How a Pellet Stove Works
A pellet stove is a great cheap heating alternative when the cold weather comes around. Understanding its parts and how they work together can make a world of difference when it comes to using yours efficiently and repairing it when necessary.
Normally, four different systems will start running as soon as the appliance is powered on.
The conical-shaped hopper filled with pellets has an opening at the bottom where the pellets drop in an auger’s helical flighting screw blade (Figure 1).
The auger is the screw conveyor that moves the pellets towards the burn pot where they’re dropped into the burn grate to combust. When the stove is started or running, the auger goes for 3 seconds and stops, its movements indicated by a pilot light. The stopped time is what is increased or decreased when adjusting the “feed rate”—increasing the delay before restarting again reduces the amount of pellets reaching the burn grate in a given time period.
At startup, the auger runs at its lowest speed, no matter what setting it’s adjusted at until the exhaust temperature reaches 120°F (50°C), where it enters a “safety delay” of 5 minutes after which it goes to its programmed setting.
2. An igniter is also part of any automatic igniting pellet stoves (Figures 2 & 3). It’s actually an electric heater of around 300 watts that heats up (indicated by another pilot light) and runs for up to 15 minutes after the stove is turned on unless the stove is turned off by the safety delay’s elapsed timer. It should ignite the pellets within 3-5 minutes.
3. Combustion Fan
The combustion fan, meanwhile, pulls air into the burn pot and up through the burn grate to ignite the pellets and maintain optimum combustion. The fan also pulls the heat up across the heat exchanger pipes (Figure 4) and then pushes the smoke out the flue venting (Figure 5).
When first activated, it will run for about 1 minute at a higher speed, before automatically dropping back to its normal speed. Although you can fine-tune by adjusting a “fan trim” when at the lowest feed rate setting, the combustion air is automatically adjusted and synchronized with the auger speed—you increase the feed rate, the combustion air goes up, and visa versa.
4. Heat Exchanger
The heat exchanger, made up of multiple metal tubes running parallel to each other above the flame (shown in Figure 4), gets heated up as the hot air from the flame is pulled up around the pipes and towards the combustion/exhaust fan.
Air Flow Parts
1. Burn Grate
The burn grate snuggly set into the burn pot is where the pellets are set ablaze (Figure 6), either manually or electrically ignited.
2. Air Intake Pipe
An air intake pipe with an adjustable damper gets air from outside of the house to provide the stove with an adequate and constant airflow necessary for proper combustion.
If there are no or few embers glowing off on both sides of the burn grate, the flame needs more air; consequently, if there is a lot of embers, glowing for 15 to 30 seconds, the air intake should be reduced by closing in the damper.
Excessive air supplied from the air inlet can burn the fuel (pellets) too quickly resulting in:
smoking or smoldering pellets,
a white/yellow flame,
burning pellets flying up into the air (popcorning),
a possibility of the stove shutting off for lack of fuel (burnt too quickly).
3. The Convection Fan
The convection fan takes the air from inside the room and blows it through every one of the heat exchanger’s heated tubes (Figure 4) and back into the room. It runs at whatever speed it is adjusted at unless taken over by the fan limit switch which puts it on high speed to cool down excessive heat level above 120°F (50°C).
There are several types of sensors within the circuitry of your appliance that will prevent fire and smoke hazards from occurring and making your appliance reliable and safe to operate.
1. The Vacuum Switch
The vacuum switch controls the auger. This safety device is designed to act upon the existing vacuum pressure inside your pellet stove from the air intake, through the firebox right up to the exhaust system. The contactor within the device is a normally opened switch that will keep the power off on the auger, whenever there is no or limited airflow in the exhaust system, the firebox, or the air intake. The system’s vacuum can be disrupted by such events as an exhaust blower failure, a plugged vent pipe, the main door opened, or the door or the ash pan not sealing properly.
2. The High Limit Switch
controls the electricity going to the vacuum switch and consequently to the auger. This switch is mounted outside the wall of the firebox and is preset to open when the temperature behind the firebox reaches approximately 250°F (120°C), thus cutting off the electricity feeding the auger motor.
3. The Convection Fan Limit Switch
The convection fan limit switch controls the speed of the convection fan. Whenever the heat inside the stove reaches 140°F (60°), it will switch the speed of the fan on high to let the stove cool back down. Such a situation can occur when the heat control is set on high and the fan speed is set to a very low or turned off. The switch will reopen and return the fan to its normal set speed as soon as the temperature goes back below 140°F (60°). If the fan keeps going from its set speed to high repeatedly, you may want to adjust the fan speed to a higher setting.
4. The Low Limit Switch
The low limit switch is mounted on the exhaust blower (combustion fan) and shuts the stove off when the exhaust temperature drops below 120°F (50°C).
The stove, however, has an initial 15 minutes “Lighting Mode” when the stove is first started, to light up the pellets and reach the 120°F (50°C) before that switch turns it off. As soon as that temperature is reached, the limit switch opens to enter a five minutes “Safety Delay” where all fans keep running and the auger stops for a proper shutdown preventing excessive smoke, heat, and fuel buildup inside the stopped unit.