Wood pellet stoves have become an increasingly popular heating option in many homes due to their efficiency, low emissions, and convenience compared to traditional wood-burning stoves. But how exactly do wood pellet stoves work?
In this comprehensive guide, we’ll cover everything you need to know about the internal components and operation of pellet stoves. We’ll discuss:
- What are wood pellets and why are they used for fuel?
- The basic internal components of a pellet stove.
- How pellet stoves work to ignite and burn pellets.
- Heat and emission transfer.
- Electrical components and controls.
- Efficiency and emissions.
- Maintenance and cleaning requirements.
- Pros and cons of pellet stoves.
What Are Wood Pellets?
Wood pellets are a compressed form of wood product that have been extruded into a uniform cylindrical shape and dried. The pellets are typically made from compacted sawdust and wood chips from lumber mills and other wood product manufacturing facilities. This recycled waste wood is an abundant and renewable resource in many regions.
Some key properties of wood pellets:
- Cylindrical shape and small size allows feeding through auger systems.
- Low moisture content (below 10%) provides high combustion efficiency.
- High density results in high heat output per volume.
- Made from renewable and sustainable wood waste products.
- Produced to conform to quality standards for reliable performance.
- Can be conveniently bagged and transported.
Using wood pellets as a fuel for heating appliances takes advantage of this renewable, locally-produced biomass resource. Pellet stoves are designed specifically to utilize wood pellets efficiently and conveniently, without the complications of burning whole logs or cordwood.
Inside a Pellet Stove: Basic Components
Pellet stoves consist of several main components that work together to automatically feed fuel, ignite it, and transfer heat into the room:
This large container holds bulk quantities of wood pellets, ranging from 35 to 130 pounds depending on the stove size. Located on top of or beside the stove, it can be filled conveniently by pouring bags of pellets directly into the hopper.
At the bottom of the hopper, an auger (metal shaft with spiral flighting) rotates to transport pellets from the hopper into the burn pot at a controlled rate. The auger length and rotational speed determines the feed rate. Augers are powered by high torque electric motors controlled by the stove’s logic.
This key combustion chamber provides a place for the pellets to collect, ignite, and burn efficiently. It may have holes along the bottom to provide the primary combustion air. Baffles help distribute air and retain the fuel.
Electric heating elements and/or hot air blowers are used to heat the pellets in the burn pot up to their ignition temperature for lighting off. Once ignited, combustion is self-sustaining.
A fan provides the required oxygen for complete combustion by blowing air into the burn pot area. Variable speed control maintains desired heat output. Secondary combustion air may enter at higher elevation to burn off emissions.
Heat Exchanger Tubes
Metal tubes pass through the center of the burn pot and combustion chamber to extract heat and transmit it into ambient room air. Tube arrangements vary by stove design. High efficiency stoves may re-burn emissions here.
Room air is circulated across the hot heat exchanger tubes to warm it up. The warmed air is then blown into the room for heating. Variable speed controls on the blower regulate room air temperature.
High temperature exhaust gases are vented safely to the outdoor atmosphere through the chimney. A pressurized vent draws combustion air from outside the home.
In addition to the motors and fans, there will also be control boards, switches, sensors, and wiring to operate the auger, igniter, and fans automatically.
How Pellet Stoves Operate
Now that we’ve reviewed the main internal components, let’s go through the operating sequence to see how pellet stoves work:
- Pellets Flow – The auger motor turns at a slow speed to gradually deliver wood pellets from the hopper into the top of the burn pot. A small capacity ensures control.
- Ignition – Electric heating elements warm up the burn pot to high temperatures needed for the pellets to ignite – around 500 to 600°F. A heat exchanger blower may recirculate hot air to aid ignition.
- Combustion – Once ignited, incoming air from the combustion fan keeps the pellets burning and hot. As new pellets are added by the auger, they burn and produce flames.
- Heat Transfer – The high temperature gases, smoke, and hot air rise past the heat exchanger tubes, transferring heat into them. The tubes get hot enough to also ignite any smoke or emissions.
- Distribution – Room air is blown across the heat exchangers by the convection blower. This heats up the room air, which then circulates through the room, transferring heat into the space for warming.
- Emissions – Gases, fumes and ash then pass into the venting/exhaust system and exit outside through the chimney. The negative pressure pulls air through the combustion system.
- Control – The entire combustion and heating sequence is precisely controlled throughout by the electronic control board and adjustable settings for feed rate, combustion airflow, distribution airflow, and target temperature.
This automatic operation sequence continues 24/7 during the heating season. The auger feed rate and combustion airflow is modulated based on the heat output required. Most stoves allow adjustment of the desired room temperature via a thermostat.
Heat Output and Efficiency
Properly operated, high quality pellet stoves can convert over 85% of the potential energy contained in the wood pellets into usable heat for warming a home. This makes them far more efficient than open fireplaces (less than 10% efficient).
Pellet stoves produce up to 50,000 BTU (British Thermal Units) of heating power, and models are available in small, medium, and large sizes. With higher heat output models, just one pellet stove can heat an entire house in all but the coldest climates. Top rated stoves will put out over 90% of their rated heat capacity.
Burning wood pellets releases their chemical energy as heat based on their composition and energy density per pound. Different grades may have slightly more or less heat potential. Premium pellets burn hotter and more consistently than lower grade or high ash content pellets.
For maximum efficiency, the combustion airflow must be properly set to provide the optimal amount of oxygen. Too much air will overcool the fire, while too little air results in incomplete combustion and higher emissions. Regular maintenance like cleaning the heat exchangers is also needed (see maintenance section below).
Emissions and Environmental Impact
When operating optimally, pellet stoves produce very little air pollution or greenhouse gases. Particulate matter, carbon monoxide, organic gases, and smoke should all be minimal with a properly adjusted, high quality pellet stove.
Compared to wood logs, wood pellets burn extremely cleanly and with less than a tenth of the emissions. This is due to the consistent size, shape, moisture content and other properties of quality pellet fuels. Pellet stoves also re-burn much of the smoke and emissions to further reduce environmental impact.
High end stoves certified by the EPA have very strict emission limits for particulate matter, carbon monoxide, and other pollutants. Look for models certified to comply with EPA NSPS (New Source Performance Standards) Phase 2 requirements. Compared to other fuels like heating oil or propane, pellet heat also emits less greenhouse gases per BTU of heat produced.
Using sustainably sourced wood pellets helps support responsible forest management practices. Overall, wood pellets are considered carbon-neutral because replanted forests re-capture the CO2 released during combustion. Pellet stoves thus have environmental advantages over fossil fuel furnaces.
Routine Cleaning and Maintenance
To keep pellet stoves operating safely and efficiently, regular maintenance is required. The major cleaning tasks include:
- Empty ash from burn pot – This needs cleaning every few days to remove ash build up which can block airflow. Scrape out debris with a tool.
- Vacuum heat exchanger and internal flue ways – Soot and fly ash will collect on internal surfaces and reduce heat transfer over time. Clean monthly.
- Gaskets and door seals – Visually inspect seals and replace worn gaskets on doors annually to maintain proper ventilation conditions.
- Igniter and fans – Clean fan blades of built up dust. Igniters may need replacing after 2-3 seasons. Make sure air intakes are clear.
- Venting system – Have the entire venting system cleaned annually by a professional chimneysweep to prevent chimney fires from creosote buildup.
- Hopper – Periodically check for sawdust buildup in hopper and clear clogged auger exits.
Following the manufacturer’s maintenance schedule and manual is highly recommended for peak performance and safety. Proper cleaning and upkeep will help ensure low emissions and high efficiency operation for many years.
Pros and Cons of Pellet Stoves
Below is a quick rundown of some key advantages pellet stoves offer over other heating systems, along with a few potential downsides to consider:
- High efficiency – can heat spaces using less fuel
- Low emissions – burn extremely clean with minimal particulates
- Convenient pellet fuel – easily handled and stored bags
- Automatic operation – consistent heating with minimal intervention
- Comfortable warmth – evenly distributes gentle heat
- Renewable biomass fuel – supports sustainable forestry
- Lower greenhouse gases – less net CO2 emissions than fossil fuels
- Purchase and installation cost – higher upfront investment
- Ongoing pellet purchases – fuel not free like cordwood
- Electricity required – auger and fans need power
- Frequent cleaning – burn pot and heat exchangers need regular maintenance
- Potential jams – auger may jam occasionally
When evaluating heating systems, be sure to consider the benefits and downsides applicable to your situation. Pellet stoves can be an excellent option for many homeowners looking for an efficient, low emission, locally-sourced, renewable heating solution.
Frequently Asked Questions
Q: Are wood pellet stoves safe?
A: Yes, pellet stoves are safe heating appliances when properly installed and operated according to the manufacturer’s instructions. They have been extensively tested and certified. Key safety features include overload protection, automatic shutdown if overheating, and close control of the combustion process.
Q: How much do pellet stoves cost?
A: The purchase cost of pellet stoves ranges from around $1,500 up to $5,000 depending on size and features. Installation costs are typically $200 – $2,000 on top of the stove price. High end appliances with automation can cost over $8,000 installed.
Q: What maintenance is required?
A: Pellet stoves need frequent cleaning of the burn pot to remove ash buildup (weekly) as well as periodic cleaning of the internal heat exchangers and venting to maintain efficiency. Gaskets should be replaced as needed. Yearly professional vent cleaning is recommended.
Q: How do pellet stoves connect to a home?
A: Pellet stoves require a dedicated 110V electrical outlet to run the motors, fans, and controls. They also need to be connected to an approved vent/chimney system to exhaust emissions and bring in combustion air. A thermostat may provide automation.
Q: What size pellet stove do I need?
A: Size your pellet stove based on the heated space you want to warm. Around 50,000 BTUs is required for every 1,000 square feet if heating the entire area. Allow extra capacity for very cold climates. Choose an EPA certified model appropriately sized for the space.
We hope this comprehensive guide has helped explain what’s inside pellet stoves, how they operate to produce heat, their benefits, and important considerations for operation and maintenance. Pellet stoves can be an excellent heating solution if sized and installed properly for your space. Please contact us with any other pellet stove questions. Stay warm!