How Does An Outdoor Furnace Work?

Outdoor furnaces, also known as outdoor wood boilers or hydronic heaters, provide an alternative way to heat homes and other buildings. They burn wood or other solid fuels like corn to heat water, which is then pumped through insulated underground pipes to provide heat.

Outdoor furnaces are very different from traditional indoor wood stoves. They are located outside, typically 30-200 feet away from the building being heated. This allows the mess and danger of an open fire to be kept completely outside.

Key Takeaway: An outdoor furnace is located outside the home or building it heats. It burns wood or other biomass fuel to heat water that circulates through underground pipes to provide heat.

The basic parts of an outdoor furnace include:

  • Firebox – This is where the fire burns and wood fuel is loaded. It is surrounded by a water jacket.
  • Water jacket – The water jacket surrounds the firebox and absorbs heat from the fire. This heated water is pumped to buildings.
  • Heat exchanger – Transfers heat from the heated water to forced air or radiant heating systems inside buildings.
  • Insulated piping – Heated water is pumped through underground insulated pipes to buildings.
  • Housing – A weatherproof cabinet surrounds the firebox, water jacket, and heat exchanger.

There are two main types of outdoor furnaces:

Standard Outdoor Furnaces

A standard outdoor furnace burns wood or other solid fuel in the firebox. The smoke and exhaust from combustion go straight up the chimney.

Standard furnaces are simple in design but are also fairly inefficient. They produce more smoke and air pollution than other designs.

To operate efficiently, they need to be burned very hot. Otherwise, they will produce a lot of smoke and creosote buildup in the chimney. Frequent cleaning is required.

Key Takeaway: Standard outdoor furnaces are simple but inefficient. They produce a lot of smoke and creosote unless burned extremely hot.

Gasification Outdoor Furnaces

Gasification furnaces use a secondary combustion chamber and forced air to burn the exhaust gases produced by the fire.

First, wood burns in the main firebox, which heats the surrounding water jacket. The smoke and gases produced go into the secondary chamber below the firebox.

Air is blown into the secondary chamber, which ignites the exhaust gases. This secondary combustion produces more heat and reduces smoke and emissions.

Computer controls can monitor temperatures and airflow to optimize efficiency. Gasification furnaces burn up to 50% less wood than standard furnaces.

Key Takeaway: Gasification outdoor furnaces use secondary combustion of exhaust gases, resulting in higher efficiency, less smoke, and lower emissions.

How A Gasification Outdoor Furnace Works

Here are the steps showing how a gasification outdoor furnace produces heat:

  1. Wood fuel is loaded into the main firebox and lit. As the fire burns, smoke and wood gases are produced.
  2. The heat from the fire is absorbed by the water jacket surrounding the firebox, heating the water.
  3. The smoke and gases rise up out of the firebox into the secondary combustion chamber underneath.
  4. Air is blown into the secondary chamber, igniting the gases and smoke. This secondary combustion produces more heat.
  5. The exhaust exits the furnace through the chimney with minimal smoke or emissions.
  6. The heated water circulates through underground pipes to buildings.
  7. Heat exchangers transfer the heat from the water to home heating systems.
  8. Cooled water returns to the outdoor furnace to be reheated.

Comparing Standard and Gasification Furnaces

Standard Furnaces

  • Simple design
  • Inefficient
  • Produce lots of smoke and creosote
  • Require very hot fires
  • More polluting

Gasification Furnaces

  • Advanced secondary combustion
  • Very efficient
  • Produce minimal smoke
  • Cleaner burning
  • Meet EPA emissions standards
  • Use less wood

Heating Multiple Buildings

One major advantage of an outdoor furnace is the ability to heat multiple buildings with a single furnace.

Underground PEX pipes are run from the outdoor furnace to each building on the property. Each building can have its own thermostat and heat exchanger to control indoor temperatures.

It is possible to heat a home, garage, barn, workshop, greenhouse, shed, pool house, and more from a single outdoor furnace.

Key Takeaway: Outdoor furnaces can heat multiple buildings with a single unit using underground piping between structures.

Heating Systems Used With Outdoor Furnaces

Outdoor furnaces are designed to work with almost any existing heating system in buildings. The main types of systems used are:

  • Forced air – A water-to-air heat exchanger is installed in the ductwork of the existing forced air furnace. The furnace blower pushes air through the heat exchanger and into ducts.
  • Hydronic radiant heat – Heated water from the outdoor furnace circulates through radiant tubing installed under floors or baseboard radiators.
  • Existing boiler system – A water-to-water heat exchanger integrates the outdoor furnace into the building’s hydronic heating system.

This flexibility makes upgrading to an outdoor furnace simple. The existing heating system doesn’t need to be replaced, just connected to the outdoor furnace.

Heating Domestic Hot Water

An outdoor furnace can also heat domestic hot water for a home. This provides endless hot water for showers, washing, and other uses.

A water-to-water heat exchanger is installed inline on the cold water line entering the home. As cold water passes through the heat exchanger, it absorbs heat from the hot water coming from the outdoor furnace.

The home’s water heater can often be turned off entirely since all hot water is provided by the outdoor furnace. This provides additional energy savings.

Key Takeaway: Outdoor furnaces can heat domestic hot water in addition to space heating, often allowing the main water heater to be turned off.

Locating The Outdoor Furnace

When installing an outdoor furnace, an important consideration is its location:

  • It should be located close enough to the building to minimize heat loss through the underground piping. Generally no more than 200 feet away.
  • Consider the direction of prevailing winds. Locate it where smoke and fumes will not blow toward the building.
  • Allow room to store a year’s supply of wood fuel nearby.
  • Keep it at least 30 feet from any building for fire safety.
  • Follow all local codes for minimum distances. Many areas prohibit locating furnaces in the front yard or close to lot lines.
  • Consider visual aesthetics. Try to tuck it out of sight behind other buildings or landscaping.

Proper location will maximize efficiency and minimize the nuisance of smoke and fumes.

Outdoor Furnace Efficiency

Several factors affect the efficiency and performance of an outdoor furnace:

  • Insulation – The unit should be well insulated to minimize heat loss.
  • Sealing – All doors and openings should seal tightly to prevent air leakage.
  • Water capacity – More water capacity stores more heat. Larger units are more efficient.
  • Burn control – Sophisticated computerized controls optimize the burn process.
  • Gasification – Secondary combustion increases efficiency substantially.
  • Wood moisture – Burning properly seasoned dry wood maximizes heat output.

With the right design and conditions, modern outdoor furnaces can provide very efficient wood heating. Gasification models convert over 90% of the wood’s energy into usable heat.

Environmental Impact

Outdoor wood furnaces have sometimes been criticized for contributing to air pollution. However, newer gasification models have extremely low emissions and meet EPA smoke standards when used properly.

Also, heating with wood harvested in a sustainable manner can be considered carbon-neutral. It releases the same amount of carbon as the trees would decaying naturally in the forest. This makes wood a renewable, environmentally-friendly fuel.

With the right outdoor furnace model and operation, wood heat can be just as clean as other heating fuels. And it provides independence from fossil fuel use.

Cost of Outdoor Furnaces

The typical costs for an outdoor furnace installation are:

  • $6,000 – $15,000 for the outdoor furnace unit itself. Higher efficiency and larger capacity units cost more.
  • $3,000 – $7,000 for installation – excavation, running piping, integrating with existing heating system, etc.
  • $1,000 – $3,000 for wood storage structure, chimney, required permits, etc.

So a complete installation often ranges from $10,000 – $25,000 in total.

These costs are offset by the ongoing savings on energy bills when switching from propane, fuel oil, or electric heat. A wood furnace can potentially pay for itself quite quickly given the high expense of heating fuel.

Key Takeaway: The total installed cost of an outdoor furnace commonly ranges from $10,000-$25,000. Energy bill savings often make the investment worthwhile.

Maintenance and Operation

Properly maintaining an outdoor furnace is important for efficiency and performance:

  • Fuel – Burn only dry, well-seasoned wood with 20% moisture content or less. Green or wet wood will lower combustion temperatures.
  • Loading – Don’t overload the firebox. Leave room for combustion air to circulate.
  • Firing – Fire the furnace hot twice a day rather than letting the fire smolder.
  • Ash removal – Remove ashes regularly to allow air flow. Don’t let ash build up.
  • Chimney cleaning – Clean the chimney annually or more often if needed.
  • Gaskets & seals – Replace worn gaskets around doors to prevent air leakage.

With proper operation and maintenance, an outdoor furnace should provide decades of reliable service.

Is An Outdoor Furnace Right For You?

Outdoor wood furnaces can be an excellent heating solution if:

  • You have access to low-cost or free firewood.
  • You want to reduce energy bills and dependence on utility companies.
  • You need to heat multiple buildings.
  • You have room to locate it safely away from buildings.
  • You don’t mind the labor of loading wood and removing ash.
  • Your area allows outdoor furnace installations.
  • You prefer wood heat or want heating diversity.

If you meet these conditions, an outdoor furnace may be a good option to consider.


What are the advantages of an outdoor wood furnace?

Some key advantages include:

  • Heating costs can be much lower than conventional heating fuels
  • One unit can provide heat and hot water to multiple buildings
  • Integrates with existing heating systems
  • Keep wood fuel storage and mess completely outside
  • Provides emergency backup heat with a wood supply
  • Reduce reliance on utility companies

What maintenance does an outdoor furnace require?

Typical maintenance includes:

  • Removing ashes from firebox
  • Cleaning chimney annually
  • Replacing door gaskets as needed
  • Painting exterior
  • Testing/replacing circ pumps
  • Keeping thermostat battery fresh

How much wood does an outdoor furnace use?

With a gasification model, an outdoor furnace will burn 1/2 to 1 full cord of wood per month on average. In mild weather, wood use drops lower. In very cold weather, wood use can be 1.5 to 2 cords per month.

What can cause smoke from an outdoor wood furnace?

Excess smoke is usually caused by:

  • Burning unseasoned wet wood
  • Not firing the furnace hot enough
  • Letting the fire smolder too long
  • Poor draft through the firebox
  • Leaks in firebox or doors

How long will an outdoor furnace last?

A high-quality outdoor furnace should last 25-30 years or longer with proper maintenance. Key longevity factors are using the right materials, controlling corrosion, and avoiding physical damage.


Outdoor wood furnaces provide an alternative way to heat homes and buildings using wood as fuel. Located outside the home, they burn wood to heat water that circulates through underground pipes into buildings.

Modern gasification furnaces are extremely efficient and clean burning. They offer lower heating costs, integration with existing heating systems, and the ability to heat multiple buildings.

With proper maintenance and use of well-seasoned wood, outdoor furnaces are an eco-friendly and cost-effective heating option. They provide independence from conventional heating fuels.