What is a Rocket Mass Heater?

by: Sara on 05/19/2016

While heating with wood is often considered a natural and economical alternative to electricity or fossil fuels, even with a fairly new and efficient stove, many cords of wood are required for a single winter and incomplete combustion can contribute to poor air quality.

A rocket mass heater is an earthen masonry heating system which provides clean, safe and efficient warmth for your home, all while using 70-90% less fuel than a traditional woodstove. In cold weather a few hours of clean, hot burning can provide 20 or more hours of steady warmth, while the unit's large thermal mass acts as a heat sink, cooling your home on sizzling days.

Today's post is an excerpt from The Rocket Mass Heater Builder's Guide, Complete Step-by-step Construction, Maintenance and Troubleshooting by Erica Wisner and Ernie Wisner

A rocket mass heater is a heavy, slow release radiant heater. It is designed primarily to heat people, secondarily to warm the areas in the line-of-sight around it. Modest tertiary functions include cooking, heating pots of water, and producing some warm air for distribution to other nearby areas.

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White heater with blue tile: built by Adiel Shnior, and Nitzan Istrovitch Photo by Adi Segal

A rocket mass heater is not a furnace or boiler; it should not be located in a hidden space, nor left to burn unattended.

The “rocket” in the name comes from a line of clean-burning cook stoves developed in the 1970s, using an insulated heat riser, to produce a very clean and efficient fire. “Mass” refers to the mass of masonry where it stores its heat. An RMH heater always includes an insulated, vertical, chimney-like heat riser as part of its clean-burning combustion chamber and a thermal mass to extract heat from its clean exhaust

An RMH can warm a home using a fraction of the firewood required by other common heaters, such as woodstoves and boilers. The large, heat-storage mass these designs incorporate can heat multiple rooms from a central location, providing


Fuel feed: Sticks and air enter downward, feeding by gravity and the heat riser's draft

overnight heat without the danger and difficulty of an all-night fire. Rocket mass heaters are most effective in “line-of-sight” and by direct contact (built-in benches or sleeping platforms), although they do warm some air for heating distant rooms.

Not all builders of rocket mass heaters use the same design. This book shows our most popular “J-style” firebox design and a heat-exchange channel that incorporates a metal pipe liner. Alternatives are described in Appendix 3, such as bigger batch fireboxes, alternative thermal mass styles, and other successful experiments.

Like other rocket stoves, RMHs use a narrow, well-insulated fire chamber to maintain a clean, hot fire. Their “whooshing” sound during full burn is pleasant (and much quieter than a propane heater). The typical fuel used is any local, dried firewood — from small branches to split cordwood. Unlike cooking rocket stoves, which have a reputation for requiring finely-split fuels, a working rocket mass heater can burn any log that fits in the feed without further processing.

Many high-end masonry heaters require custom ceramic parts, but an RMH can be built with common metal components, firebricks, and locally available insulation and thermal mass materials. Because the mass is horizontal with low, even weight distribution, they are relatively easy to lay out and install. Comfortable surface temperatures allow these horizontal mass heaters to be installed and used in a wide range of situations.

The original rocket mass heaters (see Ianto Evans and Leslie Jackson’s, Rocket MassHeaters, 2006 and 2014) were designed and built using local, earth-based, recycled, and reclaimed resources. Most examples in this book show standard components, such as firebrick and stovepipe. However, reclaimed and site-sourced materials are used by many owner-builders; Appendix 1 gives details on earthen masonry and workable alternative materials.

The purpose of this book is to allow you to build a properly working example on your first attempt — without replicating known pitfalls. For clarity, optional features and specialized designs have largely been omitted, though there is some discussion of alternative designs in Appendix 3. We encourage new builders to build a proven heater design first, and learn its ways before making any design modifications.







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