or tree within ten feet.

That seems like a good idea. Yes I have concrete floors so im hoping it will retain a small amount of heat as well in the same room as the RMH. Could I possibly have a indoor flue that makes like a 45 degree angle out of the house just before the roof. Then il make it vertical again and use a brace against the roof.I just really dont want to go through my roof since its two story...and my roof is too steep to work on. My main concern it getting the draft nice and strong. What keeps the wind from blowing into a verticle flue? It still needs a cap right? So it will have horizontal openings for wind to blow in?

Thats great you have a concrete floor to work from. Your first layer of cob you put down before your horizontal pipes , should be mixed with straw to make it an insulating mix, this will encourage your heat to rise rather than heat your floor, unless you have a foam insulating under your concrete slab any heat going down wont stop it will try to warm the earth under the slab. As far as going up with your chimney indoors then a 45 thru the wall and then more vertical rise. That would work. Yes you need a cap , I use a simple top cap (coolie cap) some locations need a more elaborate cap, depends how the wind blows. No comparison to wind blowing across the top of a stove pipe verses wind blowing straight into a horizontal run.

Okay I think I got it..im going to probably lay down some sport of brick for the bottom base then put a layer or cob. Build my fire box out of some old fire brick i have left over from my fire pit. Still not 100% on my heat riser situation. At first I was going to build it like most people with two enloping steel tubes with insulation in between but ive heard they can melt and be destroyed and are not the best choice. So I think I might make it from some sort of cast of a material or just do a brick heat riser. Then I will lay down my piping and begin to cover it in cob cover the heat riser with a 55g barrell. Ive heard of other heat dicipators being used but the barrel seems to be the most common and succesful. I just hope i have enough draft to run my pipe. Speaking of which I probably will end up needing the pull from the vertical flue to pull it through the turns. Then il cover it all up witb cob!! I think I can do this with relitively no huge issues. Knock on wood lol il be sure to post pictures as I go along and post them here. Thanks for helping me, i really appreciate the input

A 45 degree exit through the wall makes less bendiness in the pipe, but it introduces the problem of making a seriously rainproof wall penetration. It is basically the same problem as a roof penetration, and you still have to run the chimney up to 2 or 3 feet above the peak of your roof, not just above the eave where it comes out. If you are not confident about working on the roof yourself, ask some carpenters about putting in a roof jack near the peak - that will only need the minimum of chimney above the roofline. If you plan to own this house as long as it exists, you can safely run a bare stovepipe up to the roofline inside, but if you ever sell the house, a future owner might tear out the weird contraption and put in an ordinary woodstove, connect that to your bare stovepipe, and burn the house down. That would be if a prospective buyer's home inspection even allowed it to be bought with an uninsulated chimney.

You can perfectly well build a heat riser of two sheetmetal ducts/stovepipes with insulation between them, as long as the insulation can stand the 2000 degree temperatures and be self-supporting. The inner metal form will burn out, but that will be okay as long as you use the right insulation materials. A mix of perlite with just enough clay to hold it together is a common method. You can also get insulating castable refractory cement, but that is expensive (maybe $50 for a 30-50 pound bag).

If you have a two story chimney mostly inside, you should have great draft. The length of horizontal duct run depends on the size of your system and how many elbows you have. An 8" system is often right for a whole house, and that can typically support 50' of run, minus 5' for each 90 degree elbow. A 6" system is good for a small well-insulated house, or a couple of rooms, and can support 30-40' minus 5' per elbow. The elbow that turns up to the chimney does not count against this.

The whole combustion core -burn tunnel and heat riser - needs to be well insulated.

i had an affordable concept idea i used on my heat riser and have suggested it for a RMH core design and it goes like this. build your core out of sacrificial regular stove pipe (J or L core and heat riser) then buy a small pail of refractory cement for 25 dollars and make a slurry with it. once you have your slurry soak your favorite flexible high temp material like rockwool or high temp fiberglass material and soak it in your slurry real good and then layer it around your stove pipe core like paper mache to the thickness to your liking. then use a larger stove stack around the heat riser and fill the space with perlite. i am sure that you could use fireclay at 10 dollars for a 50lb bag instead of refractory cement for a fireclay slurry and still get reasonable results.

Fibrous insulation soaked in a slurry of refractory will have zero insulation value from the fibers; it will be exactly the same insulative value as plain refractory, with the benefit of toughness due to the fibers. It could work as the liner for a heat riser insulated with perlite as described.

Okay I have finally gathered all my materials and have started a mock up in the living room where my rmh will live.

I have a few more questions on dimensions A and B I depicted in my rough sketch. The drawing shows two runs but there will be a third on top of the two running back to the opposite side of my heat riser. Then I will run it straight up mostly inside for my flue. Do you think this is pushing it for as many turns as it will have?

So your chimney would end up near the cleanout/tee at the upper left of your layout sketch?

Your layout as shown has about 20' of duct plus four elbows (20'), equivalent to 40' of horizontal run. This is safely within the maximum recommended, but if you add another 10' or so plus three elbows (15'), you would have around 65' equivalent, considerably more than recommended. The draft from a tall internal chimney might overcome this, but I wouldn't bet on it.

If you really want the chimney to be at that upper left location, I would advise changing your serpentine duct to a set of half-barrel bells. I think you could fit three half-barrels in that space, or maybe four with the two in the corner mitered together in some fashion.

Would it affect anything if I turned the elbows at farthest end from the barrel so that it has a slight incline? Maybe creating a chimney like draft? I might end up having to exit and run the chimney in the mid section of the run (in the middle corner) to cut back on distance if that could help

Kyler, are you aware of bells?

Anyway, in the drawing you have made, you can't clean the pipe from the rocket, to the second cleanout, after the elbow. And After the last elbow in the corner of the room.

To illustrate what Glenn was talking about.

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/#/user/mremine/library/NYC%20Rocket%20Stove%20Build/?page=1&_suid=147573692842108856006955450718

I just do not have any access to tools to cut those things nor to even find them. I've checked my local Craigslist. So if I shorten my run with less turns and put a clean out on that far turn would it still be to short and be rather too hot coming out the exit because although it has many turns it's just not enough distance?

I know what they are I just don't understand their purpose or if you run the pipe through them or not.

Unless you are out in the wilderness, which I don't think you are, there will be businesses or industries that get materials in steel barrels/drums and have extras. Ask around and you should be able to get a lead on who might have them for sale. Here, they can be gotten for $20 or $25 each in good condition.

Cutting is another matter. You would need to buy or borrow a jigsaw with aggressive metal-cutting blades or a sawzall. These can be gotten for less than $100 new, or possibly rented for much less.

The duct does not run through the bell; it feeds into it at the bottom of one end and exits at the bottom of the other end. The bell is a big open space (relative to the duct) where the hot gases can stratify, with the hottest rising to the top, giving their heat to the mass, and sinking to the bottom as they cool. The bottom exit lets only the coolest gases leave for the chimney.

With the bell arrangement, you would need hardly any duct, just a couple of elbows and cleanouts. Could you return some of the fittings you have bought?

Okay I have one more question before I start looking into the bells more as a option because I am more or less in the wilderness. I live out of city limits and  a hour or two from any place that "might" carry something like those. If I shorten my distance and just leave it how the drawing portrays and have the flue run out just behind the 55g drum will that work? It would be about 20ft and 20ft worth of 90°turns. Would that be too far still? Then the question would be would there be enough running distance for the mass to absorb the heat leaving the exiting temperature down?

By the way thank you very much for your help! I'm kind of stressing over this situation haha

If you can go up with your chimney near the barrel, that should work well and give you enough run for heat transfer. Many benches are built with this much length and bends.

Okay thank you Glenn for enlightening me on the bell drums. This defiantly will not be my last rmh I will put one in my 30x50ft shop and in my current house I'm building from shipping containers so that may indeed come in handy. IL post pictures of my current as is comes along

Okay so a few things have changed and I've been learning the whole way. I'm going to have a indoor chimney about 25ft tall so I'm going with my original plans for the heater. For the heat riser I'm going to use 3/4 steel with cob around it. I believe where the heat riser is is about 6x5.5in. Considering my exhaust is 6in how big should the inside diameter of my heat riser be? Still using 55g drum

There are several ways to construct a very effective and long lived heat riser. But I personally wouldn't use 3/4" steel for the inner tube, that is, if you're going for a high efficiency RMH where the secondary re-burn inside the heat riser typically peaks into the range of 1500 to 1900 degrees F. The inner portion is best made of high temperature refractory, be it fire brick, insulated fire brick, or formed of clay stabilized Perlite, and etc. Do you have a copy of this book? -->

https://permies.com/t/57365/Rocket-Mass-Heater-Builder-Guide

I did but my ex gf took it. Best book I've ever seen on the subject by far. Very well constructed. I was going to do fire brick but I didn't think the squareNess was good for a smooth run

If I were to create my own mixture of material into a cylinder cast, what would I use?

Kyler Lowrimore wrote:If I were to create my own mixture of material into a cylinder cast, what would I use?

The most popular material to use is a fire-clay stabilized Perlite mix, as in Matt Walker's how-to video. Note that the inner steel duct / tubing is expected to burn out, just being a form. Cardboard "sono" tube could be used instead of steel for the inner form tube.

For my indoor stove's heat riser, it's constructed from 2600° F. Insulated Fire Brick, square configuration. Using IFB the riser goes together quickly, bricks set using fire clay mortar.

Anyway, here's Matt's video on making a DIY cast riser: