An outdoor wood furnace, also known as an outdoor wood boiler, outdoor stove, or outdoor wood-fired hydronic heater, is a heating system that provides space heating and hot water by burning wood as fuel. Outdoor wood furnaces work by heating water that circulates through insulated underground pipes to provide heat and hot water to a home or building.

How an Outdoor Wood Furnace Operates

An outdoor wood furnace consists of a firebox surrounded by a water jacket, usually located in an insulated outdoor shelter or small shed outside of the building it is heating. Firewood is loaded into the firebox and burned to heat water in the surrounding jacket. As the fire burns, smoke travels through heat exchange tubes that run through the unit’s water reservoir. The heat from the burning firewood is transferred to the water surrounding the firebox. This heated water is then pumped through underground insulated pipes to the buildings needing heat and hot water.

Once inside the building, the hot water from the outdoor furnace circulates through a heat exchanger or a coil to heat the indoor air or water supply. This heat exchange transfers the heat from the water to the home heating and water systems. After releasing its heat, the cooler water cycles back through the underground pipes to be reheated again in the outdoor furnace. This closed-loop system provides a constant flow of hot water for heating and domestic use.

Key Components

The key components of an outdoor wood furnace include:

• Firebox – This is the combustion chamber where the wood fuel is loaded, burned, and converted to heat energy. It is surrounded by a water jacket.
• Water reservoir – The water reservoir surrounds the firebox and absorbs the heat from the fire through the walls of the firebox. This heated water is circulated through the system.
• Heat exchanger – Transfers the heat from the hot water into the home’s forced-air heating system. Can be located inside or outside the furnace.
• Plenum – Channels the heated air from the heat exchanger into ductwork to distribute through the home.
• Water jacket – The hollow walls around the firebox, filled with water, which absorbs heat from the fire.
• Insulated underground piping – Transports the heated water from the furnace to the buildings. Insulated to prevent heat loss.
• Circulation pumps – Pumps that move the water through the closed-loop system.
• Chimney – Releases combustion gases and provides proper airflow for the fire.
• Outdoor furnace shelter – Insulated and ventilated enclosure that houses and protects the furnace.
• Domestic hot water coil – Heat exchanger that transfers heat from water to domestic hot water supply.
• Controls – Thermostats, valves, switches, and gauges that monitor temperatures and control furnace operation.

How Heat Generation Works

The key steps in how an outdoor wood furnace generates heat are:

1. Firewood fuel is loaded into the combustion chamber (firebox).
2. The firewood burns, releasing heat energy.
3. Hot gases from the fire release heat through the walls of the firebox into the surrounding water jacket.
4. Water in the jacket absorbs heat from the firebox walls and becomes hot.
5. The heated water circulates through underground pipes to buildings needing heat.
6. Once inside, the hot water flows through a heat exchanger, transferring heat to the building’s hot air or water heating system.
7. Cooled water returns via underground pipe to be reheated again in the furnace.
8. Combustion gases exit through the chimney.

This closed-loop system provides a constant supply of heated water from the outdoor furnace to heat the buildings. The furnace burns wood fuel as needed to maintain the hot water temperature.

Benefits of Outdoor Wood Furnaces

Outdoor wood furnaces offer several benefits that make them an attractive heating option:

• Use renewable fuel source – Burn wood, a renewable resource, instead of fossil fuels like oil or gas. Provides independence from conventional heating fuels.
• Lower heating costs – Wood fuel can be much less expensive than electricity, oil or propane. Provides substantial savings on annual heating bills.
• Supplemental heat source – Can provide primary or extra heat for spaces with an existing heating system. Allows zone heating different buildings.
• Hot water production – Heats domestic hot water along with space heating. Provides hot water self-sufficiency.
• Heating ability – Able to heat multiple buildings like a home, garage, workshop, greenhouse, swimming pool, etc. from one furnace.
• Long burn times – Large firebox capacity allows firing less frequently, with overnight burns. Requires less maintenance than indoor wood stoves.
• Constant interior warmth – Provides a steady output of heat with less temperature fluctuation than indoor wood stoves. Even, comfortable interior temperatures.
• Cleaner operation – Located outdoors away from living space. Reduces chances of indoor fire, eliminates smoke and odors inside.
• Flexible installation – Can be located away from buildings for safety. Underground piping allows flexible arrangement.
• Aesthetics – No need for a large unsightly fuel tank. Chimney is only exterior visual element.

Overall, outdoor wood furnaces can provide an affordable, self-sufficient heating and hot water system using sustainable wood fuel for many homeowners.

Types of Outdoor Wood Furnaces

There are a few main types of outdoor wood furnace systems:

Forced Air Furnace

• Heats air using a heat exchanger and blower fan.
• Hot air is distributed through ductwork to heat interior spaces.
• Can be paired with existing forced air gas/oil furnace system.
• Provides both space heating and possible domestic hot water.

Hydronic Heating Furnace

• Heats water to provide hydronic radiant floor heating.
• Heated water circulates through tubing under the floor.
• Even, comfortable radiant floor heat.
• Can integrate with existing boiler system.

Hot Water Supply Furnace

• Focus is heating domestic hot water supply.
• Heated water links to home’s hot water plumbing.
• Provides endless hot water self-sufficiently.
• Limited space heating ability.

The type of system chosen will depend on the buildings being heated, the heating distribution method, and the mix of space heating and hot water demands. Forced air furnaces offer the most overall heating versatility for typical homes.

How to Determine Correct Furnace Size

Choosing the proper size outdoor wood furnace is crucial for effective heating. The main factors that determine the correct furnace size include:

• Building square footage – The total square footage of the spaces being heated. More area requires greater heat output.
• Heating load – The number of BTUs per hour required to maintain indoor temperatures. Based on climate, insulation, and other heat loss factors.
• Number of buildings – Heating multiple buildings requires a larger furnace than a single building.
• Hot water load – If heating domestic water, the hot water usage rate must be considered.
• Supplemental heat sources – If the furnace augments another heat system, size can be reduced.
• Output rating – The furnace output capacity must meet or exceed the total calculated heating load.

Have a professional perform a heating load calculation on all the buildings to be heated to determine the required BTU/hr output. Choose a furnace with an output rating that matches or exceeds the total heating demand. An oversized furnace will burn more wood and short cycle, while an undersized unit will struggle to heat sufficiently.

How to Install an Outdoor Wood Furnace

Proper installation is crucial for an outdoor wood furnace to function safely and effectively. The main steps in installing an outdoor wood furnace include:

Select Location

• Pick a level site that is centrally located to the buildings being heated.
• Locate an adequate distance from structures for safety, usually 15-30 feet away.
• Ensure good drainage and away from flood zones.
• Visibility from the home is ideal for monitoring.

Prepare Site

• Create a concrete pad or blocks to create a flat, level foundation.
• Dig trenches 12-18 inches deep for underground piping runs.
• Install electrical supply line and outlet to power furnace.

Set Up Furnace

• Use lifting equipment to place furnace on pad. Level and shim furnace.
• Install chimney thimble and connect stack/chimney sections.
• Connect piping header and underground pipe network. Insulate piping.
• Install heat exchangers, plenums, ductwork (for forced air systems).
• Connect water lines for hot water heating coils.
• Link furnace controls to thermostats in buildings.

Finish Enclosure

• Erect and insulate the outer furnace shelter.
• Install vents in shelter for combustion air.
• Attach sheltered walkway to home.
• Consider camouflage siding or strategically placed landscaping.

Proper installation by an experienced professional ensures the outdoor furnace operates safely and efficiently. Always check local building codes for any relevant requirements.

Operating and Maintaining an Outdoor Wood Furnace

To keep an outdoor wood furnace running optimally over time, certain operating and maintenance practices are recommended:

Daily Operation

• Load wood to fill firebox below loading door level. Avoid overfilling.
• Set thermostat to desired interior temperature. Thermostat controls burn rate.
• Combustion air inlets must be unobstructed.
• Inspect for any leaks in water connections.
• Remove ashes before they reach firebox opening level.

Routine Maintenance

• Annually clean heat exchangers and chimney.
• Keep chimney top clear of debris.
• Clean plenum and ductwork.
• Drain and flush water system. Refill with fresh water.
• Lubricate circulating pumps.
• Inspect refractory bricks and mortar for damage.
• Check door seals, replace if worn.

End of Season

• Allow fire to go out and furnace to fully cool.
• Remove ash, clean out firebox.
• Disconnect piping and drain water if in cold climate.
• Clean air injection system ducts.

Proper operation and routine maintenance will prolong the life of the furnace, enhance efficiency, and provide years of reliable heating.

Costs of Installing and Operating an Outdoor Wood Furnace

The costs associated with using an outdoor wood furnace include:

Installation Costs

• Furnace unit: $5,000 to $12,000
• Underground piping: $2,000 to $4,000
• Site work, foundation, enclosure: $1,500 to $3,000
• Total estimated installation: $8,500 to $19,000

Annual Operating Costs

• Wood fuel: $500 to $1,500
• Electricity: $500 to $800
• Maintenance/cleaning: $250
• Total estimated annual cost: $1,250 to $2,550

Payback Period

• Generally 2-10 years compared to conventional heating fuel
• Depends on cost of other fuels like oil, propane, electricity

Though the upfront cost is significant, an outdoor wood furnace can pay back the initial investment in just a few years through the savings on traditional heating fuels.

Important Safety Considerations

While outdoor wood furnaces can be an effective home heating method, there are some important safety considerations:

• Install at proper clearances from buildings, trees and other combustibles.
• Maintain the furnace shelter in good condition away from elements.
• Keep the area around furnace clear of debris and other combustible materials.
• Only burn seasoned firewood, not wet or green wood.
• Avoid over-firing the furnace beyond its BTU output rating.
• Have a qualified professional perform any major repairs.
• The underground piping will get extremely hot, insulate properly.
• Keep flammable materials away from underground piping areas.
• Do not leave an active fire unattended for long periods.
• Install smoke detectors and carbon monoxide detectors as an added precaution.
• Educate all household members on emergency shutdown procedures.

Adhering to the manufacturer’s operation guidelines and taking proper precautions will help maximize the safe operation of an outdoor wood furnace.

Troubleshooting Common Outdoor Furnace Problems

Some potential issues that may arise with outdoor wood furnaces include:

Smoky fire

• Restricted air supply – Check inlets are not blocked
• Wet or unseasoned wood – Use properly dried wood
• Chimney blockage – Inspect and clean flue regularly

No hot water

• Pump failure – Replace defective circulator pump
• Airlock in piping – Purge air from system
• Leak in underground pipe – Test piping, repair leaks

Burns through wood quickly

• Excessive heat demand – Balance thermostats, check for leaks
• Improper wood loading – Don’t overload firebox
• Firebox door leak – Replace worn gaskets
• Flue leak – Inspect chimney for cracks or poor connections

Furnace water does not get hot enough

• Insufficient insulation – Add more insulation to enclosure
• Heating load too high – Make sure properly sized furnace
• Scale buildup in heat exchangers – Flush and clean system

Furnace cycling on/off frequently

• Thermostat location – Move to avoid influence of drafts
• Thermostat calibrated incorrectly – Double check temperature setting
• Excessive heat loss – Improve building insulation

With some troubleshooting and annual maintenance, potential issues can usually be identified and corrected to keep an outdoor wood furnace operating optimally. Consulting the manufacturer or an experienced service technician can also be very helpful if encountering any persistent problems.

Outdoor Wood Furnace Emissions and Efficiency

Modern EPA-certified outdoor wood furnaces are vastly improved in terms of emissions and efficiency compared to older models, through features like:

• Enhanced combustion systems – Improves burn efficiency.
• Catalytic converters – Reduce harmful emissions like particulates.
• Improved insulation – Minimizes heat loss.
• Computerized controls – Optimizes firing cycles.
• Secondary combustion chambers – Reburns smoke and gases.
• Qualified installation – Ensures proper clearances and setup.

Emissions can be further reduced by:

• Only burning clean, well-seasoned wood.
• Avoiding idling – Don’t let smolder between cycles.
• Regular maintenance and pipe cleaning.
• Proper firing and refueling methods.
• Stack height – Improves smoke dispersion.

A new EPA-certified and properly operated outdoor furnace can provide efficient, clean renewable heat for many years when maintained and used correctly.

Alternatives to Outdoor Wood Furnaces

While outdoor wood furnaces offer benefits, there are some alternative heating systems to also consider:

• Indoor wood stoves – More aesthetic, radiant heating. Less efficient.
• Geothermal heat pumps – Use subsurface earth temperature for heating and cooling. Expensive to install.
• Solar heating systems – Harness thermal energy from the sun. Limited cold-weather output.
• Natural gas/propane furnaces – Fossil fuel-based. Lower emissions than wood. No fuel storage.
• Electric boilers – Convenient. Higher operating costs. Dependent on grid.
• Pellet stoves – Use compressed wood pellets. More automated. Smaller capacity.

Each heating method has pros and cons. An outdoor wood furnace offers renewable energy self-sufficiency but requires labor for wood processing. The optimal system depends on the needs and situation of each individual homeowner.

Conclusion

Outdoor wood furnaces can provide an economical, sustainable method of heating a home and domestic hot water using renewable wood as fuel. The system works by burning wood in an insulated outdoor firebox which heats water, then pumps the hot water underground to exchange heat with the interior space.

Modern EPA-certified furnaces have enhanced efficiency and lower emissions than older models. When properly installed and maintained, outdoor wood furnaces offer independence from conventional heating fuels. However, they require significant effort in firewood preparation and loading. Overall, outdoor wood furnaces remain an attractive heating option for the self-sufficient homeowner willing to perform the necessary work to reap the benefits of heating with renewable wood.

how does outdoor wood furnace work

An outdoor wood furnace, also known as an outdoor boiler or outdoor wood stove, provides heat and hot water for homes by burning wood as fuel. Here is an overview of how an outdoor wood furnace works:

Key Components

• Firebox – This is the combustion chamber where wood fuel is loaded and burned. It is surrounded by a water jacket.
• Water jacket – The hollow walls around the firebox, filled with water, that absorb heat from the burning fire.
• Heat exchanger – Transfers heat from the heated water to the home’s air or water heating system.
• Underground piping – Insulated piping that runs from the outdoor furnace to the home to carry hot water.
• Circulation pumps – Pump water through the closed system between the furnace and home.
• Chimney – Exhausts smoke and gases from wood combustion.

How it Works

1. Firewood is placed into the firebox and burned.
2. As firewood burns, heat is generated and absorbed by the water in the surrounding water jacket.
3. This heated water circulates through underground insulated pipes to the home.
4. Inside the home, a