request for help with rmh design

Mar 08, 2020 18:35:08

Hello,
I'm working on a design for an rmh for my new house I'm building. The heated space is a rectangle about 420 square feet, 8 feet tall, well insulated, r-33 walls, r-38 floor, r-57 ceiling. Passive solar will take care of heating for 6-8 months of the year (its a year round heating climate here) but won't cut it part of the time, especially the 3 darkest months.
I'm planning a 6" system due to the small and well insulated space. Due to limited space I'm planning a bench only about 6 feet long, so it will have only 12-13' of heat exchanger before going up the standard chimney through the roof. This is over a suspended floor, but I'm confident in my ability to calculate the loads and distribute them to the bedrock below. I do have a paper copy of the Builder's Guide, and have read and studied it.
As with everything else in this house, I am trying to cut costs every possible way that doesn't sacrifice durability or effectiveness. (for example, I bought all basic porcelain light fixtures for a couple of $ each to save money because I don't care how they look- i just want light-, but I paid 5x lead acid costs for a nickel iron battery hoping to never have to replace it). This is my first time to build an rmh, and I want as much as possible to stick with a basic design that will work the first time and be safe. It needs to look decent, but not fancy, my motivation for building it is to keep me and my family warm as cost effectively and environmentally benignly as possible.
Given these parameters I wonder if some people more knowledgeable and experienced than I with rmh wouldn't mind answering a few questions, or pointing me to places they have already been answered:

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.

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?

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??

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?

thanks very much in advance for any help!!
Any other useful info given what I've shared is also most welcome!!

Mar 08, 2020 19:30:40

Congratulations on your decision to build a RMH Corey! Your really going to love it. Now, on to the questions:

Corey Schmidt wrote:1. Will the relatively short heat exchanger cause any problems other than lower efficiency?

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: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.

Mar 08, 2020 20:03:01

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.

Mar 08, 2020 20:50:00

Gerry Parent wrote:Congratulations on your decision to build a RMH Corey! Your really going to love it. Now, on to the questions:

Corey Schmidt wrote:1. Will the relatively short heat exchanger cause any problems other than lower efficiency?

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: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.

Gerry- thanks a lot for this valuable gathering of info! I've read your post and all of the links. I love the 5 minute heat riser,- fast and affordable! I have some salvaged 8" metalbestos. I'm thinking i could get some of that superwool and put it in the metalbestos and call it good.
The bell does seem like a great idea for more heat exchange. Could I go from 6" pipe to say 10" and back again twice in the heat exchanger to get some of this advantage without a total redesign? Thanks again!

Mar 08, 2020 21:12:29

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.

Thanks for your response, Orin! My coastal location is mild for the latitude so the delta T is not so great compared to interior AK. Never colder than -10f, which I haven't seen in the 14 years I've lived here. We get 0 maybe 5 times a year and 0-10f maybe 20-30 days a year, frosts are very common 7 months of the year. It rarely reaches 70f even in midsummer, maybe 10 days a year, and the all time record high is 81f. The demand for heat here is not extreme, but nearly constant. I think I will take your word for the importance of the delta T rather than study all those equations!

Mar 08, 2020 23:53:32

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.

Mar 09, 2020 12:22:36

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.

Thomas,
Thanks very much for taking the time to respond to this! These answers are helping me a lot. I think I can consider the heat riser solved now. I can see how a bell system might be superior, but I'm thinking I might save some time and money if I make use of the locally available rock and subsoil, which seems to fit better with a piped system, if I understand correctly. I'm thinking after exiting the barrel, I will have about a 6 foot run of pipe, then 2 90s with cleanouts resulting in the next 6 foot run being directly above the first, then going up the chimney which will be by the barrel. This will put all the pipe closer to the interior mass wall and keep the bench narrow (and also concentrate the load on the floor- but I haven't closed in the underside so I can add all the 2x10s I want). Could you direct me to any resources about building the core with ceramic fiber board or cast clay/perlite? Thanks again! Answers from you guys are like gold...

Mar 09, 2020 14:03:39

Hi Corey; Glad to help you on the road to becoming a Rocket scientist! Your lab coat will arrive soon, until then continue wearing jeans and a shirt!

Here is a link to Matt Walkers video page https://www.youtube.com/user/broaudio/videos all the way at the bottom you will find, making a cast core.
I made one and used it for one season. Worked great! super hot BUT the feed tube which started out as 7.5 x 7.5 square was the almost size of a 5 gal bucket by spring.

I rebuilt using heavy firebrick. My newest stove uses a ceramic core here is a link to that build https://permies.com/t/94980/Brick-Bell-Shop-Heater and another
https://permies.com/t/99519/Baby-Dragon-Roars

All of Matts videos on his incredible stoves are there. Including building with ceramic fiber board.

Here is a direct link to Matts stove site http://walkerstoves.com/ You won't find the cast core video there only on youtube

We have a cheerful crew of happy Rocket scientist Eagerly awaiting your questions!

Mar 09, 2020 14:10:58

awesome! thanks!

Mar 10, 2020 08:05:48

Do you already have all of the 6" stovepipe for the mass run on hand? If not (and even if you do), I would advise making a bell instead. For much of the bell you may be able to use local rock mortared together. What is the character of your local stone? Flat, jagged, rounded? Unless it tends to all be smooth and round, you can build a secure masonry box with only mortar to import. Given the relatively constant heating load, a thick walled bell would be appropriate and keep the heat output fairly constant. You would need some firebrick for the top third or so of the bell surrounding the heat riser, as that sees extreme thermal stresses. If you use the traditional barrel around the riser, you can get away with just the core being firebrick or other specialty thermal performance material. What I did (for about 800 sq. ft. in upstate NY) was a brick and cob bell 6' high, with a (not yet built) 6' bench bell extending from the side. The bell walls are about 9" thick, and when heated up, keep warm for up to 24 hours.

For a 6" J-tube, I think you would want a bell with interior surface area (not counting floor) of around 30-40 square feet. A 6" batch box has been calibrated to want 57 square feet ISA, and J-tubes put out heat slower for the same size system so need smaller ISA to leave enough residual heat for chimney draft.
http://batchrocket.eu/en/building#bellsizing
You could make a masonry bell 16" square x 4' high inside, for about 23 sq. ft. ISA, and a bench with interior 2' x 5' x say 6" high above the exhaust opening, another 17 sf ISA, totaling 40 sf. I would want Peter van den Berg to weigh in on this estimate. I think the constant cold climate would aid draft enough to warrant the large end of the range for ISA.

Mar 10, 2020 10:55:21

Oh, and for the bell, cheap firebrick should work fine. You only need the high grade firebrick for the actual core. You would just want the top of the bell to be at least 6" or so above the riser top... or else you could use a removable steel plate for the top to make it easy to inspect, and also usable for food/water warming or modest cooking.

Building code for masonry heaters calls for a double skin, which is wise especially in a well-sealed house. You would not want any stones (with their mortar joints) to extend all the way from inside to outside face. Two layers of stone with joints staggered, even if they are not independent walls, ought to give enough safety against carbon monoxide leakage, coupled with the fact that a well-drafting chimney will make negative pressure in the bell and any leaks will mostly go into the bell, not out.

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