Pellet Stove Evolution:
What makes them work?
What servicing is needed?
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A pellet stove is a modern heating appliance that uses sawdust compressed into wood pellets, a very green renewable and inexpensive fuel. True multi-fuel stoves use a burn pot stiring rod that keeps ash, especially corn ash from fusing together. These stoves burn compressed wood pellets, corn, soy beans, cherry pits, olive pits, bio mass fuel grains and processed silage with only a few simple adjustments.
Some early units are a hybrid design, reincarnating some wood stove features. Like a wood stove, these models have a top 6 inch flue pipe and use natural drafting. However, sawdust wood pellets need a strong air flow to keep them burning. Initial designs sometimes have a positive draft blower connected to the intake air pipe to force room air through the pot of wood pellets to burn them completely. Many of these stoves are heavy steel and cast iron, which radiates heat. They also have a convection or room blower to circulate the room air in from the back of the stove, across the top of the fire box to gain heat and out the front. The results are a very warm room.
A good example of this type of stove is the Englander 25-PFS. This stove has 2 - 140 CFM convection blowers mounted high on the back of the fire box to push as much room air as possible through the stove. The user controls are fairly simple. They include a rotary knob with 4 to 5 heat levels that release 1.0 to 5.0 lbs of pellets per hour into the burn pot depending on the setting. The controls also include a simple 2 to 3 speed rocker switch or rotary knob for setting the room blower speeds and an On/Off switch. Adjusting these controls is tricky and care must be taken in order to not overfire the stove. Overfiring occurs when the heat level is set high and the convection fan is not set high enough to keep the internal stove temperature down. This is one reason that there are safety devices to prevent a fire. Other safety measures use snap discs, vacuum switches and overload circuitry to prevent fires when the convection blower or combustion blower fails. These stoves are started by adding a handful of pellets into the burn pot and topped with starting gel, then lit with a match. These early units are not as efficient but still provide good heat without chopping and stacking a lot of wood. Also since wood pellets do not contain bark, they burn much more cleanly.
Modern units now have many more features plus more efficient designs. Most newer designs unlike the early models, have an exhaust blower at the end of the combustion air path in the stove just before the hot air enters the pellet vent pipe. Since these combustion blowers pull in clean air and have hot air containing pellet ash traveling through them, they are more rugged and better sealed to prevent ash from being released and ruining their bearings. To be more efficient, the burn air comes in from the outside of the house using an OAK (Outside Air Kit). In this manner, warm room air is no longer wasted by sending it up the chimney. The design of the convection blower path for warming room air is much more efficient now. A convection blower anywhere from 165 CFM to 265 CFM is employed and mounted low in the back of stove. This creates a long path around the fire box and out the front through a heat exhanger mounted over the fire box consisting of tubes or accordian type plenum that Harman stoves employ. The user controls are much more sophisticated. They incorporate multi-layer circuit boards with LEDs to display the heat and fan settings. These boards also control the fan speeds in conjunction with the heat levels so overfiring cannot occur. The newer stoves have all needed safety devices that prevent overfiring due to part failure. These devices are bi-metal thermo switches called snap discs and either open or close at a designated temperature or a thermocouple connected to the control board. These safety devices stop the auger from feeding pellets which will put out the fire when an unsafe condition occurs. New features include larger hoppers that hold more pellets and larger ash pans to hold more ash. Top or bottom feed auger systems that deliver the pellets from the hopper to the burn pot have been improved to prevent auger jams.
All stoves generally need professional servicing of the entire stove at least once a year. The interior of the stove has 110 volt shock hazzards and very sensitive solid state parts that can short out easily so care and experience is needed. Repairs may also be needed from time to time. Professional servicing usually consists of verifying a safe installation in accordance with the national and local fire and building codes. A thorough cleaning of the stove venting and combustion chambers is performed. The blowers should also be cleaned and lubricated and new gaskets installed when needed. The auger shaft and auger motor should also be checked and lubricated. Any worn parts or broken parts should be identified at this time and recommendations for repair and replacement or upgraded parts is made.