Editor’s note: When the TNP staff was looking for photos to illustrate Rex Ewing’s article, we contacted Greenstone Heat, a company in Denver that builds and sells masonry heaters. John Chavez, the president and founder of the company, sent us the following explanatory information.
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The vast majority of heat transferred into the home from a masonry heater is transferred as infrared radiant energy. Radiant heat passes through the air and warms the solids in the home directly. The air is then warmed by passing over the solids. On the other hand, traditional American heating systems, including gas forced air, electric baseboard, wood stoves and fireplaces, all heat through convection heat. The electromagnetic wavelength of convection energy causes it to heat the air directly, which in turn will indirectly heat the solids in the room.
TERMINOLOGY: The term “mean radiant temperature” (MRT) is used to describe the radiant temperature of a room where all objects within the space, including people, walls, floor and ceiling, are emitting and receiving sufficient radiation to sustain their heat loss.
BY THE NUMBERS: The Raber-Hutchinson comfort equation theorizes that the temperature of ambient air plus the mean radiant temperature of surrounding surfaces will equal 140 degrees Fahrenheit. In other words, if the MRT is 70 degrees, the air temperature must also be 70 degrees to maintain comfortable conditions. And for every degree drop in MRT the room temperature must increase by one degree to maintain comfort for the people in the room. This explains why you can be comfortable in your home in the summer time wearing a T-shirt with 68-degree air temperatures, while in the wintertime you can be cold in the same room with 72-degree air temperatures while wearing a sweater. The radiant energy of the sun is stronger in the summer, which raises the MRT for all the solids that make up your home.
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Similarly, a masonry heater directly heats the solids (including the people), increasing the MRT in your home, allowing you to be comfortable at lower ambient air temperatures. Since convective heating devices do little to raise the temperature of the solids in the room, the air temperature must be higher to keep its inhabitants comfortable.
IN THE HOT SEAT: As a radiant heating device a masonry heater is very effective in heating areas within a direct line of sight, but much less effective in heating those spaces that are separated by one or more walls.
Many modern homes are designed around a heat loss calculation and a resulting BTU-per-hour input requirement. Radiant heating systems are normally sized at a lower input capacity when compared to convection heating due to the different mode of heat transfer, higher MRT, thermal mass, minimal difference between ceiling and floor temperatures and the wind chill effect of convective heat.
BEFORE YOU BUILD: Greenstone Heat’s experience and industry wide testing show that the BTU-per-hour requirement for a space with a radiant masonry heater will be on average 20 to 30 percent lower than the same space with a convection heating system. However, this applies only to the area of the home within a direct line of sight of the masonry heater. The input capacity for areas of the home not within a direct line of sight of the heater should not be reduced. Furthermore, a masonry heater placed near an exterior wall of the home can lose up to 40 percent of its BTU capacity for the side of the heater facing the wall. Only a masonry heater placed centrally within a space will achieve the maximum MRT increase and lower BTU input requirement. For more information, visit greenstoneheat.com.
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This article was originally published in The NEW PIONEER™ Winter 2016 issue. Subscription is available in print and digital editions here.