Choose a Fireplace for Beauty and Warmth
(Page 2 of 6)
October/November 2003
By John Gulland
Some steel heat forms have air circulation chambers, usually with inlets under the firebox at floor level and outlets between the mantel and the ceiling. When equipped with tight-fitting glass doors and a blower to force air from the room through the heat exchanger, these fireplaces can match the heating performance of an antique potbellied stove for short periods of time. But most heat forms are not designed for, and can't stand up to, serious heating chores without failing. Commonly, the fireboxes will warp or crack from the fire's heat.
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Radiant fireplaces. The fireboxes of open fireplaces can be designed to increase the amount of radiant heat delivered to the room. Fireplace designs by Benjamin Thompson (aka Count Rumford) and later, Peter O. Rosin dominate this category. The Rumford design features a tall, shallow firebox with a narrow back and splayed sides to bring the fire forward and reflect more heat into the room. Rosin's firebox, precast with a bulge in the back, reflects the heat down, towards the fire, and then out into the room. The success of both the Rosin and Rumford designs in directing more radiant heat into the room is apparent to anyone standing in front of one of these fireplaces. But neither design overcomes the problem of heat loss from the flow of air up the chimney. Nevertheless, they're great options for a rustic weekend cabin.
"Zero-clearance" factory-built fireplaces. This style has a multi-layer sheet metal structure and usually, a firebox lined with custom firebricks. Around the firebox is a sealed space through which room air can circulate; an outer steel casing is insulated to moderate its temperature.
The name "zero clearance" refers to this style's standard positioning against the combustible wood framing into which these units are built. Tens of thousands have been installed in suburban houses during the past 30 years. They mimic the look of conventional brick fireplaces but cost much less. The heating performance of conventional `zeros' is generally poor.
FUNDAMENTAL PROBLEMS
All four styles of conventional fireplaces suffer from three inherent problems. Any one of the problems reduces their appeal, but the combined effect make them unsuitable for use in new houses for heating or aesthetics.
1. High air demand reduces efficiency. This is true even in fireplaces with special features that increase radiant heat to the room or that have air circulation chambers in which room air can be heat ed. True, a Rumford or Rosin puts out a lot of radiant heat, and true, the air pouring from the outlet grills of a heat form or a factory-built fireplace is very hot. But a sizable fraction of that heat loops through the room once and then flows right back into the firebox and up the chimney. The uncontrolled flow of excess air through an open fireplace is an efficiency robber. (See "Forego the Open Fireplace," Page 92.)
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