Practice

Rocket Mass Heater

Also known as: RMH, rocket stove mass heater

A wood-burning heater that combines an extremely-efficient rocket-stove combustion unit (vertically-stacked, insulated, internal afterburner that achieves near-complete combustion of wood gases) with a long horizontal exhaust path running through a thermal mass (cob, brick, or stone bench) that captures and slowly releases heat. The combination produces dramatically more usable heat per pound of wood than conventional stoves: typical rocket mass heaters use 70–90% less wood than equivalent-area woodstoves while producing comparable comfort. Developed by Ianto Evans, Larry Korn, Erica & Ernie Wisner, and others in the 1980s onward; widely deployed in owner-builder homes in cold climates.

Scientific

The rocket-mass-heater achieves its efficiency through three integrated principles:

1. Two-stage combustion (the rocket-stove core):

A vertical insulated combustion chamber (the “burn tunnel” and “heat riser”) forces hot combustion gases through a thermally-isolated path.
The heat riser is heavily insulated, allowing internal temperatures to climb to 1,500–2,000°F.
At these temperatures, wood gases (the volatile compounds released early in combustion) themselves ignite and burn — the “afterburner” effect.
Result: nearly complete combustion. Smoke-and-creosote output drops to near-zero; energy extraction from each pound of wood approaches theoretical maximum.

2. Thermal-mass capture (the bench):

After the heat riser, hot gases pass through a long horizontal exhaust path embedded in cob, brick, or stone mass.
The mass absorbs heat from the gases as they pass; exhaust gases exit cool (often barely above ambient).
Heat from the mass radiates slowly into the room across hours after a single firing.

3. Slow-release thermal regime:

A 30–60 minute firing of a few pounds of wood charges the bench mass; that heat releases over 8–24 hours.
The user typically fires the heater once or twice per day rather than maintaining a continuous fire.
Comfort comes from radiant heat to body and surfaces rather than convective hot-air circulation.

Practical

Standard owner-built RMH:

Combustion core — fire-brick burn tunnel and heat riser; insulation around heat riser.
Thermal mass bench — typically 6–10 feet long, 16–24 inches wide, 16–24 inches tall; built from cob, brick, or other masonry; contains the embedded exhaust-gas duct (steel pipe, often 6–8 inches diameter, embedded in the mass).
Exhaust — exits to chimney from the far end of the bench; chimney can be smaller than conventional woodstove chimney because exhaust is cool.
Fuel — small-diameter dry wood (sticks, branches, scrap wood); fed vertically into the feed tube. Doesn’t require split firewood.

Code acceptance:

Most U.S. jurisdictions have not formally adopted RMH building codes; some Pacific Northwest jurisdictions allow them under appendices or alternative-construction provisions.
Insurance and code challenges are real obstacles to formal RMH installation.
Owner-builder installations in unincorporated rural areas are common; permitted RMH installations remain rare.

Cultural

Ianto Evans (Cob Cottage Company, Oregon) and Larry Korn published the foundational Rocket Mass Heaters book in 2005 (revised editions through 2014). Erica and Ernie Wisner have built dozens of demonstration heaters and continue the technical-and-educational lineage. Paul Wheaton’s online forum (permies.com) hosts large-scale RMH discussion and design archives.

RMHs are widely deployed in natural-building homes — straw-bale, cob, hempcrete, [[off-grid-living|off-grid]] yurts and tiny houses. The combination of well-insulated natural walls and a rocket mass heater can reduce winter wood consumption by 80–90% vs conventional construction with woodstove. The technology has been particularly significant for [[off-grid-living|off-grid]] and homestead applications where wood-fuel-and-labor accounting actually matters.

Critique

Honest tradeoffs:

Floor-area cost — the thermal-mass bench takes substantial floor area (often 30–60 square feet). Small homes feel this.
Operational discipline — the fire must be tended (it’s not a “set and forget” stove); load up, light, run for an hour, walk away. Improper firing produces poor performance.
Code-permittable installation — limited; most installations are owner-built and unpermitted.
Resale and insurance concerns — non-standard heating systems can complicate property transactions.
Construction skill — requires natural-building masonry skill; not a casual DIY project.

Lenses still to grow

The Wisners and the rocket-mass-heater design lineage
Specific case studies of long-occupied RMH homes
Code-development efforts (Oregon’s pathways, regional alternative-construction codes)
Comparative analysis: RMH vs traditional masonry heater (Russian / Finnish kachelofen and tulikivi)

Sources

Ianto Evans & Larry Korn, Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build (Cob Cottage Company, 2005; revised 2014)
Erica & Ernie Wisner, The Rocket Mass Heater Builder’s Guide (New Society, 2016)
Paul Wheaton, permies.com forum archives
Cob Cottage Company demonstrations and workshops

Rooted in life.

What links here, and how

Inbound connections from across the wiki, grouped by lens and by relationship. These appear automatically — every entity page declares what it links to, and that data populates here on the targets.

Nothing yet. This entry is currently one node away from the rest of the graph — links will appear here automatically as the wiki grows. Each new entity that mentions this one in its relations frontmatter shows up here.

0 inbound links · 3 outbound