The Basics on Rocket Mass Heaters

If you think curling up next to a roaring fire during the cold winter months is pleasant, wait until you grab a book and settle on the warm bed of a rocket mass heater. The heated thermal mass remains warm for hours, offering an immensely comfortable and practical living space.

Rocket mass heaters are a contemporary adaptation to the long-standing technology of masonry stoves where the goal is to heat a thermal battery that slowly releases its warmth rather than attempting to continually heat the air in the room.

One of the primary differences is a rocket mass heater has an extended horizontal “bed” that can be utilized as a functional area. While it has a larger footprint than the vertically built masonry stoves, the simple design and fairly inexpensive materials of a rocket mass heater make it a task practically any thoughtful do-it-yourselfer can tackle.

Rocket mass heater or woodstove?

Broken down in the simplest form, a rocket mass heater consists of a vertical feed tube, a combustion chamber, a heat riser, and a thermal bank that encloses the exhaust tubes. It requires a fraction of the fuel — about a quarter of the typical amount used in a season with a woodstove — and burns cleanly.

While most people who heat their own homes are accustomed to wood heat, either with a woodstove or pellet stove, Erica Wisner says, “What we’re doing is no longer considered a woodstove.” Wisner and her husband, Ernie, teach workshops throughout the country, and focus their efforts researching the behavior of fire and improving the design features of rocket mass heaters. Rocket mass heaters eliminate the hot and cold cycles that are typical with woodstoves. A woodstove heats up, and cools off within hours, requiring more fuel.

“The real problem comes with overnight heating,” Wisner says. Most of the time people either bank the stove to keep it smoldering or they wake up to a cold house. “If you smolder a fire overnight, then you’re losing a lot of heat from the wood, and potentially creating creosote.”

Creosote buildup can cause a serious safety issue and wasted fuel. “Aside from the horrendous chimney fire scenario, the other thing that comes into play is all the smoke representing unburned fuel. There is a lot of fuel in unvolumunized gases,” Wisner says.

The rocket mass heater is a different animal. In a rocket mass heater, you might have a fire for 2 to 4 hours (depending on how cold the thermal mass is, as well as the weather conditions) to heat the thermal battery. “You get a burst of heat when the barrel heats up. Running an evening fire is very nice,” says Wisner. As the fire continues, the heat reaches the surface of the masonry in the thermal bank at roughly an inch an hour. From then on, the thermal battery heats the space and the people.

Wisner says it’s usually 75 degrees Fahrenheit when they go to bed, and it drops only 10 degrees throughout the night.

A bold experiment on the capabilities of the rocket mass heater took place in Missoula, Montana, in a tipi during the winter at the working laboratory of permaculture mad scientist Paul Wheaton.

A couple stayed in the tipi throughout the winter. But Wheaton says the test was when one particular night the temperature plummeted to 26 degrees below zero. “They slept on the mat (thermal bank) so it felt very warm, very comfortable,” he says, noting that this was no surprise since the most efficient heat is conductive, meaning you’re directly touching the heat source.

“We wanted to know what temperature it felt like when they got up in the morning,” he says. “They reported it felt as if it might be 50 to 55.” They put on warm clothes without being chilled, and headed to breakfast to join the rest of the group.

In addition to the tipi experiment, during a fall workshop, Wheaton had participants build another rocket mass heater in an office setting. “The one in the office did very well,” he says. Both heaters demonstrate the functionality of the system, even in extremely cold conditions.

How it works

The beauty of a rocket mass heater is that it’s a heat source practically anyone can build, regardless of your knowledge of fire or construction. “I have had people help build one of these who never lit a match,” says Wisner. “Masonry is not stupid simple. There are rules. There’s a whole vocabulary of terms masons use to orient things, but you can teach yourself that with a pile of gravel.” In fact, she often has students practice stacking pebbles to learn how they fit with each other.

Since the rocket mass heater is only 2 to 4 feet tall, instead of a structure towering above your head, it isn’t as technically compromised. “It’s much more doable than a taller masonry heater,” she explains.

There are a multitude of ways to construct the rocket mass heater, but a few standards remain the same. A sturdy foundation is imperative. “I do encourage a non-combustible foundation,” Wisner says. And it should have a solid foundation below it to support the weight. A floating floor above a basement or crawl space is not sufficient.

From there it’s a matter of laying out the pattern. The feed tube — where the wood is placed — and burn tunnel are two of the more critical areas since the design needs to be able to sufficiently mix the air to produce a flame going in the right direction.

Past the burn tunnel, the heat riser directs the hot air up and out through the manifold into the exhaust tubes that heat the thermal battery. The heat riser is most often created by using a recycled steel barrel focusing the heat with an interior stack that is thoroughly insulated.

The exhaust flues are typically made with galvanized pipe with elbows to form the ‘L’ or ‘J’ formations, depending on design. A cleanout box is also fashioned into the thermal mass area in order to access the tubes to vacuum any buildup once a year.

The thermal battery itself is where creativity can really shine. Many builders fill the interior with pea gravel, urbanite (broken up chunks of concrete) or earth, then finish the bed in cob, which is a mix of clay-based soil, straw and water. Brick or stone can also be used to finish the bed, depending on the available materials and your preference.

“It gives you a lot of aesthetic options,” Wisner says. “Up here we have mostly silt (which does not work well for cob construction). We have a lot of glacial till, so our stove is almost all rock. So basically any masonry or mineral-rich material will work.”

The exhaust portion of the rocket mass heater is unique. Instead of burning hot like a woodstove, the exhaust from the heater is typically around 100 degrees (right around a dryer-vent temperature) and consists of mostly steam. There are no particulates, except during the start or finish of the burn cycle.

While it technically can be exhausted through a wall, Wisner prefers to run the exhaust pipe through the roof to provide better draw, and to protect it from weather and critters. “I’m working with a lot of people who are pulling out a woodstove,” she says. There’s no reason you can’t utilize the same exhaust pipe.

Rocket mass heater construction is versatile. With these basic concepts, you can create an efficient system to fit your heating needs — as well as your aesthetic preferences.

Does it suit you?

Wisner points out that the rocket mass heater isn’t for everyone. “Electric is cheap, and people are used to adjusting the thermostat,” she notes. “Rocket mass heaters are not automated. Human operation is required.”

Learning how to start and tend to the fire can take a little practice. “The fire burns upside down and sideways. It may represent a challenge for some.” But with practice, most everyone can learn to burn it properly.

Of course, the hefty foundation that is required to support the weight is a barrier to some who are trying to accommodate one in a home with a basement or crawl space. Revising the floor plan or adding on a reinforced space may be required.

A rocket mass heater needs to be in a place where people can sit or recline on it to take the most advantage of the glorious conductive heat. It’s not something you should put in a basement or unused room.

This can pose a problem because of its size. Wisner says that rocket mass heaters are usually as big as a couch, and most rooms aren’t made for two large pieces of furniture. Some people can place it in an awkward corner, while others have to give up the space normally reserved for traditional furniture. Frankly, what couch can be a full body heating pad? It deserves an honored place in the home.

Yet, the greatest challenge is overcoming the local municipalities and their understanding of what a rocket mass heater involves. “They are difficult to permit,” admits Wisner. The Wisners have spent considerable time working with the officials in Portland, Oregon, to develop a code specifically for rocket mass heaters. In cities where inspectors have no idea what you’re talking about, the Wisners’ groundwork is a good starting point. Talk to your local officials to see what permits are required in your area and what local building codes dictate.

If you want to burn wood to heat your home, consider a rocket mass heater. While you might miss chopping and stacking multiple cords of wood each season, you’ll have a lot more time to curl up with a good book while enjoying the rocket mass heater’s long-lasting warmth.  

Read more:Choosing a Wood-Burning Stove for Your Home.

More information

For more information, including books, workshops, and permitting experience, look into these resources:

• Ernie and Erica Wisner’s website on rocket mass heater plans and workshops.
• For specific information on the Wisners’ permitting experience in Oregon: Rocker Mass Heater Permitting.
• Paul Wheaton’s discussion board on rocket mass heater projects.
Rocket Mass Heaters (Third Edition) by Ianto Evans and Leslie Jackson.

Freelance writer Amy Grisak spends about nine months of the year seeking out heat sources, and would love to install a rocket mass heater in her Great Falls, Montana, home. Follow her efforts at The Backyard Bounty.

  • Published on Dec 9, 2014
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