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How Does a Wood Burning Furnace Work?

A wood burning furnace, also known as a wood stove or wood boiler, is a heating system that uses wood as its fuel source. Wood burning furnaces work by burning wood fuel to generate heat, which is then distributed throughout a home or building via ductwork, radiators or heated floors.

How Wood Burning Furnaces Burn Wood

There are a few key components that allow a wood burning furnace to generate heat:

Firebox

The firebox is the enclosed metal container where the wood fuel is loaded and burned. Fireboxes are insulated and lined with firebrick or ceramic materials that can withstand extremely high temperatures. Air intake vents allow oxygen to enter the firebox and feed the combustion of the wood.

Baffles

Baffles are metal plates inside the firebox that slow down the hot gases produced by the fire. This allows more heat to be transferred through the walls of the firebox before the gases are vented out through the chimney. Baffles maximize efficiency by extracting as much heat as possible from the fire.

Flue

The flue is the passageway that directs exhaust gases from the firebox up and out of the chimney. Its job is to create effective draft to ensure complete combustion of the wood fuel.

Combustion Air Controls

Air control regulators allow users to manage the amount of air entering the firebox. More air creates a hotter fire, while less air slows the burn. Controlling air flow allows users to manage the heat output and burn rate of the wood furnace.

Blower Fan

Once the firebox heats up, a blower fan turns on to push the hot air through heat ducts throughout the home. This distribution system circulates heat generated by the wood furnace.

How Wood Burning Furnaces Work Through Various Stages

Wood furnaces utilize a batch burn process that involves several stages as the wood fuel is consumed:

Ignition

To initially get the furnace going, kindling and newspaper are used to ignite larger pieces of wood loaded into the firebox. Once the fire establishes, normal fuel wood can be added.

High Burn

During high burn stage, the firebox temperature climbs above 1000°F as the wood fuel burns hot with bright flames. Much of the smoke and moisture is driven out of the wood during this phase. The highest heat output occurs during high burn.

Charcoal Burn

As wood fuel is consumed, charcoal remnants build up in the firebox. The charcoal continues burning and provides a long, steady heat output. Less smoke is produced during this phase.

Coal Burn

In the final coal burn phase, remaining charcoal breaks down into coal-like embers. The coal provides lower heat output but sustains the fire. When only coal remains, it’s time to reload wood fuel to start the next burn cycle.

Idle Combustion

To achieve the longest burn times, an idle setting can be engaged after loading more wood. This restricts air flow to produce a low-oxygen, smoldering fire. Heat output drops but slowly burns remaining fuel until the next reloading.

Refueling

To sustain heating, new wood fuel must be periodically loaded into the firebox after previous wood has burned down. Ash should be removed prior to refueling. The cycle then repeats.

Heat Output Control

One of the key features of wood furnaces is the ability to control heat output based on demand:

Thermostat

Like other central heating systems, wood furnaces connect to a thermostat inside the home. Based on the target interior temperature set on the thermostat, the wood furnace regulates its burn rate and air flow to produce more or less heat accordingly.

Modulating Combustion

High-end wood furnaces can continuously modulate air flow and combustion to precisely match heat output to thermostat demand. Turning down the burner reduces fuel consumption during periods of less heat demand.

Multiple Burn Settings

Most wood furnaces have various manual settings (like idle, low and high) that alter the air flow to control intensity of the fire’s burn. Choosing lower settings during warmer weather prevents overheating.

Zone Heating

In hydronic systems, wood furnaces often connect to multiple heating zones throughout a home. Users can manage each zone independently so bedrooms, main living areas, and basements only receive heat when needed.

Heat Distribution

Another key function of wood furnaces is delivering heat generated by the firebox into the spaces needing heating:

Forced Air

Some wood furnaces tie into forced air ductwork like a normal furnace. A blower fan pushes hot air through supply ducts into rooms throughout the home with vents.

Hydronic Radiators

In hydronic systems, the wood furnace heats water that circulates through a closed loop system of pipes and radiators. Heat is emitted into rooms via convection as the hot water passes through the radiators.

In-Floor Hydronic

Heated water from a wood boiler can also be piped under floors. Heat radiates up through the flooring, providing comfortable warmth underfoot. This is known as radiant floor heating.

Hot Air Gravity Systems

In more basic systems, the natural tendency of hot air to rise can circulate heat without fans. Hot air ducting placed high on walls allows heat to naturally connect and flow into rooms.

Benefits of Wood Heat

Heating with wood offers certain advantages:

  • Renewable and Sustainable – Wood is an renewable, locally available fuel in many regions. Actively managed forests can provide a sustainable supply of wood fuel.
  • Self-Sufficient – Harvesting and burning one’s own wood provides heating self-sufficiency. Wood heat greatly reduces dependency on utility companies and the electric grid.
  • Economical – With proper thermal efficiency, heating with wood can be cheaper than heating oil, propane or electricity. Wood is among the lowest cost heating options in some areas.
  • Emergency Backup – Wood heating gives homes an emergency backup option during power outages when other systems may fail. The self-powered furnace continues heating.

Considerations for Heating With Wood

While wood heat offers benefits, some factors should be contemplated:

  • Fossil Fuels For Startup – Most wood furnaces still require some propane or gas for lighting the initial fire during startup. Wood cannot ignite itself.
  • Labor for Fuel Prep – A wood heating system demands significant physical labor to prepare, haul, stack and load fuel. This requires fitness and effort some may not have.
  • Emissions – Older or unregulated wood furnaces emit significant smoke, particulates and carbon. Modern EPA certified wood furnaces now meet strict emissions standards for cleanliness approaching other fuels. But air quality concerns remain in some regions. Properly seasoned wood and correct operation to allow complete combustion vastly reduce emissions.
  • Ash Removal – Wood ash accumulates and must be regularly emptied from the firebox. Ash management is an added chore compared to gas or oil heat.
  • Spatial Requirements – The furnace itself and fuel storage occupy significant space that could limit installation options in some homes. An adequate wood shed and room for wood stacking is required. Area must also retain access to allow fuel loading in winter.

With an understanding of the operational principles and capabilities of wood burning furnaces, you can better determine if this renewable heating system is right for your situation. If sized and operated properly for the heating load, wood furnaces can provide an economical and self-reliant heating solution.

Frequently Asked Questions

What maintenance does a wood furnace require?

  • Ash removal
  • Chimney cleaning
  • Replacing gaskets and seals
  • Cleaning air intake vents
  • Removing creosote buildup
  • Checking for corrosion
  • Inspecting combustion chamber

How much wood do you need for winter?

  • Depends on size of space heated
  • Average home uses 4-8 cords per year
  • One cord provides about 2800 sq ft BTUs
  • More insulation and efficient home lowers fuel needs

What is the cost of heating with wood?

  • Under $1000/year if you harvest your own wood
  • $2000-$4000 for purchased cordwood
  • Compare to $3000+ for oil or propane

Does a wood furnace need electricity?

  • Electricity required for blower fan operation
  • Automatic igniters also need power
  • System remains gravity-fed heat without power

What are the cleanest wood burning furnaces?

  • EPA certified wood stoves and furnaces
  • Models meeting 2020 particulate emissions standards
  • Catalytic and gasification furnaces lower emissions

So in summary, a wood burning furnace works by loading wood fuel into an insulated firebox, combusting the wood at high temperatures, controlling the burn rate via air flow, circulating hot air or water through heat exchangers, and distributing the heat throughout the home via fans or hydronic loops. With some added considerations around fuel management, wood furnaces can provide an economical and renewable heating option.

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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