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How Does a Forced Air Wood Furnace Work?

Forced air wood furnaces are an efficient and eco-friendly way to heat your home using renewable wood as fuel. Unlike traditional gravity fed wood stoves, forced air furnaces use a blower to actively push heated air through ductwork into different areas of your home. This allows for even and consistent heating throughout the house.

How Forced Air Furnaces Work

The basic components of a forced air wood furnace include a firebox surrounded by a water jacket, blower fan, heat exchanger, ductwork, and thermostat controls. Here is an overview of how each part functions:

The Firebox

The firebox is the enclosed metal chamber where the wood fuel is burned. High temperature firebrick lines the inside to withstand the extreme heat. There is an insulated steel door with a handle to access the firebox for loading more wood.

Air vents on the bottom allow oxygen into the firebox to fuel combustion. The firebox may have optional features like a glass viewing window or secondary combustion chamber to further improve efficiency.

The Water Jacket

Surrounding the firebox is a water jacket – a hollow metal shell that gets filled with water. As the firebox heats up, the water absorbs the heat and gets hot itself. This hot water later helps transfer heat to the air via the heat exchanger.

The water jacket essentially acts as a “thermal battery”, storing heat energy over time for gradual release. This is a key feature that makes wood furnaces burn cleanly and efficiently.

Blower Fan

Behind the firebox sits a squirrel cage blower fan. This fan actively pulls cool air from the room through the heat exchanger. The air gets warmed as it passes over the hot heat exchanger surfaces, and then gets pushed back out into the home through ductwork.

The blower fan is what makes forced air furnaces different than passive wood stoves. The fan enables active circulation of warm air.

Heat Exchanger

The heat exchanger is a series of metal tubes, plates, or chambers located where the firebox and water jacket meet the back of the furnace. As air from the blower fan passes over the heat exchanger, the heat from the hot water jacket gets transferred to the air.

Some designs may incorporate secondary combustion chambers in the heat exchanger to further incinerate smoke and gases, improving efficiency.

Ductwork

Insulated ductwork connects to the back of the furnace and branches out to vents in different rooms. This ductwork allows the heated air from the blower fan to circulate throughout the house. Dampers in the ductwork can control airflow to different zones.

Thermostat

A thermostat mounted on the wall acts as the control center. You set your desired room temperature on the thermostat. When the room dips below this set temp, the thermostat signals the furnace blower fan to turn on and pump heat. Once up to temp, the blower stops.

This on/off cycling continues automatically to maintain your set temperature. Advanced programmable thermostats allow custom heating schedules.

How to Operate a Forced Air Wood Furnace

Operating a wood furnace requires some manual work to load wood and remove ashes, but can heat a home effectively with less effort than a plain wood stove. Here are the basic steps for operation:

Load Wood

Load split firewood into the firebox, leaving some space for air circulation. Hardwoods like oak, maple and ash provide the most heat. Softer woods like pine burn faster but require more frequent loading.

Start with a base of larger logs on bottom, then stack smaller kindling on top in a crisscross pattern allowing air gaps.

Light Fire

Light the kindling using Firestarter or newspaper. Leave the door slightly cracked open initially to promote air flow and combustion until the fire establishes.

Adjust Air Controls

There are adjustable air inlet vents on the firebox to control the burn rate. Open vents provide more oxygen for a hotter, faster burn. Close them partially to slow the burn for longer heat. But be sure to leave some air so the fire doesn’t smother out.

Let the Furnace Cycle

Once lit, a full firebox can heat for 6 hours or more. The blower fan will automatically turn on and off to maintain the thermostat set temperature.

Refill with more wood as needed when fire dies down. Don’t let fire go out completely to avoid restarting a cold furnace.

Remove Ashes

Ash will build up in the firebox over time from burnt wood. Occasionally rake ashes into the ash pan for disposal once cooled.

Clean Smoke Ducts

The exhaust smoke ducts and chimney should be cleaned annually to remove creosote buildup which can cause chimney fires.

Service Water Jacket

Drain and refill the water jacket fluid as specified by manufacturer. This prevents corrosion and keeps the heat transfer working efficiently.

With a properly installed and maintained wood furnace, following this basic operation routine will keep your home comfortably heated throughout cold winters.

Types of Forced Air Wood Furnaces

There are a few main designs of forced air wood furnaces to choose from:

Indoor or Outdoor Models

Forced air wood furnaces can be configured for indoor installation or outdoor installation.

Indoor models allow the furnace to be installed in a basement or utility room, saving space outdoors. The smoke ducts vent horizontally through an exterior wall.

Outdoor models sit in a weather-proof enclosure outside like an A/C condenser. The smoke vents vertically with a chimney. This keeps wood smoke and ash outside. Ductwork runs underground to the home.

Steel or Soapstone Construction

The firebox and water jacket can be made from either steel or soapstone.

Steel conducts heat quickly for a fast warm up. Soapstone absorbs and radiates heat more slowly and steadily. Soapstone furnaces don’t cycle on and off as frequently.

Soapstone is naturally fireproof and can last longer than steel fireboxes. But soapstone is more expensive.

Catalytic or Non-Catalytic Combustion

Some furnaces include a catalytic combustor which helps incinerate remaining smoke and gases at the end of the burn process, reducing emissions.

Non-catalytic furnaces rely solely on firebox design for clean burns. Catalytic models provide an extra level of smoke control.

Key Benefits of Forced Air Wood Heat

Switching to a forced air wood furnace can make practical and economic sense for many homeowners. Benefits include:

Lower Heating Costs – Wood is typically much less expensive than propane, natural gas or heating oil. And it’s a renewable, locally available fuel.

Self-Sufficient Heating – Grow your own wood onsite or source it locally to avoid reliance on external fuel distributors.

Even Heating Distribution – Ducts allow heat to easily reach every room unlike basic wood stoves. No more cold spots!

Advanced Efficiency – Features like catalytic combustion provide cleaner burns, using fuel fully.

Backup Heating Ability – Many wood furnaces can also connect to a propane or electric burner for automatic backup.

Reduced Fossil Fuel Use – Heating with renewable wood reduces your carbon footprint. It’s a sustainable choice.

Power Outage Protection – Wood heat works even during electrical outages when other systems may fail.

Recommendations for Installation

Proper installation is crucial for forced air wood furnaces to function safely and effectively. Here are some key recommendations:

  • Hire an experienced, licensed installer familiar with local codes.
  • Ensure adequate clearance to combustibles per manufacturer instructions.
  • Use appropriate UL-listed Class A all-fuel chimney sections.
  • Install to manufacturer’s specifications for chimney height. An ideal draft is essential.
  • Connect properly sized ductwork for good airflow and heat circulation.
  • Install electric backup burner if desired for automatic heating redundancy.
  • Include a hearth pad to protect combustible flooring underneath.
  • Position outdoors if possible for cleaner operation and safety.
  • Provide sufficient indoor make up air for combustion.
  • Install CO monitors and fire/smoke detectors for safety.
  • Inspect annually and clean as needed to prevent creosote buildup.

Taking the time to correctly install your wood furnace will pay off for years of effective, trouble-free heating.

FAQ

Many homeowners have additional questions when considering a new wood heating system. Here are answers to some of the most frequently asked questions:

How much wood does a forced air furnace consume?

An average furnace may use anywhere between 4-8 cords of wood over a typical heating season, depending on climate, home size, and insulation. Harder woods like oak last longer.

What maintenance is required?

Basic annual maintenance involves removing ashes, cleaning smoke ducts, and draining/refilling water jacket fluid. Check door gaskets/seals periodically.

How long should a wood furnace last?

With proper maintenance, a high-quality wood furnace should provide 20 years or longer of usable life. Key components can be repaired or replaced as needed.

Can you burn coal in a wood furnace?

No, wood furnaces are not designed to safely handle coal combustion. Burning coal will damage the firebox. Only use recommended solid fuels like cordwood.

Do I need to hire a chimney sweep annually?

Annual professional chimney sweeping is recommended to prevent dangerous creosote accumulation and ensure proper draft. Do-it-yourself cleaning is difficult.

How often do you add wood?

For peak cold periods, expect to reload wood every 4-6 hours. Milder weather may allow 8-12 hours between loading. Adjust burn rate using air inlet controls.

What temperature can a wood furnace heat to?

Most furnaces can heat evenly up to 80 degrees Fahrenheit. Sizing the furnace properly to your home’s heat loss rate is important to reach desired temps.

Can a wood furnace heat my domestic water too?

Yes, some wood furnaces allow connecting a water heating coil or heat exchanger to also provide hot water alongside heating. This uses the wood heat for multiple needs.

Conclusion

Forced air wood furnaces provide an efficient, self-sufficient heating option using renewable wood fuel. With thermostatically controlled circulation of warm air throughout a home, they offer an upgrade over traditional wood stoves. Models with catalytic combustion also burn clean for reduced emissions. With proper installation and maintenance, a wood furnace can be a smart investment for reliable winter heating for years to come.

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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