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. This results in a 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 pellet stoves have internal moving parts that can fail and may need replacement. 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. Always unplug the unit before replacing parts.
Below is a list of some parts and when they may need replacing:
Door and window gaskets get old, frayed then get brittle and break. If the flame gets lazy the door and window gaskets become leaky which causes poor heating performance.
Hopper gaskets get old, frayed then get brittle and break and let air in the hopper that can feed a hopper fire or back burn.
Ash pan gaskets get old, frayed then get brittle and break and let air in that can cause a lazy flame and poor heating performance
Ignitors wear out as it may take longer than 4-8 mins to light the pellets or on some startups, they may not light.
Auger motors get sluggish and also may skip which means the gears in the gear box are stripped.
Convection and Combustion blowers may become noisy, cease and not turn at all, or if the windings are bad the fan blades may need a push to get going.
Control boards or panels can get suffer damage from AC line surges and get flakey or intermittent. This can cause pellet overfeeding, the triacs can fail so the devices they control (Blowers, auger motor and ignitor) will stay on or not work at all. The power button may not be able to turn the stove on anymore.
Snap discs are temperature sensing bi-metal strips that open or close at the designed temperature so when they fail thet either stay open or remain closed.
Vacuum switches close when they sense firebox vacuum pressure or exhaust blower pressure and will not change state when pressure is applied.
Firebox door and hopper safety switches fail and stop the auger from turning which makes the fire go out.