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rocket mass heaters

RMH to quickly heat small space

Posts: 39

Location: Central Connecticut - Zone 6

posted 3 years ago

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I'm interested in building a small RMH to heat an earth-bermed tiny house I use as an art studio. Only about 100 sq ft, lots of windows so low insulation.

Often it will be unheated, and then I might just go out there for only a couple hours, so I want it to get up to heat quickly.

Other times I might spend all day and even want to sleep in the space, and I will want to employ the slow-release heat of a RMH.

Any suggestions on how to design for both of these conditions?

Can there be some kind of valves so that the mass heat battery charging switches on and off?

Is a RMH even a good choice for this application?

Any suggestions on plans to look at for a starting point? I also don't want it to take up a whole lot of space. Thanks.

thomas rubino

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posted 3 years ago

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Hi Jon;
Yes and Yes
An RMH is perfect for your art studio.
A bypass allows the heat to head up the chimney before heating the mass.
A 6" J-Tube RMH would make you want to open a window for fresh cool air!

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Jon Piper

Posts: 39

Location: Central Connecticut - Zone 6

posted 3 years ago

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Good to know! Thanks.

I dug around looking for designs with a bypass. Maybe something like this 6" daybed with bypass plan would be a good starting point for me? I'm a bit lost about where to begin.

I don't think I want it to take up so much floor space, but I can probably shrink it down.

thomas rubino

master rocket scientist

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posted 3 years ago

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Creating a bypass is simple.
A pipe connection from either your transition area ( Where your horizontal pipes start)
Or from the bottom of your barrel directly across to the exit chimney.
In your application, a regular 6" wood stove damper with a short connection between the barrel area to the pipe would work fine.

Your bed/bench can be as wide as you would like.
A 6" J-Tube can push 35' of horizontal pipe with a 5' deduction for each 90-degree turn not counting the last one when you go vertical.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Jon Piper

Posts: 39

Location: Central Connecticut - Zone 6

posted 3 years ago

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Looking more into this, it sounds like a lot of heat radiates from the barrel even before it gets to the mass. Maybe that would be enough even without the bypass, although the bypass sounds useful for starting when the space is cold.

Anyway, what is the general rule for how far that barrel has to be from the combustible wood walls? Are there ways to reduce that spacing? With such a small room, I'd like to tuck the heater as far toward the wall as possible.

thomas rubino

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posted 3 years ago

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Yes, easy to do.
1" spacers and a sheet of metal will deflect the extreme heat.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

C. Letellier

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posted 3 years ago

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Are you building the bench into the floor? You won't have much room for bench otherwise. If your entry door uses nearly 9 square feet. The barrel uses another at least 9 square feet by the time you get clearances from the walls. By the time you get a J tube, barrel and chimey in that is probably closer to 15 square feet. I assume this is outside measurement so the walls probably eat another 15 to 20 square feet. So you have already used basically at least 40 square feet of your 100. Doesn't leave much room for a bench. If it was 2 feet wide and ran one wall there goes another 20 square feet meaning over half your building is already used up.

Suggest drawing you building up roughly on graph paper and then drawing in the various pieces. So you can picture what is left.

Country oriented nerd with primary interests in alternate energy in particular solar. Dabble in gardening, trees, cob, soil building and a host of others.

Glenn Herbert

Rocket Scientist

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Location: Upstate NY, zone 5

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posted 3 years ago

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In your space, I would advise building the combustion core into a brick bell with the front wall metal for quick radiation, instead of the barrel and bench. You could make a good one in a 2' x 3' space, 5 or 6' tall, and the brick walls would only need about 4" clearance to the exterior walls.

An example: tiny house rocket mass heater: the cyclone batch style

Jon Piper

Posts: 39

Location: Central Connecticut - Zone 6

posted 3 years ago

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Thanks for the advice.
Glenn - your brick bell example does look great in how it would use up less space. I also really love how you can see the fire! I've never really worked with brick before, although I've done a bit with other masonry materials so I feel like I could handle bricks ok. My biggest concern is the mortar. I have no idea what I'm doing with cob or where to find clay or crushed fire brick and worry about making my own mortar It sounds like it takes some experience to make a strong formulation. I wouldn't want my whole stove to be held together by a material I didn't make right. Lots to think about and learn..

thomas rubino

master rocket scientist

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Location: latitude 47 N.W. montana zone 6A

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posted 3 years ago

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Hi Jon;
Do not let the mortar worry you, its very easy.
Home depot sells Sakrete 1-cu ft 100-lb Commercial Grade Medium Sand for $12 or so. (edit just checked, its now $19 a sack)
A masonry supply will carry #50 sacks of fireclay but if not then any pottery shop will have dry bagged clay for sale.

One scoop of dry clay with three scoops of sand, add water, and mix by hand.
I generally put 3 scoops of clay and 9 of sand in a 5-gallon bucket.

Clay mortar is your friend! Easy on the hands, no gloves needed, can sit for months in the bucket, just add water when needed.
Excess mortar dry or wet can get tossed in the bucket!
It really is a pleasure to work with.
Refractory mortar is not!

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Glenn Herbert

Rocket Scientist

Posts: 4779

Location: Upstate NY, zone 5

738

posted 3 years ago

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One caveat with clay mortar: the construction needs to be strong and stable on its own, as clay acts purely as a bedding and sealing agent and not as any sort of "glue". Plastering over the brick construction will significantly help the stability of a clay mortared structure.

Jon Piper

Posts: 39

Location: Central Connecticut - Zone 6

posted 3 years ago

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Thanks for the help and encouragement guys. That sounds doable and actually quite nice.

I called the masonry suppliers and pottery shops around here and they don't seem to carry dry clay, but looks like I can order some
https://euclids.com/products/50lbs-hawthorn-fire-clay-35-mesh?pr_prod_strat=use_description&pr_rec_id=410d39fd0&pr_rec_pid=5133823311917&pr_ref_pid=5133823508525&pr_seq=uniform

There's a place nearby where I can get sand by the bucket pretty cheap. Does it matter what type of sand? They have: Bedding/utility sand, Washed sand, Concrete sand, Mason sand, Infield clay mix, Septic sand, Bank run sand, Screened C-33 sand, Screened sand

I still have lots to learn about the design. I wish there was a fool-proof how-to guide, but I guess that is also the beauty of RMHs. They are each unique and custom to their environment.

thomas rubino

master rocket scientist

Posts: 7114

Location: latitude 47 N.W. montana zone 6A

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posted 3 years ago

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Hey Jon;
You will want mason sand or screened sand.
The most important is no rocks and a uniform size.
You will need three times as much sand as clay.

I suggest that 2 #50 sacks of fireclay should be more than enough.
If you are paying a high price then start with just one.
I pay a whopping $7.50 for a #50 sack but I drive 120 miles one way to do so.
I also suggest you search out a local contractor who builds fireplaces and ask them where to source Fireclay in Ct.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Jules Silverlock

steward

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Location: Queensland, Australia

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posted 3 years ago

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You may be interested in the new Free Heat movie by Paul and a bunch of other expert collaborators all about rocket mass heaters. I particularly like how beautiful this one is - great for an art studio if you can scale it down:

Explore the Permies Digital Market - ebooks, movies, building plans, courses, and more. Oh my!

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Pat Melchor

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posted 3 years ago

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Rocket Mass Stove Heater: I want to build something very small (that takes up limited floor space in a conventional home ) that can heat maybe 100-300 sf. Is it possible to build a 1) very small, 2) possibly portable/movable unit?
I live in a conventional home with vinyl flooring in the den/kitchen area, with limited floor space for this unit. I have no building experience.

Glenn Herbert

Rocket Scientist

Posts: 4779

Location: Upstate NY, zone 5

738

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Look at the "tiny house rocket mass heater: the cyclone batch style" thread link in my previous post here, for something that could be made to fit your conditions.

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