PELLET STOVES WOOD ENERGY FOR ALL
(Page 2 of 9)
October/November 1995
By John Vivian
The wood fiber that goes into pellets is dried in processing to a water content of as little as 5 percent, and being about the size and shape of rabbit pellets (incoming or outgoing), each exposes a large surface area (compared to mass) to passing oxygen. They beg to burn, but in the fire basket, pellets settle into a compact bed that offers no edges or splinters to ignite, so—like coal—will not burn without a strong draft.
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A network of tiny, convoluted air channels snakes up between the pellets. When the pellet bed is ignited (with a flammable paste, much as you use starter fluid to ignite charcoal in the backyard barbecue), a small fan begins to push a constant supply of air through the pellets.
During the burn, the surface of the fire bed is constantly replenished with fresh pellets by an electric auger, so the fuel burns hot, evenly and thoroughly from the bottom up. Additional oxygen is introduced at the proper level above the fire and at the correct velocity to burn the smoke thoroughly before it leaves the fire box. Pellets burn so clean, emitting less than the EPA's no-test cut-off level of 800kg of particulates/hr, they are effectively exempted from the smoke-emission testing program forced on log-burning home heaters. Though the forced-draft process is the result of precision engineering, it follows the same principle as when you blow on coals to brighten up a campfire or a blacksmith uses in applying a bellows to his forge.
Hot as the fire is, the venting system keeps the exterior of the stove cool enough that it can be located nearer combustible walls and furniture than the vast majority of conventional stoves. With added heat-shielding, most models are Insurance and Fire Code-approved for use even in the close and flammable confines of a mobile home.
Venting the Stove
Hot exhaust gases circulate through a network of thin-walled heat exchanger channels in the body of the stove that permit transfer of heat energy to a counter flow of room air forced through a separate set of channels by another fan. Heated room air exits to circulate through the room or be directed via optional hot-air conduits through the whole house.
Once drained of their heat energy, the fan-forced exhaust gasses exit the stove through a rear- or top-mounted vent pipe. Unlike a conventional chimney, the flue need not run straight up and above the roof peak to take advantage of gravity (hot air rises, but this exhaust is so cool it needs fan energy to remove it). The exhaust system cannot be hidden in enclosed walls or crawl spaces, though, and should be designed for easy access, inspection and occasional disassembly inspection and—if needed—for cleaning.
A precision thermostat on the stove, or optionally, a low-voltage wall-mount unit, varies heat output by increasing or decreasing fuel flow. In contrast to a conventional stove that manages heat output by altering air supply with draft and exhaust dampers, a pellet stove maintains a constant, full air flow so long as the fire is hot. Heat output is increased by augering in more fuel. Fully oxygenated combustion of a super-dry fuel guarantees a highly efficient burn, easily exceeding the 80-something percent efficiencies that some catalytic stoves boast of.
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