Masonry Magazine February 1996 Page. 18
Masonry Heaters
Masonry heaters, once largely unknown to the general public, are gaining in popularity with ecologically-minded homeowners. Awareness of the value of recycling, renewable energy and healthy living is increasing nationwide. For educated homeowners desiring the warmth of a fireplace and energy-efficiency, masonry heaters are an appealing alternative. Masonry heaters are wood-burning appliances.
Construction of Masonry Heaters
Masny heaters are wood burning appliances, similar to masonry fireplaces, but with important differences. Masonry heaters take the heat retaining benfits of masonry fireplaces a step further.
By CHRISTINE A. SUBASIC, P.E.
Consulting Architectural Engineer
similar to masonry fireplaces, but with important differences. Masonry heaters take the heat-retaining benefits of masonry fireplaces a step further. The result is a wood-burning masonry appliance which is both energy-efficient and clean-burning. Fig ure 1.
Masonry heaters can have a variety of appearances. They can include custom features such as benches and wood nooks, or be selected from a manufacturer's catalog. Masonry heaters are available as factory-produced units, as units constructed entirely on-site from unit masonry and as units which are a combination of prefabricated components, assembled on-site using unit masonry to complete the design.
Masonry heaters constructed in a factory are typically UL-listed appliances. Masonry heaters constructed on-site are not UL-listed, but are covered by an ASTM standard. Approved in 1994, ASTM Standard E 1602, Guide for the Construction of Solid Fuel Burning Masonry Heaters [Ref. 1], covers fire-safety and general performance issues of concern with site-built units.
The Standard provides the following definition for a masonry heater.
"A masonry heater is a vented heating system of predominantly masonry construction having a mass of at least 800 kg (1,760 lbs), excluding the chimney and heater base. In particular, a unit designed specifically to capture and store a substantial portion of the heat energy from a solid fuel fire in the mass of the appliance through internal heat exchange flue channels, enable a charge of solid fuel mixed with an adequate amount of air to burn rapidly and more completely at high temperatures in order to reduce emission of unburned hydrocarbons, and be constructed of sufficient mass and surface area such that under normal operating conditions, the external surface temperature of the heater (except in the region immediately surrounding the fuel loading door(s)), does not exceed 110°C (230°F).
As a guide meant for all masonry heater types, ASTM Standard E 1602 does not specify plans or dimensions, other than for safety. However, diagrams grat of typical construction for four general masonry heater types are included. Figs. 2-5.
Different from Fireplaces
One of the primary characteristics which distinguishes masonry heaters from masonry fireplaces is the inclusion of heat exchange flue channels leading from the combustion chamber to the chimney flue. Energy-efficient fireplaces often incorporate channels around the firebox to heat room air, but these are external to the firebox, and only circulate room air around the heated core. Masonry heaters, on the other hand, channel the combustion gases from the firebox in a winding route on their way to the flue. The higher temperature of the combustion gases and direct contact with the masonry results in better heat transfer by the masonry heater.
In addition, masonry heaters are designed as heat-storage systems. They should be centrally located with an inside chimney for proper