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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 generate heat and distribute it throughout a home or building. Understanding how a wood furnace works can help homeowners decide if this type of heating system is right for their needs. This article will provide an overview of the key components and operation of a forced air wood furnace.

What is a Forced Air Wood Furnace?

A forced air wood furnace, sometimes called a wood furnace or wood stove furnace, is a central heating system similar to a gas or oil furnace. However, instead of using fossil fuels, it burns wood to provide heat. The furnace consists of a firebox surrounded by a water jacket. A blower fan pushes air across the hot water jacket, warming the air, which is then distributed through ductwork into the rooms of the home.

Forced air wood furnaces are available in a range of BTU outputs to heat spaces of different sizes. Many can provide both heating and hot water by integrating with a home’s existing water heater. High efficiency EPA certified wood furnaces provide more complete combustion and lower emissions.

Key Components of a Forced Air Wood Furnace

The main components of a forced air wood furnace include:

Firebox

This is the chamber where the wood fuel is loaded, ignited and burns. It needs to be large enough to hold wood and allow proper air flow for efficient combustion. The firebox is surrounded by the water jacket.

Water Jacket

The water jacket surrounds the firebox and absorbs heat from the fire. It contains water that is heated as the wood burns. This heated water is then circulated to heat exchangers.

Heat Exchangers

These transfer the heat from the heated water into the air stream produced by the blower fan. They consist of a series of metal tubes or plates that maximize heat transfer efficiency.

Blower Fan and Distribution Ductwork

An electric blower fan pulls cool air from the room and forces it across the hot heat exchangers. This warms the air, which is then pushed through ductwork to circulate throughout the home.

Chimney

This vents combustion gases and smoke safely outside. Most wood furnaces require a chimney, typically 6 inches or more in diameter.

Outdoor Air Intake

Brings in fresh outside air for combustion instead of using already heated indoor air. This makes the system more efficient.

Thermostat

Allows the furnace’s operation and heat output to be controlled by setting the desired interior temperature.

How Does a Forced Air Wood Furnace Work?

The operation of a forced air wood furnace relies on basic heating principles and thermodynamics. Here are the key steps in how the system provides heat:

1. Loading the Firebox

Wood fuel, typically cut into logs or split cordwood, is manually loaded into the firebox. Hardwoods like oak or maple are ideal. The wood must be dry, with a moisture content below 20%, for proper burning.

2. Lighting a Fire

Paper, kindling and firestarter materials help ignite the wood in the firebox. Sufficient air flow must be allowed into the firebox for the fire to light and burn properly.

3. Heating Water in the Water Jacket

As the fire burns, it releases heat. The hot gases from the fire are contained in the firebox so their heat is transferred into the surrounding water jacket. This heats the water inside the jacket to 145-180°F.

4. Circulating Heated Water

The now heated water in the water jacket is pumped via convection or through a circulation pump to the heat exchangers in the plenum.

5. Forced Air Across Heat Exchangers

The blower fan activates, pulling cool air from the room through return air ducts. It forces this air across the hot heat exchangers, warming it 30-80°F above room temp.

6. Distributing Warm Air

The blower fan pushes the warmed air out through the main supply ducts and into vents throughout the home, raising the indoor temperature.

7. Venting Exhaust

Smoke, gases and excess heat exit through the chimney, while room air is replenished through the outdoor air intake.

8. Thermostat Control

The thermostat monitors indoor temps and activates the furnace’s blower to run as needed to maintain the desired temperature. It will signal more heat is needed based on the home’s heat loss.

This cycle repeats as the firebox is reloaded and burned to match the heat demand. The furnace will run daily during the colder months to keep the home comfortably heated.

Key Factors in Proper Wood Furnace Operation

Several factors are important for a wood furnace to operate efficiently, cleanly, and safely:

  • Dry wood fuel – Needs 20% moisture content or less
  • Proper air flow – For clean combustion in the firebox
  • Clean heat exchangers – For maximum heat transfer
  • Sealed ductwork – Prevents conditioned air leaks
  • Insulated home – Reduces heat loss requiring fuel
  • Regular chimney cleaning – Prevents creosote buildup
  • Outdoor air intake – Provides sufficient oxygen for the fire
  • Thermostat setting – Balances comfort and fuel consumption

Benefits of a Forced Air Wood Furnace

Heating with a wood furnace has several potential benefits:

  • Lower heating costs – Wood can be much cheaper than propane or heating oil
  • Self-sufficiency – Doesn’t rely on any utilities or fuel deliveries
  • Use renewable fuel source – Burning wood is carbon-neutral
  • Uninterrupted operation – Not affected by power outages
  • Thermal mass heating – Provides more even, consistent heat
  • Ambiance – Cozy look and smell of burning wood
  • Support local economy – Can use wood from local sources

Disadvantages and Considerations

While they can provide an affordable and self-sufficient heating option, wood furnaces also have some downsides to consider:

  • Labor for fuel – Cutting, splitting, stacking wood is hard work
  • Wood storage – Adequate covered space is needed to season wood
  • Ash removal – Firebox needs periodic cleaning out
  • Chimney maintenance – Requires annual professional cleaning
  • Home unattended – Potential safety risks if fire is left burning
  • Dirtier than gas/oil – More particulate emissions
  • Learning curve – Proper operation takes experience
  • Upfront cost – More expensive than gas or oil furnaces

Overall, a wood furnace can be an effective and money-saving heating system if sized properly for the space, operated safely and efficiently, and fueled with good quality wood. Considering installation and operating costs along with maintenance requirements will allow homeowners to decide if a wood-burning furnace is their best heating option.

Frequently Asked Questions

How much wood does a furnace use?

On average, a wood furnace might use between 4-8 cords of wood per year, depending on the size of the home, insulation levels, and local climate. More efficient furnaces may use less.

What temperature should the water jacket run at?

The ideal water jacket temperature range is 145-180°F. Too low reduces heat output, while too high can be unsafe.

How often does the chimney need cleaning?

The chimney and flue pipes should be professionally cleaned at least once a year to remove creosote deposits and maximize safety.

Can a wood furnace be the sole heat source?

Yes, a properly sized wood furnace is capable of being the only heating system for most homes. Supplemental heat may be needed at times.

How long will a forced air wood furnace last?

A quality wood furnace can provide 20 years or more of service life if properly maintained and operated. Replacing components like blowers may extend lifespan.

Can I heat my home and water with one wood furnace?

Yes, many wood furnace models allow connection to existing water heaters to provide both hot air and domestic hot water.

This overview covers the main components and operation of a forced air wood furnace heating system. With an understanding of how they work, homeowners can better determine if this renewable energy heating option makes sense for their specific needs and comfort. With proper maintenance and use, a wood furnace can be a durable and cost-effective way to heat a home.

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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