How Does an Outdoor Wood Furnace Work?

Outdoor wood furnaces, also known as outdoor boilers, outdoor wood boilers, or outdoor wood-fired hydronic heaters, have become an increasingly popular option for residential heating in rural areas over the past few decades. An outdoor wood furnace burns wood to heat water that is then piped underground into a home to provide radiant floor heat, as well as domestic hot water.

What are the Benefits of an Outdoor Wood Furnace?

There are several key advantages to using an outdoor wood furnace:

  • Lower Heating Costs: Wood is typically a cheaper fuel source than electricity, natural gas, propane, or fuel oil. With the rising costs of fossil fuels, an outdoor wood furnace can provide major savings on monthly heating bills.
  • Self-Sufficiency: Harvesting and burning your own wood allows you to be less reliant on utility companies and traditional fuel sources. This can be especially beneficial in rural areas.
  • Renewable Resource: Wood is a renewable resource, so heating with a wood furnace can be more sustainable than using finite fossil fuels. As long as trees are replanted, wood will continue to be available.
  • Aesthetic Appeal: Many homeowners enjoy the rustic aesthetic of an outdoor wood furnace and boiler. It evokes a traditional way of living off the land.
  • Even and Efficient Heat: The radiant floor heating system creates an even distribution of heat throughout the home. The wood furnace can also heat domestic hot water in the summer when heating is not needed.

How Does an Outdoor Wood Furnace Work?

The key components and operation of an outdoor wood furnace work as follows:

  • Firebox and Combustion Chamber: This is the main enclosure where the wood fuel is loaded, ignited, and burned. It contains the fire itself and allows air flow for combustion.
  • Heat Exchanger: Also called the boiler, this component transfers the heat from the fire into the water. It contains a series of tubes or pipes with water running through them to collect the heat.
  • Wood Burning System: This consists of the adjustable air intakes, baffles, and flue that control the rate of burning and airflow through the furnace.
  • Water Jacket: The water jacket surrounds the combustion chamber and heat exchanger. As heat is produced, it warms the water in the jacket.
  • Thermostat: This allows the furnace to be set to a target temperature. Once the target temp is reached, the air supply shuts down to prevent overheating.
  • Insulated Underground Piping: Heated water from the furnace is pumped through insulated pipes running underground to the home’s radiant floor heating system.
  • Heat Load Calculation: Proper sizing of the outdoor wood furnace is vital and based on the heat loss calculation of the home. Oversizing can lead to excessive cycling and creosote buildup.

How the Burn Process Works

Here is an overview of the wood burning process in an outdoor furnace:

  1. Wood fuel (logs, cordwood, biomass pellets, etc) is loaded into the firebox.
  2. Combustion air enters through lower vents, allowing the fire to ignite and burn.
  3. The fire heats the walls of the combustion chamber, releasing volatile gases from the wood.
  4. Secondary air is injected above the fire, which burns the released gases. This causes the glowing coals.
  5. As fire burns, heat is transferred through the walls of the heat exchanger.
  6. Water circulating through the heat exchanger tubes absorbs the heat.
  7. Heated water flows into the water jacket then out to the home’s heating system.
  8. Loading door seals shut off air supply when closed, slowing the fire.
  9. Ash falls to the bottom of the firebox and is removed during cleanings.
  10. Thermostat controls rate of burn via draft induction to maintain target temp.

This wood combustion and heat transfer allows the outdoor furnace to supply heat and hot water to the home. Monitoring and fine-tuning the burn process is key for clean, efficient operation.

What Are the Components of an Outdoor Wood Furnace?

The major components of an outdoor wood furnace include:


This is the chamber where the fire burns. It needs to be large enough to hold a long burn time of up to 12 hours. Common construction materials are stainless steel or heat-resistant alloys.

Fuel Loading Door

A tight-sealing insulated door allows wood to be safely loaded into the firebox while in operation. Air control valves often link to the door.

Combustion Chamber

Surrounding the firebox, the combustion chamber allows the fire and gases to be contained. Baffles help control airflow.

Heat Exchanger

The heat exchanger transfers heat from combustion into the boiler water. It typically uses steel tubes or a cylindrical vessel.

Water Jacket

This sealed outer shell surrounds the heat exchanger, filled with water to be heated. Insulation helps minimize heat loss.

Smoke Outlet & Chimney

To vent combustion gases and draw air into the fire. Chimney height creates the draft.

Ash Box

Collects ash and coals fallen from the firebox. Emptied periodically during cleaning.

Air Intake Damper

Adjustable vent that controls air inflow to the firebox. Linked to thermostat.

Circulating Pump

Circulates heated water from furnace into home heat system and back.

Outdoor Thermostat

Allows the burn rate and boiler temperature to be modulated based on heating demand.

What Are the Different Types of Outdoor Wood Furnaces?

There are a few main classifications of outdoor wood furnaces based on the boiler design:

  • Water Jacket Boiler: The classic and most common style. Has a water jacket enveloping the firebox and heat exchanger.
  • Fire Tube Boiler: Uses water-filled tubes running through the firebox to collect heat. Compact design.
  • Heat Exchanger Boiler: Heated gases pass across a steel heat exchanger that warms the boiler water.
  • Hybrid Boiler: Uses a heat exchanger plus a water jacket for very high efficiency. More complex.

Many custom designs are also available, along with different piping and heating system configurations. But the water jacket boiler remains the simplest and most affordable option for heating a home.

What Are the Basic Steps for Installation?

Installing an outdoor wood furnace requires careful planning and preparation:

1. Permits & Codes

Check local building codes and obtain required permits. Zoning laws may limit outdoor furnace placement.

2. Equipment Selection

Choose reputable brand and properly sized model based on home’s heat load. Select boiler type, features, etc.

3. Site Selection

Pick a level site safely away from structures, trees and foot traffic with access to wood supply.

4. Foundation

Pour a reinforced concrete base as foundation for furnace. Ensure it is level.

5. Boiler Installation

Bolt boiler onto foundation. Insert chimney through roof. Install well-insulated piping underground to home.

6. Interior Heating System

Connect boiler piping to existing or new radiant floor system. Install hot water connections.

7. Electrical and Thermostat

Wire furnace safely to dedicated electrical circuit. Install and program thermostat.

8. Permit Inspection

Have installation inspected by local code officer when complete.

Proper installation by a qualified installer is highly recommended. This ensures safe, efficient operation and compliance with building codes.

What Maintenance and Safety Practices Are Required?

Operating an outdoor wood furnace safely and efficiently requires some maintenance best practices:

  • Chimney Inspection: Annual professional chimney inspection to check for creosote buildup.
  • Ash Removal: Frequently remove ash to prevent accumulation. Store ash safely away from combustibles.
  • Door Gaskets: Replace worn out door gaskets that can compromise air seals.
  • Water Treatment: Use boiler chemicals to adjust pH and control scale/corrosion in water jacket.
  • Tune-ups: Periodic service checks by technician to ensure components are functioning optimally.
  • Good Wood: Burn only dry, seasoned firewood with 20% moisture content or less.
  • No Hazardous Materials: Never burn trash, painted/treated wood, plastics, gasoline, etc.
  • Per Code: Maintain proper clearances to combustibles and maintain smokestack height.
  • Learn Proper Operation: User training on starting/maintaining clean, efficient fires.

Following the manufacturer’s operation and maintenance guidelines is vital for safety. Also check insurance requirements.

What Are Some Common Problems or Issues?

Some potential issues to be aware of with outdoor wood furnaces include:

  • Overfiring: Can lead to dangerous pressures, leaks, and damage. Follow loading limits.
  • Creosote Buildup: Unburned wood gases condense in chimney, creating flammable creosote. Have chimney cleaned annually.
  • Smoke and Odors: Improper firing techniques can create excess smoke. Only burn seasoned wood.
  • Flue Fires: Highly flammable creosote ignites in chimney. Have chimney cleaned if buildup exceeds 1/4″.
  • Thermal Stress: Metal components expand and contract with huge temperature swings. May lead to leaks if furnace cycles frequently.
  • Scale Buildup: Hard water can leave mineral deposits inside furnace. Use boiler treatment chemicals.
  • Wood Moisture: Wet, unseasoned wood lowers heat output and causes deposits. Always burn dry, seasoned wood.
  • Overheating: Boil-overs occur if water is lost and metal overheats. Have pressure valve.

With careful operation and maintenance, potential issues can be minimized or avoided. But professional help is recommended for serious problems.

Frequently Asked Questions

What are the different ways to heat the home with an outdoor furnace?

The main options are in-floor radiant heating, baseboard hot water radiators, or forced air systems. Radiant floor heat is the most popular method.

How often does the furnace need to be loaded with wood?

Frequency depends on heat demand, but expect to load every 4 to 12 hours during the coldest winter months. Milder weather may allow once daily loading.

What is the average cost of an outdoor wood furnace?

Installed cost typically ranges from $8,000 – $15,000 depending on the boiler size and type. Cost varies by brand, dealer, and location.

How much wood do you need per season to heat with an outdoor furnace?

An average home uses 8-12 cords per year. Well-insulated, newer homes may use less. Much depends on climate zone and owner operation.

What is the best wood to burn in an outdoor furnace?

Dry, well-seasoned hardwoods like oak, ash, maple and beech are ideal. Softwoods like pine are acceptable but may require more frequent loading. Avoid wet wood.

How far can the furnace be from the home?

Up to 200-300 feet is common. Further distances are possible but require larger size piping to minimize heat loss. Insulation is key.

In summary, outdoor wood furnaces provide an earth-friendly, self-sufficient way to heat a home at a lower cost by utilizing a renewable fuel source. With proper installation, maintenance, and operation, an outdoor wood furnace can provide years of reliable performance. Check with local authorities for regulations in your area. With a quality boiler and good burning practices, you can stay warm through winter in an efficient, sustainable manner.

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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