Turn Waste Wood into Home Heat: Buy a Pellet Stove
(Page 4 of 6)
February/March 2009
By Steve Maxwell
Theoretically, a vent for a pellet appliance could just be poked through the wall to dump the exhaust outdoors because the exhaust is fan-forced, but in practice this so-called “direct venting” is not a good idea. To explain why, we need to explore what happens when electrical power is removed from an operating pellet stove. Because pellet appliances depend on electricity to run the fans, auger and control system, power outages can cause difficulties beyond the obvious result of not operating.
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If you pull the plug on standard models that are direct vented, the flames slowly die when the fan and auger stop. Smoke builds up in the appliance and can seep slowly into the room through small leaks around the door or hopper. Although the smell of smoke can be alarming to users after the first power failure they experience, the smoking stops when the small amount of fuel in the burn pot is consumed. There are two common ways to prevent this smoking problem.
One option is to install a battery backup system to provide electricity to operate the exhaust fan until the burn pot is emptied, usually 15 to 30 minutes. But, most pellet stoves and inserts draw up to 400 watts of power during start up and about 100 watts in normal operation, so a small battery backup system won’t provide enough electricity to keep them heating for long.
A better solution is to add some vertical rise to the vent. A 5- to 10-foot rise of the vent up the outside wall of the house can provide enough natural draft, even if the fan is not running, to vent residual smoke until the fire goes out. A vertical section of vent on the outside wall resolves the problem of smoke leakage if the power fails, but it doesn’t solve all the potential problems.
Pellet stoves connected to vents that terminate part way up a wall can be vulnerable to shut down if a strong wind blows against that wall. The pressure sensor mentioned earlier may activate to turn the appliance off. A better approach is to run the pellet vent right through the roof overhang so it is not susceptible to wind effects around the house. If the vent is run above roof level, there is almost no chance a high wind will shut the stove down. Note that professional installers increase vent diameter from 3 inches to 4 inches for systems that include an elbow and more than a few feet of vent. Experience has shown that 3-inch vents fill up too quickly with fly ash and the resulting flow restriction may interfere with operation. Leonard is so certain that pellet vents should include a vertical run that his company now refuses to install so-called direct vents.
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