RMH vented vertically in a city home

I've been thinking of switching from wood stove to a RMH. I noticed that the designs usually have a horizontal exhaust pipe, which is usually embedded in the thermal mass bench or floor or whatever. Does anyone know if that is merely just for spacial/layout reasons or does it have to do with the physics of the rocket effect and/or exhaust effectiveness? The reason I am asking this is because in the city it might have to be vented upwards through the house chimney. My thought was to run the horizontal line through a heated bench, then end it into the existing chimney, where it would go up another 30 feet to the exit through the house chimney. Would this be OK?

Hi David.

well, it is better to use a vertical chimney after the horizontal "heat transfer" bench or part of the rocket mass heater. OIf you want a shorter run horizontaly, either snake your pipe, or search into bells. You need to keep a bit of heat to power the chimney.

And concrete makes a good mass. If you plan staying there. Make a brick wall around your pipes, chuck rubble in there, pour liquid concrete to fill it up. You want your mass to be either monolithic, or have air gaps big enough to have convection between the blocks of your mass. No trapped air between gravell or sand which turns out to be an insulator.

Tho, imho, bells are cleverer.

Wow, that's great information, and great news! That design will probably work perfectly here then! I actually have a pile of fire bricks that I pulled out of a giant old coal furnace that I cut up from the basement when I got this house, so I can probably use those bricks for the portions that will be touching the fire. If I build the heat riser out of fire bricks, would that do away with my need for perlite insulation and cob completely? Or will I still need those for mortar on the heat riser?

Also, I want to get a good book to help with some of the technicals of the design. Any suggestions? The comments on "Rocket Mass Heaters" says that book is getting outdated...

David, you need insulation around the burn tunel and heat riser. You can build it out of refractory bricks, but it takes longer to get to full working temps than say half bricks. Or refractory chamote flue tubing.

http://www.stove.ru/index.php?lng=1&rs=3

http://www.stove.ru/index.php?lng=1&rs=16

David Blackton : You did not give us a location for yourself, we need a little climate information,and we can not comment more than generally about your local ordinances
and how this will affect any insurance considerations!

Ionto Evans searched for and found a region with a sheltered valley with stable constant low velocity winds, tending to be out of one direction year round ! Yes, such places
do exist, further all off his houses were ~15 ' tall~ or less, with all these near ideal conditions, his horizontal chimney works very well for him and the Intentional Community
that is Cob Cottage ! The high Burn temps and the heat differential engine effect created at the Heat Riser / Barrel guarantees 20'-30' or more of horizontal flow through any
standard smooth wall stove pipe !

Your situation merely requires a look at the condition of the existing chimney and its internal shape and structure by a chimney sweep, ask for and check references, and like
going to see a Lawyer or Doctor, take their advice even if its not good news, almost often you can get away with a reline or a flex pipe chimney placed inside, I would not
ever recommend Flexpipe for horizontal use but properly sized for the size of your system 6'' or 8'', flex pipe will work in your tall chimney !

I understand where our friend Max is coming from, However there are a few problems with using Cement/ Concrete or Concrete blocks at the start of your Rocket mass heater
that you need to know and understand, also, for now and the foreseeable future Rocket Mass Heaters and Thermal Mass Benches will turn-off potential home buyers, who see
any home as their single largest investment !

For that reason I would build out of the more traditional cob, simply to protect your investment ! Cob can be removed with little or no damage showing after 20 + years, concrete
is forever and will probably always announce its presence or its former presence to all who come to look at your home as a future investment !

We have also not discused the special requirements for installing a heavyweight Unit on wooden floors, will that be a consideration for you ! A simple layout of the floor that you
hope to install your Rocket on and your understanding that you want this heater to live the the very heart of your home where you will spend most of your time is important !

Yes, it is true that Rocket Mass Heaters has been the standard for so long that improvements have been made on its slightly dated design, Of the ~100,000 R.M.H.s~ that
have been built, most of them have been built "from the BOOK!" And 95% of all the first time builds (that worked ) came out of 'The Book'! Literally there is STILL No other book
with as much Rocket Mass Heater information in any Language, if there is no replacement for it, Anywhere, I can't call it outdated !

If you go to rocket stoves.com you can download multiple PDF Copies of 'the Book' for $15 and this will allow you to speak about the parts of the R.M.H. in the same language as
your fellow members, and to understand the need to keep a Constant Cross Sectional area within your rocket, well enough to explain it in your own words to a third party !

You are welcome to come back here at any time with over 19,000 fellow members you can come here nearly 24 / 7 and get a response, If a R.M.H. is in your future, we will help
you build one! For the Craft, Think like fire, flow like gas, Don't be the Marshmallow! Your comments and questions are Solicited and Welcome ! PYRO - Logically Big AL !

Thanks for the responses everyone!

To answer your questions Allen, I am located in Philadelphia. This is a row home basement with concrete floor poured over dirt and a high basement ceiling. I own the house next door as well, which is where we currently live. Although it is possible I might build on wood floors in the future, these old houses are build so much stronger than new houses, so the weight is not nearly as much of an issue. I always design my rooms to center any heavy weight near the side (structural) walls rather than near the middle of the rooms or the front or back sides of the rooms (unsupported, as the joists go from side to side, not front to back). Also, I have been pouring concrete radiant slabs over the wood floors and they have no trouble with this weight. At several points we actually had several tons of sand piled in the middle of the living room floor inside a dumpster, and the floor didn't seem to notice (that's not to mention another several tons of cement, furniture, and other supplies stacked around it! Oh yeah, and about 10 more tons of bagged rubble, around 800 bags total!)!

This brings up another question, are RMHs usually built along the outside wall of a room like conventional heating systems usually are? I could probably make an L shape from the front wall of the house to the side wall where the chimney is, but the thermal bench would only have a single run of stove pipe running through it then, which is why I was thinking of having the bench on the side wall only with a the pipe going out and back again for a smaller but warmer bench. Is the reasoning for putting a heater on the exterior wall of a house mainly for uniformity of heat distribution, or are there other factors as well? I often think you would lose more heat that way, because locating the hottest part of the house next to the coldest part would create the biggest heat differential, which would cause the greatest heat loss over time. I like and agree with the concept around here that you should focus on heating the person, not the house, so why not locate the heat bench AWAY from the exterior walls, and then just agree to locate your body on the heat bench rather than over by the cold window?

Regarding insurance, I am much more concerned with it actually being safe than keeping the insurance company happy. If they really have a problem with it, I can either find another insurance company or change the design to appease them. The state of the house right now (and any of the other 100+ year-old houses around here) is so bad that there would be a dozen other much bigger insurance concerns, and whatever I do is going to be a MAJOR improvement on how things are now. I've also seen the extremely shoddy work of most contractors, and although I don't consider myself an expert, I know I am doing MUCH higher quality work than most professionals! In fact, I hired two contractors to do my electrical and plumbing on my first house, and when the electrician came to inspect it, he made me totally tear out and redo everything they had done. I did it myself the second time, and he was happy with my work. As I'm living in my own house, I know the work I've done is good quality, and I'd much rather have it ACTUALLY work than to have someone who does "certified" work that is actually a mess of leaking pipes and shorting wires. I realize that there are legal risks with anything though, which is why I am moving forward with caution.

The existing chimney is brick, starting in the basement, going through the first, second, and third floors, then out through the roof. The roof is a slightly sloped flat roof with torch-down rubber on top of wood decking. There is an existing 6" round metal chimney sleeve in the chimney now. It's the same as the chimney on the house that we are living in, which we sweep out ourselves every few months. We are mostly burning old wood that we collect around our block for our wood stove, so there is very little build-up anyway, but always better to be safe than sorry!

One thing I haven't been able to figure out is why people here keep saying that concrete is forever or that it is hard to work with. Am I missing something? I mean if you don't want it in the future, why not just take it out again? I understand the comments on using cob to withstand the temperatures though, so I'll have to look into how to build cob.

Here is a picture that I found of what my fire bricks look like: http://www.hotelrestaurantsupply.com/TWN-225044.html?utm_source=froogle&utm_medium=shopping&utm_campaign=products&gclid=CIvWzvX2r7oCFYqi4AodajsAsw
Mine are probably bigger than the ones in the picture, as this furnace was maybe three feet across, but they look exactly the same! I actually burned out several diamond metal-cutting blades cutting into the fire brick before I realized it was there! I thought I had run into some super-strong metal or something! I was really excited because I'd been wanting to build a RMH for a long time, but needed to find an affordable source of fire bricks, and there it was! They are shaped a little funny though to fit a large round chamber, but I was thinking of maybe breaking them.

So I still wasn't quite clear on the fire-brick portion of the design. Would I just build the feed tube, burn tunnel, and heat riser out of fire-brick without using any mortar, then coat just the outside with cob? And which parts can be normal brick, just the outside wall of the bench?

I did look at that page again for the book download, but there doesn't seem to be any download link, only a link to buy a physical copy of the book. Is the e-version no longer available?

.

David Blackton : The only reason that I can think of for heaters on outside walls is to deal with Vertical Chimneys most easily !

My limited experience is mostly with cement pours attached to houses or poured pads for equipment You can have the type of staining that marks its presence long after the
concrete was removed, Cob does not Rot, will not burn, Termites won't eat it, and it is as cheap as dirt ! I do understand about the class of construction you are talking about,
I would keep in my mind the slow pyrolysis of joists exposed to moderate heat energy loads around brick chimneys mostly in the area of the basement / 1st Floor ,
and 1st / 2nd floor, you can end up with a joist beam end that has turned to charcoal, as these are well hidden they are hard to spot, not like dry rot or insect damage !

Concrete, and C.M.U.s will start an unrelated failure if exposed to heat above 400 dF. Famously, paper starts to burn at 451 dF

The type of fire brick you want is usually 9'' X 4.5'' X 2.5'' (sometimes 3''), there are splits- 9'' X 4.5'' X 1.25 and shorter pieces, and they seem to split at least as easily as
regular brick !

If you were cutting them up wet and were going through multiple blades you probably have the harder type, super durable, and are built to let heat radiate through them to the
metal jacket they are surrounded by ! We want softer brick that re-radiates most of the heat back into the Burn Tunnel Combustion Chamber to quickly raise the temperature
the wood is 'burning' at to the freakishly high temps that gives your RMH its super efficiencies !

when you go to the site - ' rocketstoves.com ' - you will see the picture of 'The Book' , click on the 'add to cart' directly below ! you can down load all the copies
you want, one for every job site ! Yours in haste Big Al

Thanks Allen! I just downloaded the book, so I'll read through it and see if the information there helps! That's strange though; every time I went to the site before this, it was a completely different site with videos and things on it, and the book link took you to an Amazon page which only offered the physical book. At least I have the book this time!

I'll read through the book before coming back with many more questions.

But to quickly respond to what you said about the cob versus cement, I THINK that cement would be cheaper, judging by the price of clay that this site linked to on Amazon, but of course if I can find wild clay, then it's free! I usually get the torn bags of cement and lime for $1 from Lowe's, then I get sand for $30/ton, so I can do a tone of cement for about $30. So from a cost and availability standpoint, cement is cheaper and much more easy to find. But the more important question is the functionality. I guess I should get some clay anyway to make the clay "slip."

And yes, the fire bricks that I found are VERY strong! The diamond blades were supposedly designed for cutting through masonry as well as cast iron and steel, but they burned right up as soon as I hit the fire brick! I do have some of the softer ones inside my wood stove, but the wood stove probably needs those ones! Were you implying using just normal masonry (non fire) bricks? What is the benefit of fire bricks in a stove setting over just normal bricks? I guess the normal bricks would crack over time? And cracking is why you also don't reccomend the thinner fire bricks? I think the kind I have in my wood stove are the thinner type anyway (although not too many of them have cracked yet, but then again it wasn't a rocket stove!).

I don't think pyrolysis should be a problem with my joists, as the brick chimney shields most of the heat. I don't think the outside of the brick even gets hot enough to be more than warm to the touch. And it sounds like the RMH should actually emit COOLER air than our wood stove is! And after over 100 years of wood, coal, oil, and finally gas fire exhaust, none of the joists seem damaged, so I'm not too worried about that (I've looked at a lot of old row homes, and 100 percent of them had rotten joists under the bathrooms, always in the exact same part of the house, but usually everywhere else the joists are almost perfect after all these years!). Are you saying that I could have charcoalized joist ends but not realize it?

OK, well let me look through the book before I ask any more questions!

David Blackton : I only have anecdotal or third hand information on using the thinner bricks ( 9'' X 4.5'' X 1.25'') in place of the regular fire brick, I will only decide to go that way
if I find myself in possession of cheap fire brick Spilts.

If all I had to hand were the 100 year old orange - red bricks, some times called soft bricks, I am dealing with a physical thing I have experience with, I would choose soft red
brick over hard "Fire Brick", and soft fire brick over the used,old, soft, red/orange House brick !

An interesting thing about Cob vs. Concrete, vs Urbanite (recycling old hunks of concrete) filled concrete, vs rocks in concrete, vs, Heavy chunks of iron ore/slag is the heavier
they are, the more heat they will take up, and the more Heat energy will transfer rapidly through the mass from the core !

The only way I have ever found to detect Pyrolysed wood is by the same trick you have probably taught yourself to detect in those bathrooms, the floor squeak that others do
not hear, and a too springy floor ! Every squeaky floor has a story to tell though sometimes the archeology is panful to explore ! For the Good of The Craft ! BIG AL !