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How Does an Outdoor Wood Furnace Work?

Outdoor wood furnaces, also known as outdoor boilers or hydronic heaters, are standalone units that provide heat and hot water by burning wood. They work by heating water that is then circulated through insulated underground pipes to provide radiant floor heating or heat exchangers to warm air.

How an Outdoor Wood Furnace Works

An outdoor wood furnace consists of a firebox surrounded by a water jacket. A wood fire is lit inside the firebox which heats up the water in the surrounding jacket. This hot water is pumped through insulated underground pipes into the home’s existing heating system. Here are the main components and how they work together:

Firebox

This is the combustion chamber where wood fuel is loaded and burned. It is normally lined with high temperature refractory bricks or castable refractory cement. The firebox has an insulated door and air vents to control air flow.

Water Jacket

The water jacket surrounds the firebox and holds water that captures heat from the fire. It is typically made of welded carbon steel plates sealed to be watertight. The heated water flows out through outlet pipes near the top.

Heat Exchanger

This device transfers heat from the hot water into forced air for duct heating systems. It consists of a series of tubes or plates allowing air to pass over the hot surface to absorb heat.

Plumbing Connections

Insulated PEX or PVC pipes connect the outdoor furnace to the existing heating system in the home. One pipe loop provides hot water supply while a second returns cooler water back to the furnace.

Circulation Pump

A pump is used to circulate the hot water generated by the outdoor furnace through the insulated underground pipes into the home’s distribution system. It is sized to overcome the pressure drop through the system.

Water Jacket Thermostat

This sensor monitors water temperature in the furnace jacket and triggers the circulation pump when the target temperature is reached. It prevents circulation until there is sufficient heat.

Draft Fan

The draft fan pulls air into the firebox to feed oxygen to the fire. It provides a small amount of forced draft to improve combustion efficiency and heat output.

Chimney

Exhaust gases and smoke exit through the chimney. The chimney height creates draft to pull in combustion air. Proper chimney design is critical for good performance.

Aqua stat Control

This acts as the thermostat, monitoring water temperature leaving the furnace and calling for heat if the temperature drops below the setpoint. It controls the burn rate based on demand.

Safety Sensors

High temperature and pressure switches shut down the furnace safely if operating conditions exceed safe limits. These protect against overheating or inadequate water flow.

How an Outdoor Wood Furnace Burns Wood

An outdoor wood furnace burns wood fuel in a process called gasification combustion. This occurs in stages as wood is heated and broken down into combustible gases that burn more cleanly and efficiently than a plain wood fire. Here are the stages of wood gasification:

Heating and Drying – Initially wood fuel is heated to drive off moisture. Temperatures reach 212°F releasing absorbed moisture from the wood.

Pyrolysis – As temperatures increase to 500-1100°F, pyrolysis occurs where heat breaks down the wood into char, condensable vapors, non-condensable gases, and water vapor.

Gasification – At 1100-1600°F the char begins to release combustible gases including hydrogen, methane, carbon monoxide, and other hydrocarbons. Air flow is limited at this stage.

Combustion – The combustible pyrolysis gases mix with air and ignite above 1100°F, producing flame above the char bed. This secondary combustion stage provides most of the heat.

Charcoal Burnout – Remaining char combusts with oxygen to produce ash, carbon dioxide, and water vapor. Heat output diminishes as char is consumed.

This multi-stage gasification enables clean, efficient burns. Combustion of the wood gases occurs at very high temperatures for more complete burns. The process also reduces creosote buildup and allows use of less seasoned wood.

Heating Modes of Operation

Outdoor wood furnaces have different operating modes depending on the heating demand:

Idle – When no heat is required, the furnace remains lit but the burner output is minimized by closing down air supply to produce a low, smoldering fire.

Startup – To raise water temperature, air supply is increased to produce a hotter fire. More wood may be added until the target temperature is reached.

Steady State – At normal operating temperatures, the air supply is regulated to match heat demand and maintain the hot water set point.

High Demand – During peak heating loads, a vigorous fire is maintained by maximizing air flow. More wood is burned to meet the increased energy demand.

Burnout – As firewood is consumed, the remaining charcoal burns down marking the end of a burn cycle. The furnace remains off until the next firing.

Refractory Cure – When firing a new furnace for the first time, low temperature curing fires help condition refractory surfaces and prevent damage.

Warm Weather Shutdown – During warmer seasons, the furnace may be shut down completely with no fires. The heating system can run off domestic hot water.

Understanding these operating modes allows properly managing the burn process for efficiency and longevity. An outdoor furnace should not be left to smolder in idle mode for extended times.

Benefits of Outdoor Wood Furnaces

Outdoor wood furnaces offer several advantages over indoor wood stoves:

  • Whole Home Heating – A single furnace can provide both space heating and domestic hot water for an entire house.
  • Central Heating System Integration – They tie into existing ductwork and radiators using standard heating components.
  • Constant Heat – Outdoor furnaces maintain water temperature 24/7 for consistent heating without temperature swings.
  • Increased Safety – With the fire located safely outdoors, risk of fire or carbon monoxide poisoning is greatly reduced.
  • Flexibility – They can burn scrap and lower grade woods like branches and tree tops other stoves can’t handle.
  • Cost Savings – For properties with access to free firewood, outdoor furnaces can provide huge savings on heating bills.
  • Off-Grid Capability – They allow heating homes completely independently without electrical or fossil fuel dependency.
  • Environmentally Friendly – Burning wood is considered carbon neutral since re-grown trees recapture the CO2 released.

With proper care and maintenance, a quality outdoor wood furnace can provide decades of reliable, affordable whole home heating.

Choosing an Outdoor Wood Furnace

Important factors to consider when selecting an outdoor wood furnace include:

  • Efficiency – Look for high combustion and heat transfer efficiency ratings to maximize heat output from the wood fuel burned.
  • Output Capacity – Match furnace output BTUs/hr to the heat load requirements of the home based on climate and square footage.
  • Burn Controls – Advanced secondary combustion systems extend burn times and regulate heat output.
  • Insulation – A heavily insulated firebox and water jacket improve efficiency and reduce wood consumption.
  • Construction – Durable welded steel construction and refractory lining prevent corrosion and premature failure.
  • Chimney Height – A proper draft is needed to prevent smoke spillage so sufficient chimney height is critical.
  • Warranty – A long warranty on critical furnace components provides peace of mind on the investment.
  • Cost – Consider both purchase price and long term operating costs when comparing furnace options.

Selecting a high quality, correctly sized outdoor wood furnace suited to the heating application will ensure many years of optimum performance.

Installing an Outdoor Wood Furnace

Proper installation of an outdoor wood furnace is critical for efficient, safe operation. The key steps are:

Foundation – A reinforced concrete pad keeps the furnace elevated and stable.

Clearances – The furnace must be located at least 25-50 feet from any structure with ample room around it for loading.

Chimney – A chimney that meets height requirements should be installed, typically 15-30 feet above ground depending on location.

Underground Piping – Insulated PEX or PVC piping rated for outdoor use is buried below the frost line to connect to the heating system.

Permits – Building permits and inspections are typically required for the installation. Zoning laws may impact placement.

HVAC Integration – The furnace is tied into the existing heating and hot water systems with all necessary components.

Start-up and Testing – Proper start-up by the installer and testing for leaks and performance ensures everything is working correctly.

Refractory Curing – Initial low temperature fires season the refractory panels. This is critical for castable refractory linings.

Following the manufacturer’s detailed installation instructions combined with local building codes allows the furnace to be brought online safely and correctly.

Operating and Maintaining an Outdoor Furnace

To keep an outdoor wood furnace running cleanly and efficiently, proper operation and maintenance practices are required:

  • Burn only seasoned wood with a moisture content below 20%. Wet wood leads to excess smoke and creosote.
  • Inspect door seals and replace if worn to maintain tight door closure.
  • Empty ashes regularly before they build up and block air flow.
  • Keep the area around the furnace clear of vegetation and combustible materials.
  • Inspect the chimney and pipes annually for creosote and clean as needed.
  • Check for any water leaks and repair immediately to avoid corrosion damage.
  • Lubricate circulation pumps and motors according to manufacturer specifications.
  • Monitor refractory and repair any cracks or damage to prevent further deterioration.
  • At end of season, clean out interior and drain water completely if shutting down for warm months.

With good operational habits and preventative maintenance, an outdoor wood furnace can provide many years of trouble-free heating.

Troubleshooting Common Outdoor Furnace Problems

Some typical problems encountered with outdoor wood furnaces include:

Smoking or Burning Improperly – This is often due to wet wood or poor draft caused by an issue like chimney blockage.

Not Reaching Temperature – A system that won’t get up to temperature may have pump problems or air leaks allowing heat loss.

Overheating – Common causes are a failed aquastat or a pump not circulating water through the heat exchanger as expected.

Creosote Buildup – Excess creosote indicates incomplete combustion from issues like poor draft, bad fuel, or operating problems.

Leaking – Leaks in the water jacket or plumbing must be repaired quickly to prevent corrosion damage, freezing, or other dangerous situations.

Burns Too Fast – Fast burning suggests leaks allowing too much air into the firebox or burn settings needing adjustment.

Noise or Vibration – Worn motor mounts, bad pump bearings, and loose components can cause abnormal mechanical noises.

Careful troubleshooting and maintenance keeps problems at bay. But major issues beyond DIY repair may require a qualified technician.

Safety Best Practices

When operated properly, outdoor wood furnaces are safe systems, but important safety precautions should still be followed:

  • Keep the furnace area clear of leaves, brush, and other combustible materials.
  • Never leave for extended periods with the furnace unattended or in idle smoldering mode.
  • Do not burn trash, painted or treated wood, or anything other than firewood.
  • Watch that children and pets keep a safe distance from the furnace.
  • Install smoke and CO detectors as an added safety measure.
  • Handle ashes carefully and correctly to prevent fires. Allow ashes to fully cool before disposal.
  • Keep fire extinguishers nearby and ready in the rare event of an external fire.
  • Always follow the manufacturer’s operation instructions.

With sensible precautions, outdoor wood furnaces can be operated for many years with minimal risk. But complacency or neglecting proper practices can lead to potential hazards.

Frequently Asked Questions

Q: How much wood does an outdoor furnace burn?

A: Actual consumption depends on unit size, home heat loss, and climate but a typical rule of thumb is 4-6 cords of wood per heating season.

Q: What size outdoor wood furnace do I need?

A: Size it based on the heat load requirements of the home using around 60,000-80,000 BTU output capacity per 1000 sq ft of space heated. An oversized furnace will cycle more and burn less efficiently.

Q: How do I obtain a permit for an outdoor furnace installation?

A: Contact your local municipality about permitting requirements which normally entail an application with details on the installation scope and equipment. Permits help ensure compliant, safe installations.

Q: Can an existing home heating system work with an outdoor furnace?

A: In most cases, the outdoor furnace can integrate with existing forced air or radiant systems with the addition of a heat exchanger or plumbing into the existing hydronic system.

Q: What maintenance does an outdoor wood furnace require?

A: Ash removal, chimney cleaning, replacing door gaskets, checking for leaks, lubricating pumps, inspecting refractory, draining water, and other basic tasks are required annually.

Conclusion

Outdoor wood furnaces are a time-tested way to heat homes using renewable wood fuel. When sized, installed, and operated correctly, these hydronic heating systems provide economical whole home heating for years.

Following best practices for installation, maintenance, and operation will ensure optimal performance and longevity from an outdoor wood furnace. With a properly functioning outdoor furnace, stay warm and save money while heating with sustainable wood.

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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