Rocket Mass Heaters: 10 Things You Need to Know
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.