November 07, 2007 11:44 AM CST

Heating Green

By Norbert Senf

Masonry heaters are firing up green building practices. A masonry heater is a unique type of masonry fireplace that is able to burn wood cleanly. By rapidly burning a large batch of wood (up to 60 pounds), the heater stores the heat in a masonry thermal mass. The masonry then radiates heat gently into the building for up to 24 hours or more from a single two-hour firing.

Several aspects of masonry heaters tie in directly to green building, including greenhouse gas reduction, low output capacity and radiant heat:

Wood is a renewable fuel, particularly if harvested as part of a sustainable forestry operation. The downside of burning wood is air pollution. Wood is potentially a clean fuel. Pollution from wood burning is almost entirely the result of incomplete (smoldering) combustion. It can be greatly reduced by improved burn methods, and masonry heaters achieve this by burning the wood rapidly in a hot fire and then using the masonry mass to store the heat. Chimney flues have been known to stay clean for 20 years and longer with proper operator practice.

In a large, 8,000-pound heater, the heat from a two-hour burn of 60 pounds of wood can be radiated into the building over a span of 24 hours, at roughly 10,000 BTUs per hour. In many locations, this is a typical heat load for a superinsulated building on an average day. Other cordwood burning appliances have difficulty targeting such low heat loads without resorting to smoldering (dirty) combustion.

A typical large heater will have about 100 square feet of radiating masonry surface at about 150 degrees farenheit. This is widely regarded as a comfortable and healthy type of heating, similar to what is obtainable with a hydronic floor. There is minimal air movement, and a reduction in dust circulation.

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About the Author

Norbert Senf is a past president of the Masonry Heater Association of North America.