Rocket Mass Heaters: 10 Things You Need to Know
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Rocket mass heaters can save money you money on heating costs, reduce your environmental impact, and keep you warm in the coldest winters.
When I first heard about rocket mass heaters I was kind of skeptical. I read that they could eliminate something like 80-90% of the wood of a conventional stove and I thought, “Bah! That seems like a pretty unreasonable claim to me.” So I put on my investigative pants and got pretty obsessed.
One of the first articles that I came across was Paul Wheaton’s article on rocket mass heaters. He does a good job of breaking down the technology in a way that anyone can understand. So, I am going to relay some of his information as well as some other details I scrounged up!
I want to tell you about several of the reasons why rocket mass heating is so important. Let’s jump in.
#1 – What is the difference between a rocket mass heater and a wood stove?
Rocket Mass Heaters are like masonry heaters, except that people are building them for a LOT less money. They are super efficient, and hold the heat much longer than a conventional wood stove.
First, the sticks are put into the wood feed vertically. They are standing straight up. Then, only the bottoms of the sticks burn, and the fire burns sideways!
How does the fire burn sideways? The heat riser in the barrel is so insanely hot that it creates a convective current. This pulls the fire sideways into it.
Next, the thermosiphon draft pulls the smoke from the fire up into the riser and into the barrel. The sharp turn mixes that smoke with fire so it burns up the smoke in the secondary combustion chamber! Thus, it only leaves a tiny bit of CO2! It’s nearly purely steam. The image above illustrates this process.
#2 – How well do rocket stoves work?
Rocket stoves or heaters work exceptionally well. They are able to heat large areas while being very fuel efficient and clean burning. This makes them both economical and environmentally friendly ways to heat an interior space.
When the exhaust runs through the cob mass, the mass absorbs the extremely high heat. Then it slowly releases the heat over the course of a few days. This enables the rocket mass heaters to continue to provide heat for many hours after the fire has stopped.
This drastically reduces the amount of wood needed to sustain these bad boys. Some people heat their home using only dead yard debris and scrap wood, eliminating over $150/month in natural gas bills.
How much energy can rocket heaters put out?
The overall energy output can range drastically depending on the style and size. Some rocket mass heaters put out an equivalent of 250,000 BTUs.
How many square feet can a rocket mass heater heat?
Can a rocket mass heater heat your entire home? Probably! The max square footage rocket mass heaters can comfortably warm ranges from 800 square feet to 3000 square feet. The amount of heat depends on the type, size, and design of the unit.
What are the benefits of a rocket mass heater vs natural gas?
This one surprised me, because natural gas is supposed to burn really clean. I didn’t know wood COULD burn that clean. At high temperatures, wood burns down to steam and CO2, with a little bit of mineral ash left over.
Consider the environmental footprint of extracting natural gas (like fracking), refining, and the miles of pipes or delivery tanks that have to be in place for you to use it. By comparison, harvesting local wood looks pretty clean.
#3 – They are super comfy to sit on, like a full body heating pad.
Did you ever crouch over the heating vent in your nightie or pajamas, trying to get warm enough to get dressed? Good way to be late to school wasn’t it?
Moving hot air around is not an efficient way to heat things. Air doesn’t hold a lot of heat, plus it causes evaporation, which makes you colder. Forced-air heating is pretty inefficient. The reason it’s so common is that it’s relatively easy to control with thermostats and vents, and not as dangerous as steam heat.
These rocket mass heaters are more efficient like steam heat, but way less dangerous. The surface temperature is just warm to the touch. They are usually sculpted into heated seats or beds. If you need a warm-up boost, you just curl up and tuck your toes under the cushions.
Over-worked friends just kind of melt onto the bench when they come over for dinner.
#4 – Rocket mass heaters can be completely DIY.
These rocket mass heaters can most definitely be built by you and yourself alone. Ernie and Erica Wisner have taught tons and tons of workshops on how to build them. Respected teachers all over the world have, too!
This is not to say that is is 100% simple or fool proof. You definitely need to do your research and especially need to figure out which kind of rocket mass heater is best for your situation. However, with research and careful planning, you can definitely make one by yourself.
What is the best material for a rocket mass heater?
A big part of these rocket mass heaters being so efficient is the actual thermal “mass” part of them. There are several types of mass that have been experimented with, but the most common mass is made out of cob. (Cob is a natural building material made from sand, clay, water, some kind of fibrous or organic material (straw) and earth.)
#5 – You keep far more heat inside your house with a rocket mass heater than with a conventional wood stove.
A conventional wood stove uses a lot of the heat to push the smoke out of the house. With a rocket mass heater, the heat gets absorbed into the mass. This extracts as much heat as possible before the exhaust leaves the house.
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Can you burn a rocket stove indoors?
Yes, as long as your rocket stove has the proper venting. Large rocket mass heaters are designed specifically for indoor use to heat your whole house, tipi, or barn. Portable rocket heaters are typically not designed for indoor use. This brings us to…
Do rocket stoves need a chimney?
Rocket mass heaters or stoves must have an exhaust. This can be a chimney or pipe that creates the proper airflow and venting needed in the home.
#6 – People are living comfortably in a rocket mass heated tipi through Montana winters.
Like I said, up at Paul Wheaton’s land there is a ton of experimentation going on. One of the coolest rocket mass heater innovations he has is the one that is inside of the tipi. It is a traditional cob style rocket mass heater that heats a completely uninsulated canvas tipi.
Each of the people that have lived in it have reported utmost comfortability. This is huge because Montana gets down to -25 degrees Fahrenheit in the winter. I actually visited Paul’s land in December and got to hang out in the tipi. It was completely warm and toasty in there!
#7 – Rocket mass heaters use old technology with a new twist.
This taps into what I was saying earlier about the heat getting absorbed into the mass. What it does is mix modern science with knowledge from hundreds of years ago. We burn the smoke and capture heat from the exhaust, while focusing on utilizing the more efficient forms of heat. Radiant and conductive heat are favored over convective heat.
The new twist is using modern materials to make the building process cheaper and easier. We also make the heaters more effective at getting the heat where people want it. The heat is right where they can sit on it and tuck their cold toes under the cushions.
- Convection is heat exchange via moving air or water, which is how most of our standard heating systems work.
- Conduction happens when two surfaces touch each other – such as your body sitting on the warm stove exhaust.
- Radiant heat transfer happens when warmth radiates from a hot surface to strike another surface. It doesn’t require air movement. Examples include warmth radiating from a fire or from the sun.
#8 – There are many different kinds of rocket mass heaters.
The most popular design for a rocket mass heater is a 6” Annex Variation, but there are many different options. Here are a few:
- Daybed Bypass Variation
- Bonny Convection Bench Variation
- Batchbox Variation
- Cob Style
- Pebble Style
- Wood box Style Shippable Cores
- Small Molded Shippable Cores
- Rocket Mass Heater that doubles as a BBQ, cooker, and smoker
- Rocket Stove with a griddle, oven, and hot water
- Batch Box Variation with heat directed into the floor
Portable Rocket Wood Stoves vs Built-in Rocket Mass Heaters for Home
Portable rocket wood stoves are used for camping or just being warm while outside on your homestead. The overall concept is similar to that of a rocket mass heater – burn hot and fast, and trap heat in the mass.
Rocket mass heaters are designed to warm larger areas for a longer period of time, due to their larger size and mass. Portable rocket wood stoves are much smaller, so they don’t hold as much heat.
See the Ecozoom Rocket Stove Review for more information on portable rocket stoves.
#9 – How much do Rocket Mass Heaters Cost to Build
The cost for these heaters varies greatly. DIY options can be anywhere from $50-1,000 to build yourself, while hiring a contractor can cost upwards of $5,000 in some cases.
#10 – What are the disadvantages of rocket stoves?
Some people may not love the large size of these stoves and the look of the mass used to retain the stove’s heat. The DIY models definitely have a more rustic look that’s not a fit for every home.
You also need a solid foundation, as the mass of the stove is heavy.
Final Thoughts
Rocket mass heaters provide a multitude of opportunities for heating your home in a clean, efficient, cheap way. They create a uniquely comfortable living area, and use local fuel.
To get a quick look at how a rocket mass heater works and “meet” Paul, Ernie, and Erica, you can watch the video below.
You may also enjoy:
5 Things You Need to Know Before You Buy a Wood Burning Stove
Cassie Langstraat grew up under the big blue skies of Montana. She has a fire in her soul for all things regarding literature, permaculture, and feminism. When she isn’t writing or poking around in her garden, she enjoys reading philosophical novels, fishing in the dark, and eating cheeses with names she can’t pronounce.
Originally posted in 2015, last updated in 2023.
Thank you for this article…and the links. We had to take down our outdoor wood burner last year…lost heat to part of the house because of it and the way a previous owner had installed the furnace system. We have not been able to come up with anything that wouldn’t cost a small fortune to solve the issue. This is the first glimmer of an idea to seriously look into.
I would like to know if the combustion portion of a RMH can be built on the outside of a building and the heat sync portion on the inside of the building. I have not seen any of these split installations. If the designs are out there can you please direct me to them.
You’d lose a lot of heat with that setup. I have not seen any installations like you describe.
Hi Jeff, Matt Walker created the Walker Boiler for hydronic heating with the rocket portion outside of the house. You can watch a video at Youtube:
https://www.youtube.com/watch?v=7XsK08XuT10
You did not explain one of the crucial features of the rocket stove in your illustration, without which it is just a mass heater . The heat from the fire is NOT extracted from the combustion chamber. The combustion chamber is insulated so that it reaches very high temperatures producing more complete combustion. The heat is extracted from the exhaust flue downstream of the combustion chamber. This is the design flaw of the majority of wood stoves, heat is extracted from the combustion chamber resulting in a low temperature fire resulting in soot and unburned exhaust gases.
Does anyone know if anyone out there has figured out a way to make a RMS work in a travel trailer as of yet? If so where might I find instructions? I have read several articles that mentioned possibility using the water tanks as the mass but can not seem to find any info from anyone who has tried it. I am knew at all of this but have read a lot. I have yet to see one in action in person though. I am desperate to find an alternative reliable heat source that does not rely on fossil fuels or electricity. I grew up using wood stoves and fireplaces and prefer them over other types of heat but they do require a lot of work and use a lot of fuel ⛽. I love the idea ? behind the RMS and would love to use one if possible in my travel trailer but weight is the obvious issue I am having. Is there anyone that can help me find simple instructions if they exist?
I know there are portable rocket stoves that are mostly used for cooking, but don’t know if they would give you enough BTUs for heating, plus some sort of ventilation would be required. We have a post on portable rocket stoves here.
I just discovered RMH after watching a show. Amd I came across this article while investigating deeper. After reading both the article and the discussions I’ve got a few comments. 1st I think the article needs a small edit. I use to write emissions forms when cars had carbs before computers so I know a little. Combustion produces CO which is bad, CO2 is what is in your glass of soda, it’s what we exhale and plants breath to produce O2. I’m sure the minor mistakes were in the articles used to compile this article and went un-noticed. Now like the CAT on your car Higher temps cause CO to combine with O2 to form CO2 and if you could add say an aquarium air pump hooked to a small metal tube to force more fresh air into the combustion chamber you could lower the emissions even lower but you want it added after the burn process toward the top of the burn chamber so it adds to the exhaust burn-off. 2nd the Steam aspect comes in as the exhaust cools and the left-over Oxy combines with Hydrogen that didn’t burn to form H2O which if any is minor. Now as to Comments: As far as outside air, you wouldn’t want to add an outside air source unless it’s added before your feed box or your fire won’t draft correctly. Now if you add a 3rd chamber outside the burn barrel assembly with an outside air source and a convection out pipe that only collects heat off the outside of the burn barrel and forces it back into the room that would be an add-on not actually part of the furnace, it would be the same as wrapping the burn barrel with copper tubing and using water to supply base boards. As far as weight goes 1 solution is the use of lava rocks in the external heat sink (bench) frame your bench and fill with lava rocks. Their light (some even float) and mix crushed rock with cement powder to seal the rocks into the framework. You can get both lava rocks and lava soil at most supply centers for as low as $1lb. That would reduce your weight considerably. And if you use concrete backer board for the face of your bench you will have a nice surface for tiles. Someone questioned the ceramic radiant heat tubes across the ceiling, walk into any Walmart garden center in the winter and look up to see where the excessive heat is coming from, they also make the little furnace at the input end that burns waste oil from your car and kitchen. But thats something different, I mentioned it because that system has a small fan at the exhaust end that starts the updraft effect. They make inline pusher fans for duct work, something like that could be added to the top of your chimney with a switch next to your stove used to prime your furnace for lighting. It could be off/on or connected to a timer like some gas furnaces that run a few minutes and shut off. Of course it would have to be an all metal fan due to the exhaust heat and require maintenance.
Anyway these are just a few comments and ideas, I hope some help.
Are there also larger radius of curvature stove pipe options available that would work in typical designs?
Chimney pipe is available in a variety of angles (15, 30, 45, 90), depending on the type of pipe. The trick is getting around multiple bends.
Hey, I’m thinking about building my own for green house, but I have to admit the only thing stopping me is not knowing how you clean it. No matter how clean the burn, there will be some soot to clean, and nowhere on web I could not find directions how to clean all those curvy pipes set in masonry. Any ideas where to find directions for cleaning?
You’ve hit on one of the “gorillas in the room” with some of these designs. Our stove has a relatively simple flue with access ports. Many of the designs I’ve seen have no ready access for cleaning. A brush that’s flexible enough to handle the corners would have a hard time putting up with the force required to make it go around those corners. Plan for access ports.
Thanks. That’s the most useful tip I got on this subject.
Lifetime costs and maintenance was a big part of our planning with our home. I’m glad you asked the question.
The rocket stove heater, with pellet hopper, that I purchased, has passed the Underwriter Labratory testing and is insurable in a residence. The family has a factory building them a few at a time. Mine was delivered by truck. I’m very happy with it.0
I have seen a video where an owner tried to bring in air from outside and regulate it with a homemade valve to the burn chamber, I read in the book RM heater builders guide [E &E] they speak of how critical CSA [cross sectional area] should be consistent to the successful operation of the RMH.
so has anyone had success with bringing air in from outside as opposed to drawing air from the room?
the video I saw was on youtube and this family built a earth bag home with the RMH.
we are excited to build one for our shop/studio. we had hoped to have a masonry heater built but finances have been used elsewhere etc. and I am anxious to work with cob.
thanks for the site!!
I think the R.M. needs a few adjustments, if it will perform as they say. Why are they putting the cheep galvanized pipe in all that work. I would use something such as concrete tiles or pipe. A 55gal barrel will last approximately 10 years, if kept inside. The weather is what rusts them out and some stove pollish would help preserve them. I am planning to build one, they seem to be very economical. Seems they just need a little perfection.