The Basics on Rocket Mass Heaters

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

| January/February 2015

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.”

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