thomas rubino wrote:Hi Corey;
First, a RMH will work for you. Way Better than a box stove ever could. Your home is well built , well insulated and a not overly large single story. It's perfect for a mass heater!
Now I'll take a shot at answering your questions. Although my answers are going to sound quite a bit like Gerry's.
1) A short mass run is OK. Changing pipe size however is not a good idea.
With a traditional piped mass your pipes must maintain a constant size to maintain flow direction, same size as the core and riser. 6" in your case.
You can create a heated wall of sorts.
When you go with a brick bell over a piped mass, the only stove pipe is the exhaust pipe leaving from the very bottom of the bell. It is called a stratification chamber.
This is a very good way to maximize a small foot print by going vertical. It does use several hundred clay bricks
2) Excellent choice going with a five minute riser. You won't be sorry. Your metalbestos pipe will work but it is overkill if you have standard pipe about.
3)You can cast a core from fireclay and perlite. They work very well, they also are very fragile.
If you can mail order, then ceramic fiber board would be the material of choice to build your core. You would still need some split firebricks and some plain clay bricks would be very handy to have.
4) Fire clay #50 sacks you will need some. Creek sand will work if you can find some.
I taped every joint on my systems but with the pipes buried in the mass, I would not worry about it.
For your mass, use subsoil encasing the pipes . Then large rocks , as large as you can move and not crush your pipes. Encase each with subsoil for zero air gaps.
Orin Raichart wrote:Hi Corey,
I've only made two rocket stoves and they weren't the best compared to the experts' here on this site....so I'll reply with a beginner's answers rather than an expert's answers with the hope the experts chime in.
The first thing you could tell would be how cold it gets for the three darkest months every night. This will tell us what delta T is for your environment (read on to see what delta T is if you don't know).
Corey Schmidt wrote:Hello,
1. Will the relatively short heat exchanger cause any problems other than lower efficiency? If I can consume half the fuel that I would with a cheap epa woodstove I would be ecstatic.
Here's a thought experiment to do before you read the next couple of lines:
imagine taking a torch and heating up a piece of metal until it is red hot -as soon as you remove the torch, the metal rapidly cools until you can touch it in about 15 minutes;
now imagine taking some very thick dense rocks and heating them up until they are very hot on all sides as well as the center -as soon as you remove the torch, the rocks will start to cool, but like a potato, will still burn your hand long after the piece of red hot metal has cooled to where you can touch it.
It's my view point the point of the rocket mass heater is that it has as much mass as possible to heat up. Otherwise you're attempting to use the rocket mass heater like a piece of metal; I believe it will fail in this role. Experts can tell you if your short bench is sufficient for a heat sink.
Here's another concern: I've heard that in some parts of Alaska, a RMH is not appropriate. Why? Because you can't burn enough wood to the heat the rocks up because even the rocks loose too much heat in such a large temperature delta. The transfer of heat is directly proportional to the temperature difference between outside and the temperature you want inside. I've attached a pdf with some equations to describe heat flow due to radiation, conduction and convection; you can see that the difference in Temperature (delta T or dT) is the driving (overwhelming) part of the equation.
Basically, in really harsh conditions, -60F for months, will have the heat leaving the rock faster than you can put it in the rock.
So if you're seeing -60 for a month at a time, I hope an expert chimes in and tells you if a RMH will even work in those conditions like you need it to.
Corey Schmidt wrote:
3. A firebrick split costs $6 locally. I can get 'low duty 1700f' standard size firebrick for $4.12 and splits for $3.21 and used regular bricks for $1.83 if I go to Anchorage (a multi day trip for me). Is there any good way to substitute any of these cheaper materials for the fuel feed, burn tunnel, or heat riser, like by putting a clay slip or refractory plaster inside, or??
If your delta T isn't too great and a rocket stove will still work for you, I would pay for the good firebrick...but hey, again the experts here will tell you what other materials might work instead.
Corey Schmidt wrote:
4. Clay and sand are hard to get here, they would need to come by boat (like everything else) and a lot of effort or expense, so I am planning on using subsoil (mostly silty and rocky) from the site for the mass, which will be bounded on one side by a nonflammable interior wall (steel studs, cement board, and filled with subsoil for mass) and elsewhere by local stone. Do i need to tape or cement the stovepipe seams in the heat exchanger?
Cob isn't made with silt usually. What is the specific heat of your silt? The Fischer Price House at Wheaton Labs has a rocket mass heater whose mass is pea gravel; maybe you can dump your soil through a screen and get the rock you need for mass if the small rocks can touch each other on most of their surface area.
Again my answers are only a beginner's concern and not definitive.
Gerry Parent wrote:Congratulations on your decision to build a RMH Corey! Your really going to love it. Now, on to the questions:
Not a problem. As a suggestion, since your limited in your horizontal space, have you considered going up with your heat exchanger instead/also in the form of a bell? They do mention masonry bells a bit in the guide on page 8 and 175..., and you can also get a lot more info here: Bell Theory
Corey Schmidt wrote:1. Will the relatively short heat exchanger cause any problems other than lower efficiency?
Corey Schmidt wrote:2. I'm thinking of making a heat riser from stove pipe and clay stabilized perlite, like in the Paul Wheaton portable rmh video where they haul it on bicycle trailers. What is the life expectancy of such a heat riser, with heavy use? would lifetime cost be less with a firebrick heat riser due to greater longevity?
I've had my clay/perlite heat riser now for 3 years. I certainly wouldn't say I used it as much as you will but its still holding up well. Here is a picture of it during a more recent autopsy: heat riser This is the old school way of making a heat riser that is still being made and/or used by some but was never really a lifetime kind of thing.
Firebrick would certainly hold up better. It can be the split dense firebrick that would then be wrapped in insulation or full size insulated bricks with no extra insulation needed.
Now the Cadillac of heat risers is what is called a 5 minute riser developed by a fellow named Pinhead over at the proboards.com forum. It consists of a 1" superwool blanket put inside of a pipe. That's it!
One place that it was mentioned here: Working-Morgan-Superwool-ceramic-blanket
Corey Schmidt wrote:3. A firebrick split costs $6 locally. I can get 'low duty 1700f' standard size firebrick for $4.12 and splits for $3.21 and used regular bricks for $1.83 if I go to Anchorage (a multi day trip for me). Is there any good way to substitute any of these cheaper materials for the burn tube or heat riser, like by putting a clay slip or refractory plaster inside, or??
The original RMH's created by Ianto Evans used mostly recycled or inexpensive materials. The core was often built with those soft clay bricks which worked quite well.
Erica Wisner gives a good detailed description or many types of brick that may also help with your decision: Fake-fire-brick
Corey Schmidt wrote:4. Clay and sand are hard to get here, they would need to come by boat (like everything else) and a lot of effort or expense, so I am planning on using subsoil (mostly silty and rocky) from the site for the mass, which will be bounded on one side by a nonflammable interior wall (steel studs, cement board, and filled with subsoil for mass) and elsewhere by local stone. Do i need to tape or cement the stovepipe seams in the heat exchanger?
Definitely not needed with clay as sealer, but not 100% sure when it comes to using other types of soil. Best to keep away from an expensive aluminum foil type tape if you can avoid it. For the most part (except perhaps at the start up), the stove is operating under negative pressure. Meaning, any leaks or small gaps will draw inwards rather than leak outwards.
If you can get a bag of fireclay you can stretch it far enough to cover all the important areas that need sealing or adhesion and then just fill and tamp in the rest. Leave as few air gaps as you can as trapped air is insulative. Review bells though before deciding on a piped bench.
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