The Basics on Rocket Mass Heaters

Take wood heat to a new level with rocket mass heaters.

| January/February 2015

  • Part of the appeal of this heating system is that the thermal mass becomes part of the furniture, and it's aesthetically pleasing.
    Photo by Calen Kennett
  • Doesn't get much more cozy than Gritty and his barn cat, curled up on the rocket mass heater.
    Illustration by Brad Anderson
  • Adding wood to the firebox.
    Photo by Calen Kennett
  • Make sure to check local building codes and ordinances.
    Photo by Kristie Wheaton
  • According to proponents, these heating systems use less wood than a woodstove, and provide ample warmth.
    Photo by Kristie Wheaton
  • Permaculture enthusiast Paul Wheaton tested a rocket mass heater in the tipi above during a winter near Missoula, Montana.
    Photo by Kristie Wheaton
  • Rocket mass heaters are similar to a woodstove, but with a built-in heat storage trap that radiates more slowly.
    Photo by Kristie Wheaton
  • The firebox loaded up and ready to go.
    Photo by Kristie Wheaton

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.”

10/8/2018 8:28:37 AM

I do not see an ash clean out, is there one?

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