Outdoor rocket mass heater concerns

I love sitting around a firepit with friends. I don't love constantly stoking it with wood, always having a cold back side, showering before I go to sleep because I stink, and breathing in smoke all night.

So I want to try building an outdoor rocket mass heater but I have some concerns.

Will the freezing temps in Connecticut cause cracking or total disintegration of the cob (or anything else for that matter)?
How weather proof is cob out I the open? Will it withstand rain and snow?
I thought it would be a good idea to vent the exhaust through small vents running along the base of the front of the bench to warm your feet. Would this be of any concern due to poisonous gasses?
Is there anything else you can think of that may be a stumbling block to building an outdoor rocket mass heater that I have not thought of?
I'm sorry if this has been asked elsewhere but I have searched and not found anything.
Thanks in advance for your help!

bumping this thread, since I'm also wanting to build one or two RMHs outside.

I'm planning mine for an outdoor kitchen. there will be a roof, but no walls. so there won't be any direct precipitation to erode the cob. my biggest question is will it work? I know that the conductive and radiant heat from a big mass work without having to heat the air, but what if the air isn't heated at all? I would like this to be a usable space year round, but our winters only get down to 15 Fahrenheit rarely. will I be pissing in the wind, or will we end up with a nice, cozy outdoor space?

I'm not planning to do anything crazy. just the standard design and maybe try to work an oven and/or stove top into the design. should there be any considerations specific to an outside heater?

On any cob installation you need a roof... thats it. Cold doesn't bother it heat doesn't bother it but water does. You can potentially oil it till it turns into linoleum and wax it till it has a shell. To date i have not met nor seen any bench that either of these things has been done on.
You like to sit around the fire with friends then a small covering roof, A pergola, or a courtyard might do.
I personally like the court yard with a rocket bench and a rumford fire place. Its a bit of over kill but you have a wall around you a warm bench under you a nice fire in front of you and if done with care a roof to keep you dry and still be able to look at the stars. I am a rain baby i like to watch a good rain, and being able to watch and smell the rain warm and and dry, till i feel like dancing and warm up after is great.

Side blown rain is not a problem Splash can be, but is easy to remedy as you build (a tall foundation is best but you can also work flat stone into the wall and have that for run off.) I have seen lots of folks try finishes on benches only two worked. hydraulic lime plaster and some kind of blood, oil and cactus juice mix in plaster. hydraulic lime is expensive, and i dont remember the mix.

I recently built a cob oven and earthbag bench. After I have my oven up-to baking temperature I take out the red hot coals and put extinguish them in a metal can. It seems like a waste of heat and I'm pondering building a rocket mass heater to put these nice hot coals into to kick start the heater. The only downside I can see is that the rocket mass flame is not visible and provides very little light. Are there any rocket mass designs that give you a little better view of the flame? The earthen bench is curved and calls out for a fire in the center but I do like the efficiency and less issues with smoke of the Rocket Mass heater. Although the Rumford fireplace sounds great it does seem like overkill to have a Cob Oven, rocket mass heater and at a Rumford going to enjoy a late night baking session outdoors.

Well no there are no rocket stoves with windows but i am working on the problem. a rumford fire place would be the baby for using the coals. it provides plenty of light and will heat you very well.

Sure there are rocket mass heaters with windows. Here's one...

http://www.permsteading.com/viewtopic.php?f=6&t=79&start=30


The window works well, and the viewing tunnel doubles as an oven. Works a treat.

well as i live and breathe; i guess there is now one with a window. where did you put the window in the system? I am assuming end of the feed tube.

Yep, I extended a tube off the front of the feed. It's almost a foot from the feed area to the window. I used a little window/door off of a little marine diesel heater I had in my shop. At first the seal was not so good, and I had to restrict the air flow at the feed to compensate, but I used some fiberglass rope seal and it works great. It will soot up if I have to cover the feed, which right now I sometimes do as my wood is too long and wet. With a proper load that doesn't need covering, it doesn't soot at all. I love staring at it. I'll post a video here shortly of the flames turning the corner at the back of the tunnel. It's cool.

I believe you could go off of any of the three sides of the feed, and also create ovens around there if desired. Actually, I don't see why one couldn't do the same looking into the bottom of the riser.

hmm dont know how that all would effect the stove operation over time; and i hate to tell folks something works unless i have it tested for over a year. I will defiantly try it. One foot, huh?

have you tried it closer to the burn? hmmm i seem to remember us talking about this. good job getting it done!
Thank you

Wow that looks great Matt. I'll keep an eye out for a wood burning stove window assembly. It's not super cheap but a 12" x 12 " Pyrocerm glass costs about $70 http://www.fireglassonline.com/Order-Wood-Stove-Glass.html . Has anyone experiment with that material? I was thinking about cobbing it in but the access door plus mini oven idea looks pretty useful.

I'm attaching a photo of the oven and bench.

I see this is a pretty old post, but it hits what i am looking for so ill make an attempt to write my thoughts out here.

First off, i love this alternative building stuff, awesome!!

I currently live in Norway (Northern Norway) and my father wants me to build a rocket mass heater outdoors and build a pizzaoven on top of the barrel (my hopes where that the heat from the barrel would preheat the oven to a resenable temp before i would fire it internally)
I do have some cool materials availible to me, (i don't think cob/clay would be easily availible, but i might be wrong) There is clay in the shore, but this is seawater so i don't know how this is for building, also there will be no roof over the RMH and my thought was to use Slate, i think its called slate, as a finish and some special stone to hold the heat, my thought was to start off with a layer of foam insulation, then some sand, then the exaust pipes on top of this, and then perlite or something else that holds heat, a layer of the specialty stone that is designed to hold heat and then the layer of slate as a finish on top, so maybe a gap of about 10-12 inches from the top of the exaust pipe to the top of the finish.

Does this sound like a workable ide (i almost wrote good idea... but i left that one out:)

ALso had the thought of insulating the barrel so that i could build the exaust longer and so that more heat would get up into the pizzaoven on top and through the exaus (building a longer bench), so would this work? to inslutate the burnbarrel?

feel free to make any suggestions or ask questions, i will try to be more specific with the rock types that are here.

How does Leca Balls work as a thermal mass?

thank you guys

with love

KJ

Kjartan Rumpsfeld wrote:I see this is a pretty old post, but it hits what i am looking for so ill make an attempt to write my thoughts out here.

I currently live in Norway (Northern Norway) and my father wants me to build a rocket mass heater outdoors and build a pizzaoven on top of the barrel (my hopes where that the heat from the barrel would preheat the oven to a resenable temp before i would fire it internally)

Why don't you incorporate the pizza oven into the riser (barrel) itself?

I do have some cool materials availible to me, (i don't think cob/clay would be easily availible, but i might be wrong) There is clay in the shore, but this is seawater so i don't know how this is for building, also there will be no roof over the RMH and my thought was to use Slate, i think its called slate, as a finish and some special stone to hold the heat, my thought was to start off with a layer of foam insulation, then some sand, then the exaust pipes on top of this, and then perlite or something else that holds heat, a layer of the specialty stone that is designed to hold heat and then the layer of slate as a finish on top, so maybe a gap of about 10-12 inches from the top of the exaust pipe to the top of the finish.

I'd consider the foam to be risky. If it gets hot enough, it'd lose it's ability to support the weight.

Does this sound like a workable ide (i almost wrote good idea... but i left that one out:)

ALso had the thought of insulating the barrel so that i could build the exaust longer and so that more heat would get up into the pizzaoven on top and through the exaus (building a longer bench), so would this work? to inslutate the burnbarrel?

It's generally a bad idea to insulate the barrel, because part of it's function is as a radiator. It permits the hot gasses that have just existed the internal riser/chimney to cool somewhat, becoming heavier and thus contributing to the difference in density that pushes the exhaust gases the rest of the way through the horizontal mass channels. It's important to how the RMH can work without a normal chimney, which wouldn't work anyway with the cool exhaust at the end of the mass channels. If you were to install a pizza oven at the top of the riser, presumedly with a bottom baffle/reflector/fireback or two to prevent the very hot gasses from direct contact with your pizza, the external surfaces of your pizza oven could serve the same function, replacing the barrel altogether. You'd need to make it out of exposed steel, however, which is contradictory to what tradition would suggest; since most ovens are built to keep heat in, while your's would have to be built to permit heat to escape.

You'd have to be selective about the species of wood you chose to run the RMH whenever you used the oven, since there might still be enough unburnt smoke to affect flavor.

Creighton Samuiels wrote:

Kjartan Rumpsfeld wrote:I see this is a pretty old post, but it hits what i am looking for so ill make an attempt to write my thoughts out here.

I currently live in Norway (Northern Norway) and my father wants me to build a rocket mass heater outdoors and build a pizzaoven on top of the barrel (my hopes where that the heat from the barrel would preheat the oven to a resenable temp before i would fire it internally)

Why don't you incorporate the pizza oven into the riser (barrel) itself?

I do have some cool materials availible to me, (i don't think cob/clay would be easily availible, but i might be wrong) There is clay in the shore, but this is seawater so i don't know how this is for building, also there will be no roof over the RMH and my thought was to use Slate, i think its called slate, as a finish and some special stone to hold the heat, my thought was to start off with a layer of foam insulation, then some sand, then the exaust pipes on top of this, and then perlite or something else that holds heat, a layer of the specialty stone that is designed to hold heat and then the layer of slate as a finish on top, so maybe a gap of about 10-12 inches from the top of the exaust pipe to the top of the finish.

I'd consider the foam to be risky. If it gets hot enough, it'd lose it's ability to support the weight.

Does this sound like a workable ide (i almost wrote good idea... but i left that one out:)

ALso had the thought of insulating the barrel so that i could build the exaust longer and so that more heat would get up into the pizzaoven on top and through the exaus (building a longer bench), so would this work? to inslutate the burnbarrel?

It's generally a bad idea to insulate the barrel, because part of it's function is as a radiator. It permits the hot gasses that have just existed the internal riser/chimney to cool somewhat, becoming heavier and thus contributing to the difference in density that pushes the exhaust gases the rest of the way through the horizontal mass channels. It's important to how the RMH can work without a normal chimney, which wouldn't work anyway with the cool exhaust at the end of the mass channels. If you were to install a pizza oven at the top of the riser, presumedly with a bottom baffle/reflector/fireback or two to prevent the very hot gasses from direct contact with your pizza, the external surfaces of your pizza oven could serve the same function, replacing the barrel altogether. You'd need to make it out of exposed steel, however, which is contradictory to what tradition would suggest; since most ovens are built to keep heat in, while your's would have to be built to permit heat to escape.

You'd have to be selective about the species of wood you chose to run the RMH whenever you used the oven, since there might still be enough unburnt smoke to affect flavor.

Cool, thank you, i am not quite sure how i would incoorporate the oven instead of the barrel, the thought i am having right now is to build the oven on top of the barrel and leave half of the barrel exposed to the elements and to use the barrel and stones around parts of the barrel as support for the oven, my feel was that this would give a couple of 100 degrees farenheit as a startup making it easier to fire up the pizzaoven after a while, and still having the normal effect on the RMH with most of the barrel exposed.

Do you have any pictures or links to something like i am talking about?

Also, what do you think of a slate finish, instead of a cob finish?

My thought was also to use some soapstone as a layer above the exaust to hold heat for a while and to use Leca balls (don`t know if this exists anywhere else then here , but they are basically Burnt Clay balls that are puffed up so they hold quite a bit of air, and are used as insulators) would this be a good thermal mass to use? the way i am seeing it is that airgaps would hold quite a bit of heat and if there is a layer of soapstone covered with a layer of slate it would be a good surface that would hold heat for a while?

I would also use granite to build my "exterior walls" and then plaster slate on top of that, giving it a slate/tile finish.

what are your thoughts on this?

Kjartan Rumpsfeld wrote:
My thought was also to use some soapstone as a layer above the exaust to hold heat for a while and to use Leca balls (don`t know if this exists anywhere else then here , but they are basically Burnt Clay balls that are puffed up so they hold quite a bit of air, and are used as insulators) would this be a good thermal mass to use? the way i am seeing it is that airgaps would hold quite a bit of heat and if there is a layer of soapstone covered with a layer of slate it would be a good surface that would hold heat for a while?

if a substance has a lot of entrapped air, it isn't likely to function well as thermal mass. the 'mass' part of thermal mass really is pretty important. if looking up the specific heat of a substance is impractical or just too much work, the weight or density of the substance is a pretty good clue to how much heat energy it can store: heavier=more heat storage. which is to say that I don't think Leca balls will work well in this capacity.

they might work for the parts of the system that need insulation, though, like under and around the burn tunnel and around the upright of the J.

Kjartan Rumpsfeld wrote:
the thought i am having right now is to build the oven on top of the barrel and leave half of the barrel exposed to the elements and to use the barrel and stones around parts of the barrel as support for the oven,

Do this, but cut a 4" hole in the top of the barrel and fabricate an adjustable baffle to enable you to open or close it as desired. In this way you can quickly heat the oven with the column of flame from the heat riser, adjust the heat into the oven with very fine control possible, and route the heat you don't use for cooking into the seating.

Kjartan Rumpsfeld wrote:
Also, what do you think of a slate finish, instead of a cob finish?

This will also work well. My theory on the outdoor systems is that contrary to an indoor RMH, in an outdoor system I want quick response in the seating area. I achieve this by using the half barrel system to create low mass benches with thin slate tops. In this way the seating can be warm within an hour of lighting the fire, even when it's close to freezing outside and the bench started cold.

I love these outdoor systems, and would love to see your project when it is complete. I usually use a small BBQ grill on top of the barrel, but they make great modular systems where you can cook on a grill, or boil a large pot or canner, or heat an oven, all while enjoying a fire and using as much of the heat as possible. Here's pic of my current set up. It's a work in progress, hopefully I'll finish up the bench and backrest in the next month or so for the summer outdoor cooking season.

Your heat exchanger's outer wall doesn't need to be metal: it just needs to cool the air coming out of the heat riser in one way or another. Metal does that job well, but metal outdoors will corrode. Using a thin wall of non-insulating brick should work too.

bumping this out of interest. going to do this as well this spring. anything new popped up on this subject?

would mostly rocks in the bench be better than cob for outside? modifications and repairs would be easier. maybe crete board and tile for seat/cover?

i like the barrel idea. does it take away the performance?

cant you paint or seal the cob after baked out? does it absorb moister anyway?

Ronald, cementitious board and tile is a great finish for outdoors, in my opinion. I've had some in a test installation outside for a year now and it's holding up great. The half barrels do not hinder performance, rather, they allow you to use fairly light covering construction so you can get fast response for outdoor use, where you might not want to heat a large mass for days on end. There are ways to seal cob, but frankly, I think it makes more sense to use cob for installations of a season or two, and switch to your first idea for longer lasting installations. Play with some cob builds until you know what you want, then cover with the board/tile or something similar. Here's how my system looked at the end of last summer. It's still in this configuration and holding up great, although I'm getting ready to redo the oven side system for a new layout.

thank you Matt! that is an awesome bench set up!

Wytze Schouten wrote: Your heat exchanger's outer wall doesn't need to be metal: it just needs to cool the air coming out of the heat riser in one way or another. Metal does that job well, but metal outdoors will corrode. Using a thin wall of non-insulating brick should work too.

Two concerns people mentioned that I wanted to chime in on:
1) the masonry "alternative barrel: above:
Non-insulating (dense) brick still doesn't shed heat nearly as fast as the metal would. Insulating materials would, of course, be worse in this area (stoves that have their barrels cobbed over don't work as well, and one example that was mistakenly insulated hardly worked at all afterward). A non-metal barrel would likely affect design parameters, e.g. exhaust not as cool, more resistance to downward movement, so you'd need a shorter bench for less drag and a taller, warmer exhaust chimney for compensating draft.
It's also not that simple to join ordinary firebrick or tile in a way that provides a complete air seal, and we know that leaky barrels (except when they vent directly out the top like a cooking rocket) can cause draft problems. The air leak effectively means too much air is entering the bell, and reduces the draw at the firebox.
If you need a non-metal barrel / bell-type design, I'd look into established, well-tested contraflow masonry heaters (Swedish, Finnish, and Russian models exist, and some of the German kachelhofen may also be contraflow designs). Their proportions will be more what you'd need for this type of draft balance.

2) Outdoor bench protection:
While Matt Walker's bench is indeed gorgeous, and we've seen a lot of attempts to 'waterproof' earthen benches with slate, concrete mosaic, wax, etc., this is just not a great way to go.
I had the privilege to sit in on the City Repair Project's builder discussion about weatherization of outdoor benches, mostly cob benches without metal components.
- They had tried linseed oil and wax (not suitable for a heated bench as the wax gets sticky) and it needs to be renewed almost yearly to remain effective. The gorgeous "angel" bench, despite being in a neighborhood with 4 builders who knew the maintenance needs, got water-saturated and collapsed.
- They had tried slate and cement; the water tends to wick into the back of the seat and over the lip; some success was reported with a bead of aquarium cement to force a drip-line instead of surface wicking, but it wasn't 100% reliable (the water tends to dislodge the cement over time). Cement stops the cob from drying out between wettings, also, effectively trapping moisture within the bench. Numerous collapses of ancient (and formerly stout) earthen buildings have been reported after a recent cement stucco "upgrade" which trapped moisture within the walls.
- They had tried waterglass (sodium silicate) with some encouraging results as to hardness, but it tends to get an unattractive cloudy bloom, and is not 100% effective for the same reasons as above.
- They had tried little tile 'roofs' on the back of the bench; the overhang needed to be substantial to prevent drip erosion below. And this doesn't address the seating area of the bench; sitting on a ridgeline or tile roof is not very attractive.
- There was a proposal for a cob crescent shape with a wooden bench seat, which would tend to collect less water at the critical seat-back joint, and the plank could be removed or tilted up in winter to further protect the bench back.

The universal agreement among all builders present was that a roof is the only reliable weather protection for a cob bench.
All buildings need a good hat, and a good pair of boots (old Devon, UK reference to cob farm buildings). The earthen materials must be protected by a roof with a good overhang, especially on the weather side if known; and they must be lifted up away from contact with ground damp and dripline splash.

Since rocket heated benches involve vulnerable metal components in contact with hydroscopic clay or corrosive lime-based (cement) materials, keeping them dry will vastly extend their lives. You could spend $200 for a stainless-steel barrel, and do all-ceramic-flue-lined heat-exchange channels... or you could spend about the same money (as the barrel, not even counting corrosion-resistant flues) to build a simple shed roof. Might only need hardware and shingles (or greenhouse panels for daylit) if you have roundwood on site. And then you have a year-round heated bandshell-style garden seat, or even a heated gazebo.

You can of course build the bench first and the gazebo after, but I think it's usually simpler if you get both designs finalized and then build the roof first. This makes sure the foundations support the posts, and gives you have a sheltered place to work.

Outdoor benches without weather protection should be regarded as temporary. Same goes for cob ovens.
Nothing wrong with a temporary project - some villages re-built clay ovens every year as part of the soil-building process (fired straw-rich clay has better drainage than straight clay soils).
But if it works, you'll probably want to make it permanent. So give a little thought up front to what Phase 2 might look like - allow room / support for post and pier, cob walls, or a shed roof supported off an existing outbuilding. Whatever fits with the existing structures and future plans.

Weather protection obviously varies by region - check older local buildings for eave extensions, masonry rain damage, drip-line protections, etc. What you want is a building old enough, that has not been maintained or plastic-coated to hide the damage, that you can see visible patterns from the ordinary weather conditions of the area.

Depending on your region, I'd guess you could expect an unprotected bench to last maybe 3 to 5 years, before it would need massive repairs. With a basic roof (no walls), you might be looking at 10 years, up to 50 or more if you get the details right. Cob buildings in damp climates are still in use 400+ years later, and archaeological sites routinely find intact portions of pre-historic earthen buildings that were evidently used for centuries or millenia. Without a roof, however, all but the most arid sites tend to degrade once exposed.

In Connecticut / New England (and most areas east of the Rockies) you have nasty winter weather, both wet and freezing, and relatively humid summers too. From the damage I saw that sidewalks take in CT and MA, I'd be skeptical about unprotected masonry benches. Brick with an overhanging concrete cap might do it, but that doesn't protect the metal components or firebox, or firewood.

On the wet side of the West coast we may have 9+ months of rain and high humidity; again, a roof that not just protects but actively ventilates underneath is a routine part of local projects.

In the arid Southwest, on the other hand, there are ancient examples of adobe built directly on soil (which, in Tucson anyway, might as well be adobe - it bakes brick-hard). There may be problems around gutters and shower drains, but not from "weather" - freezing weather rarely follows rain exposure, so protection from sun, radiant frost, and winds are your main considerations there.

Wherever sub-freezing temps and wet conditions may occur together (including during the spring thaw), you have potential for frost-spalling of even the hardest masonry materials. Good buildings are designed to keep themselves, as well as their contents, dry and protected. If you want a design that can handle all weather while soaking wet, you are looking at bridge piers, basement walls, and boat construction as your relevant examples. You could probably cast components out of concrete for skate-park chic, if it was more attractive than doing an actual roof. But concrete's not that fun to sit on, I find, even when it's warm.

Dry wood storage is also a concern - without dry wood, tending the fire is a lot more of a pain (as Matt testified). If you need a woodshed anyway, or at least some kind of dry wood storage, the roof really does start to look like the easiest option.

We do have a greenhouse heater plan set in draft, waiting on beta-testing, that covers some detailing options for damp (not saturated) conditions and the expected durability of different options. $20 until I get confirmed reports from current builders, probably $35 after. PM or email me if you are interested. I might be willing to swap for good, detailed pictures of an existing greenhouse or outdoor build of very similar dimensions, if you've been working with a prototype that was based on our other 8" plan sets.

-Erica W

I want to build an outdoor system like Matt shows in the video above. In a perfect world I will have a roof, but there are so many projects at this new place in the city, I may move forward on the stove before there is a roof. I like the idea of the half-barrel bench that heats up fast, for a nice evening's lounging. I love the idea of windows to see the flames, and I really, really want to be able to cook outside.

Basically, I'd just like a complete copy of what's in the video!! Matt, any interest in a lovely getaway to Portland?

I can see that Home Depot sells the Hardiebacker board for $12/sheet (right now there's a guy much closer to Seattle selling tile backer board for $6/sheet, just FYI). I also see flagstones for sale, so maybe I spend a couple hundred dollars on a ton of flagstones (delivered), and the biggest ones become the base/flooring and the smaller ones go on top of the bench. I also like the idea of getting tile at the Habitat ReStore, various natural colors will look good, or I could go totally colorful, depends on what is there. . .

Hmmmm, I will keep scheming. I think I should make a materials list, I know I'm only partway there with the items above.

I'm always interested in lovely getaways to Portland! It sounds like you have a good handle on the materials list Julia. Let me know if/how I can help.