How do we come from a press on the pellet stove thermostat button to cozy 80°F room temperature?
Back-to-nature pellet stoves are all the rage. They are both environmentally-friendly and easy to use.
The reason why pellet stoves are so convenient is hidden in the way how they work.
The inner workings of pellet stoves follow a few basic principles universal for all automated stoves that use pellets for fuel.
We are going to cover how does a pellet stove work in 7 distinct steps. Automated stoves function by just one press on a button; we’re going to look at what is happening inside the pellet stove.
You’ll see that metrics such as BTU, energy-efficiency, and hopper size are directly connected with how well pellet stoves work. Based on these key specifications, we have made a list of the best pellet stoves you can check here.
Inner Parts Of Every Pellet Stove
The inner workings of the pellet stove cover the function of their integral parts. These are:
- Hopper that holds the pellets. We distinguish between top hoppers and bottom hoppers.
- Auger. A motorized screw that transports pellets from the hopper to the burn pot. The thermostat primarily controls the auger’s speed, which in turn delivers a certain quantity of pellets per time unit into the burn pot.
- Burn pot (combustion chamber). A cast-iron ‘combustion’ chamber where the pellets are ignited and burn at a controlled rate. Here all the heat of the pellet stove is created (up to 120,000 BTU).
- Ash tray. Located below the burn pot, here is where all the ash is gathered. It needs to be cleaned periodically; pellets create a minuscule amount of ash, making cleaning ash less frequent (about once a week).
- Convection blower fan and heat exchangers. The fan pulls in the colder room air; it is passed over the hot burn pot where it heats up. The now heated air is circulated over heat exchangers. These are cast-iron or steel tubes that transfer the generated heat to warm up your house.
- Exhaust blower fan. Pushes the gases created by the pellet-burning out of the combustion chamber. The gases are expelled outward via chimney (in the case of insert pellet stoves) or a small hole in the wall (in the case of free-standing pellet stoves).
- Thermostat. Used for setting the temperature of the room/house. It directly controls the speed of auger feed; if you increase the thermostat temperature, the auger will increase the transport of pellets from the hopper to the burn pot, and the stove will generate more heat.
All in all, these are the parts that give you an insight into how the pellet stove works. Let’s look at each of the 7 steps that create a pellet stove cycle:
Step 1: Feeding The Hopper With Pellets (Fuel Container)
The hopper is the simplest to understands. It is a metal box that serves as a container of pellets. In most cases, you buy 40 lbs bags of pellets and store them in your garage (they need a 100% dry environment).
When you want to use them to fuel the pellet stove, you take a bag, open the hopper and empty the whole bag of pellets into the hopper. Hoppers can hold anywhere from 40 to 70 lbs of pellets, depending on the size of the hopper and the density of the pellets.
Essentially, a hopper is like a fuel tank in a car. The more pellets you feed into the hopper, the longer you will be able to burn the stove. When the hopper is low on pellets, you resupply the pellets, and you’re good to go.
There are two types of hoppers:
- Top hopper. Located above the burn pot, a top hopper is more fire-resistant but presents a chamber where gases can be accumulated.
- Bottom hopper. Located below the burn pot, a bottom hopper is more likely to catch fire but less likely to present a chamber where the gases would accumulate.
In general, HVAC specialists prefer top hoppers because they are less likely to catch fire. Homeowners, however, might prefer bottom hoppers because they are easier to reload (closer to the floor, less heavy lifting).
Step 2: Auger Feeds Pellets To The Burn Pot (Fuel Injector)
How easy it is to increase or decrease pellet stove heat production largely comes down to the auger. Auger is a cast-iron screw that pushes pellets available in the hopper to the burn pot.
Every modern pellet stove has a sensor that measures how much pellets are needed to achieve or maintain the required temperature (we set that temperature on the thermostat). When the combustion chamber sensor detects a lack of pellets, it shuts a signal to the auger. Auger turns to deliver just the right amount of pellets of the mechanized tray that delivers the pellets into the combustion chamber.
If you have a top hopper, you will actually hear pellets delivered by the turn of the auger falling on the tray. In the case of a bottom hopper, the auger will lift the pellets to the tray.
The speed of the auger feed determines how much heat the stove will produce.
Example: If you want to maintain a 75°F indoor temperature, the auger will deliver about 1 pound of pellets per hour. When you set the thermostat to 80°F, the auger will start delivering pellets at a quicker rate – let’s say 5 pounds per hour – until the indoor temperature reaches 80°F. After that, it will maintain that temperature by delivering about 1.2 pounds of pellets per hour to the burn pot.
Step 3: Burn Pot Ignites And Burns Pellets (Combustion Chamber)
The integral part of how a pellet stove works is the burn pot. Here is where the pellets – sawdust and waste wood compressed under high pressure – are burned. The process of burning pellets is part of the process that creates all the heat pellet stove produces.
Pellets are delivered into the cast-iron burn pot, located in the combustion chamber, by the mechanical tray. As we’ve learned in the previous step, the auger loads the tray that in turn delivers the pellets to the burn pot.
For every burning process, two things are needed:
- Fuel. In the case of pellets stove, the fuel is pellets (heavily compressed wood).
- Oxygen. Indoor air is composed of about 20% oxygen. As we will see in Step 4, it is the job of a convention blower fan to deliver the air needed to start the combustion.
The principal part is the ignition. All modern pellet stoves have electrical ignition; press a button on the thermostat, and the electrical current will create a spark that will ignite the pellets on the burn pot.
Pellets are compacted pieces of wood. When they burn, they give away an immense amount of energy. Here is how much energy you get from different fuel sources:
- Wood: About 2500 kcal per 2 pounds.
- Pellets: About 4500 kcal per 2 pounds.
- Fuel oil: About 9500 kcal per 2 pounds.
As you can see, pellets give away almost twice the amount of energy per pound as wood.
Step 4: Ash Tray Gathers Ash (Emission Levels)
Almost every part of the pellet will be used as fuel. However, there is still some ash that will be produced as a result of burning pellets.
To gather all the ash, every pellet stove has an ash tray below the burn pot. One of the advantages of how pellet stoves work is that the amount of ash that is produced is minimal.
For example, some pellet stoves can burn 24/7 for a whole week without the need to draw out the ash tray and to remove ash. In most cases, the ash tray has to be cleared of ash once or twice per week during the heating season.
Step 5: Convection Blower Fan Moves Indoor Air Over Heater Exchanger To Heat It Up
Up till now, we have been burning pellets to produce energy. Now it’s time to transfer that energy and use it to warm up our house. This is where the convection blower fan comes in.
Convection blower sucks in the indoor air (at about room temperature – anywhere between 60°F and 72°F). This air is passed over the burn pot where it heats up to above 120°F.
Now, this is the heater air we’re looking for, but it’s not clean enough. This heated air, therefore, moves across a series of iron bars – also known as a heat exchanger – that are designed to heat clean air and transfer it indoors via a blower.
In effect, here is how the effect of the pellet stove is most evident:
- BEFORE: Room temperature air between 60°F and 72°F is pulled into the pellet stove.
- AFTER: Air that comes out of the pellet stove is heated significantly; typically, we get between 90°F and 110°F steady airflow from the stove.
Step 6: Exhaust Blower Fan Expels Unclean Air Outdoors
As air that was sucked into the pellet stove and passed over burn pot contains gases and other emissions. These are a result of burning pellets.
The heat of this now unclean air was already transferred to heating our house via heat exchangers. In short, the job of this air is done. Now we need to expel it outward.
Most of the pellet stoves work by having installed piping in the back of the stove. The exhaust blower fans the dirty air toward that piping.
If you have an insert pellet stove, the piping continues toward the chimney. All the gas produced by burning pellets is expelled through the chimney.
If, on the other hand, you have a free-standing pellet stove, the piping will continue specially designed exhaust pipe. In most cases, they are installed horizontally through the wall.
In short, the chimney is not a 100% necessity when it comes to how the free-standing pellet stoves expel the gases created by pellet-burning. Nonetheless, having a vertically installed chimney is the best way to ensure the highest pellet heat yield for maximum efficiency.
Step 7: Thermostat (Remote Controller For Your Pellet Stove)
In practice, we don’t really need to know how a pellet stove works to operate it. All these processes can be modulated via a simple thermostat.
Some advanced thermostats may have additional functions, such as directly controlling the auger speed or directly controlling the airflow throughout the pellet stove.
Most thermostats, however, include only one truly important function. And that’s the indoor temperature.
Just use thermostats to set the temperature, and you’re good to go.
Of course, don’t forget to:
- Load the hopper (Step 1) when it’s empty.
- Empty the ash tray (Step 4) when it’s full.
If you have any additional questions about how a pellet stove works, you can address them in the comments below.