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How Does a Forced Air Wood Furnace Work?

A forced air wood furnace is a heating system that uses wood as its fuel source to provide warmth for a home or building. Wood furnaces work by burning wood to heat air which is then distributed throughout the building via ductwork. Understanding how these furnaces operate can help homeowners decide if this renewable energy system is right for their needs.

How Wood Furnaces Differ from Traditional Furnaces

Traditional gas or oil furnaces burn fossil fuels to heat air or water for distribution. In contrast, a wood furnace burns renewable wood as its energy source. Some key differences include:

  • Fuel type – Wood furnaces use wood, typically cordwood logs or wood pellets, while traditional furnaces use natural gas, heating oil, or propane.
  • Heat distribution – Most wood furnaces are paired with ductwork to distribute warm air around the home, while some traditional furnaces distribute heated water.
  • Efficiency – Wood furnaces can reach up to 90% efficiency by capturing heat that would otherwise be lost up the chimney. Their efficiency often exceeds that of some traditional furnaces.
  • Cost – Heating with wood, when harvested sustainably, is significantly less expensive than burning fossil fuels. However, the furnace itself requires a higher upfront investment.
  • Environmental impact – Wood is a renewable fuel source when trees are replanted at the same rate they are harvested. Fossil fuels are non-renewable resources that emit greenhouse gases.

How Forced Air Furnaces Work

All forced air furnaces, including wood-burning models, operate on the same basic principles:

Combustion

Like any furnace, the firebox contains the fuel source which is ignited by an electric spark, pilot light, or match. As wood burns inside the firebox, it releases heat energy.

Heat Transfer

The hot gases from the fire transfer their heat to the metal walls of the firebox, heating them up. Air from the home is then blown across the hot firebox walls by the blower fan. As this air passes over the heated metal, thermal energy transfers from the metal to the air, warming it up.

Heat Distribution

The blower fan pushes the now heated air into ductwork that distributes it throughout the home. Vents release the warm air into different rooms. The cooler air from the rooms then cycles back to the furnace to be reheated.

Control System

A thermostat monitors the interior temperature and controls the furnace’s operation. When the thermostat calls for heat, it sends a signal activating the blower fan and telling the furnace to continue burning fuel. Once the desired temperature is reached, the thermostat halts operation.

Key Components and Operation

While wood and traditional furnaces share some basic components like blowers and ductwork, wood furnaces have additional parts unique to burning solid wood fuel:

Firebox

This insulated steel box holds the fire and includes components like the fuel load door, combustion air inlets, and heat exchanger. It should be properly sized to the heating needs of the home. Too large wastes fuel, while too small cannot provide enough heat.

Burn Chamber

Also called the combustion chamber, this is where the wood fuel burns, releasing gases and heat. Primary combustion air enters through inlets on the bottom and facilitates an efficient fire.

Heat Exchanger

While the fire burns in the chamber, this network of tubes or pipes sits inside or around it. Air from the home circulates through the exchanger, preventing dangerous combustion gases from entering the home while absorbing heat from the firebox.

Baffles

These metal plates redirect the path of combustion gases before they exit the heat exchanger tubes. This causes the gases to take a longer path, ensuring they transfer more of their heat energy to the exchanger rather than letting it escape up the chimney.

Blower Fan

This fan activates when the furnace receives a signal from the thermostat. It pulls air from the home and forces it through the heat exchanger, warming it, before pushing the heated air through ducts.

Log Retainer

This optional component sits inside the firebox and helps properly position logs so adequate oxygen reaches all sides of the wood for improved combustion efficiency.

Chimney Pipe

This ductwork vents combustion gases and smoke safely out of the home through the roof. It should be properly sized and installed at the correct height to promote draft.

Ductwork

Usually contained inside walls, floors, and ceilings, these channels distribute the heated air from the furnace throughout different areas of the home. The duct system should be well-insulated.

How to Operate a Wood Furnace

Operating a wood furnace requires some work, but getting the hang of it will allow homeowners to keep their homes comfortably heated:

Fuel Selection

Choose seasoned firewood with a moisture content below 20%. Softwoods like pine kindle hot, fast fires, while hardwoods like oak burn longer and more steadily. Using wood pellets requires less work and produces less ash.

Starting a Fire

Open the air inlets and stack several pieces of kindling topped with a few small logs in a crisscross pattern with air gaps between. Light the kindling and keep the door slightly cracked until the fire is burning strongly before sealing.

Maintaining a Fire

Check the furnace every few hours to remove ash and add more wood. Add larger logs once the fire is established. Do not let the fire smolder without flame, as this wastes wood and causes creosote buildup in the chimney.

Controlling Heat Output

The thermostat automatically controls the burn rate and blower fan. Burn speeds can also be adjusted manually by closing down air inlets or opening the bypass damper to reduce heat.

Shutting Down and Restarting

Let the furnace burn through most of the wood during shutdown to minimize smoke when restarting. Leave a bed of hot coals to help reignite a new fire. Partially close air inlets and the chimney damper to preserve coals.

Ash Removal

Remove ash regularly from the firebox and ash pan to prevent buildup. Only remove ash when the furnace is cold. Place the ashes in a metal bucket with a lid and store outside away from combustibles.

Selecting a Forced Air Wood Furnace

Heating needs, design, efficiency, and options can vary significantly between models. Consider the following when selecting a wood furnace:

  • Efficiency rating – Look for a model with high heating efficiency – ideally 80-90% or more.
  • Output capacity – Match the furnace’s BTU output to the square footage you need to heat. An undersized furnace will struggle while an oversized unit wastes fuel.
  • Construction – Opt for a sturdy steel firebox with few welds or seams that can leak air and heat. Refractory lining inside the firebox improves heat retention.
  • Insulation – A well-insulated firebox increases efficiency. High-density fiberglass or ceramic wool are good insulators able to withstand high temperatures.
  • Blower size – A more powerful blower circulates air faster. Look for a blower sized appropriately for the furnace’s BTU output.
  • Heat exchanger design – Longer tubes and more baffles retain heat better while stainless steel resists corrosion.
  • Log retainer – This positions fuel properly for maximum combustion efficiency.
  • Warranty – A longer warranty provides more protection. Look for at least a 5-year manufacturer’s warranty on parts.
  • Safety certification – Verify the furnace meets certification standards like UL, CSA, or EPA.
  • Maintenance needs – Basic maintenance like ash removal is required. Some models are more accessible for cleaning than others.
  • Price – Initial cost is higher than traditional furnaces but lower operating costs help offset this over time. Consider total lifecycle costs when evaluating price.

Installing a Forced Air Wood Furnace

Proper installation is critical for safe and efficient operation. Consider the following guidelines:

  • Hire an experienced professional installer certified for wood furnaces. Do-it-yourself installation can void warranties.
  • The furnace should have its own dedicated chimney that meets local codes. Use high temperature chimney liner.
  • Locate the furnace interior to the home in a central area for best heat distribution.
    • The furnace needs adequate clearance from combustible surfaces. Follow the manufacturer’s instructions.
    • Sealing ductwork joints and insulating all ducts improves efficiency. Avoid excess bends and length.
    • Return ducts should draw air from a central, open area of the home.
    • Install a barometric draft regulator to control draft up the chimney.
    • An outside air intake prevents pressure issues and improves efficiency.
    • A qualified inspector should evaluate the installation before use.

    Maintaining a Forced Air Wood Furnace

    Routine maintenance ensures safe, reliable, and efficient operation:

    • Inspect door seals and replace them if worn or damaged. Keep seals clean.
    • Empty the ash pan regularly and remove accumulated ash in the firebox.
    • Clean creosote buildup from the heat exchanger tubes annually using brushes.
    • Inspect for corrosion and repair any damaged parts like baffles.
    • Service the chimney and outdoor air intake annually.
    • Have a professional service the blower motor as needed.
    • Only burn well-seasoned cordwood with low moisture content.
    • Run the furnace hot periodically to prevent creosote accumulation.

    Pros and Cons of Forced Air Wood Furnaces

    Wood furnaces have both advantages and disadvantages depending on a home’s needs:

    Pros:

    • Renewable fuel source that can lower heating costs
    • Heat output can be sustained for hours with minimal tending
    • Highly efficient units available
    • Even heat distribution through ducts

    Cons:

    • High upfront installation cost
    • Requires storage space for wood fuel
    • More maintenance than gas or oil furnaces
    • Operation generates more ashes to remove
    • Potential for smoke, creosote, and environmental impact if improperly used

    Frequently Asked Questions

    Q: What size wood furnace do I need?

    A: Size it based on the square footage you need to heat. An inaccurate sizing will decrease efficiency. Have a heating professional help with sizing.

    Q: How much wood does a furnace use?

    A: This depends on the size, but figure 1.5-3 cords per year for a typically sized home furnace. Monitor your actual use the first year to know exactly.

    Q: Where should the thermostat be located?

    A: Install it on an interior wall away from drafts, doors, windows, and direct sunshine which can influence its accuracy.

    Q: How often does the chimney need cleaning?

    A: Inspect it monthly and clean it as needed, especially if creosote accumulation exceeds 1/4″. Annual professional cleaning is recommended.

    Q: What maintenance does the furnace require?

    A: Ash removal, replacing door gaskets, cleaning heat exchangers, servicing blower motors, inspecting for corrosion, and testing safety sensors.

    Conclusion

    Forced air wood furnaces provide an alternative to traditional heating systems by using renewable wood fuel. Their operation revolves around efficiently burning wood to transfer its heat energy into circulating air. When properly maintained and used, these furnaces can safely and economically provide comfort for many years. Considering installation and operating costs, maintenance, and other factors can help determine if a wood furnace is the right choice for your home heating needs. With an understanding of how they work, homeowners can utilize wood furnaces to their full potential.

Laura Kassovic

Laura Kassovic, a former engineer at Intel SOC, now dedicates her efforts to mentoring startups in the realms of Wearables and AI. As a co-founder of New Tech Brake, she spearheads a wireless sensing solution enterprise catering to diverse applications including product development, research, location tracking, and people monitoring, as well as asset and cargo supervision. The platform empowers developers to craft an array of innovations such as fitness trackers, temperature-monitored cargo systems, medical trial tools, smart running garments, or even straightforward transmission of unprocessed accelerometer data to cloud-based repositories.

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