Introduction
A outdoor wood furnace, also known as an outdoor wood boiler or outdoor wood-fired hydronic heater, is a heating system that burns wood to heat water which is then circulated through insulated underground pipes to provide heat and hot water to a home or building. Outdoor wood furnaces allow the noisy, dirty mess of burning wood to be located outside while easily providing heat and hot water inside.
Outdoor wood furnaces have grown in popularity in recent years as a cost-effective and self-sufficient heating method, especially in rural areas. However, they have also generated some controversy regarding their environmental impact and potential health hazards from smoke emissions.
In this comprehensive guide, we will examine how an outdoor wood furnace system works, its components, types of outdoor wood furnaces, tips for safe and efficient operation, cost considerations, environmental impact, pros and cons, and comparisons to other heating systems. Whether you are considering purchasing an outdoor furnace or just curious how they operate, this article covers all the key details about how outdoor wood furnaces work.
How Does an Outdoor Wood Furnace Work?
The basic operation of an outdoor wood furnace involves burning wood in an insulated firebox to heat water in a surrounding jacket. The heated water is circulated through underground insulated pipes to transfer heat to the home’s existing heating system and provide hot water. Here is a step-by-step overview of how an outdoor wood furnace works:
1. Loading the Firebox
The firebox is loaded with wood through a front-loading door. Many larger capacity furnaces can be loaded less frequently by using large, full logs. Smaller furnaces may need to be loaded more often with shorter logs or wood splits. The wood should be seasoned and dry for clean burning.
2. Combustion and Heat Transfer
As wood burns in the firebox, combustion releases heat which transfers into the water jacket surrounding the firebox. This heated water reaches temperatures of 180-200°F.
3. Circulation Pumps
This hot water is circulated by pumps through underground insulated pipes to the building. One set of pipes provides hot water to the existing heating system’s return. Another set may connect to the domestic hot water heater.
4. Heat Exchange
The circulating hot water releases its heat through heat exchangers in the existing heating system, whether a forced air furnace, boiler for radiators, or air or water-source heat pump.
5. Domestic Hot Water
If connected to the domestic water lines, the hot water from the furnace can also provide direct hot water for faucets, showers, washing, etc. This can drastically reduce water heating costs.
6. Thermostat Control
The existing thermostats in the building control the circulating pumps. When heat is called for, the pumps turn on to circulate the hot water from the furnace through the system. The furnace itself operates independently to maintain water temperature.
This basic concept allows an outdoor wood furnace to harness the heat from burning wood to provide warm floors and hot water inside your home, all controlled conveniently from existing thermostats. The separate firebox and hot water circulation allow the smoke and ash to be eliminated outside.
Key Components and Systems
Outdoor wood furnaces incorporate a number of important components and design factors:
Insulated Firebox
The firebox is heavily insulated to maximize heat transfer efficiency into the water and minimize heat loss. Firebrick refractory lining helps retain heat. High temperature insulation gaskets seal the doors tightly.
Water Jacket
Surrounding the firebox is a water jacket which holds and heats clean water (with antifreeze for freeze protection) up to 180-200°F to provide the circulating hot water supply.
Draft Controls
Air intake dampers and flue gas outlet dampers precisely control the airflow and draft through the firebox for clean, efficient combustion.
Air Supply
Air is drawn into the unit through a separate outside air intake pipe to supply oxygen for combustion. This avoids depleting indoor air quality.
Smoke/Heat Exchanger
Some models optimize efficiency by routing the smoke through a heat exchanger that absorbs additional heat before the smoke exits the chimney.
Low Water Cutoff
A crucial safety device shuts down the furnace if water drops too low to avoid hazardous overheating.
High Temp Limit Switch
Similarly, if the water overheats above 210°F, this switch will immediately stop the combustion process for safety.
Backflow Preventer
An important code requirement protects against cross-contamination of potable water with a special valve on the domestic hot water supply.
In addition to these internal components, the external connections and circulation required for an outdoor wood furnace include:
Underground Piping
The heated water is piped underground through heavily insulated pipes (R-16 or higher) 4-6 feet below grade to prevent freezing.
Supply and Return Connections
The pipes connect to supply and return lines of existing heating system and hot water heater (if used for domestic hot water).
Circulation Pumps
Pumps designed for the high water temperatures reliably circulate the hot water from the furnace into the building systems.
Thermostatic Control
The existing thermostats in the building operate the circulation pumps when heat is required. No power is needed at the furnace itself.
With these key components and systems in place, an outdoor wood furnace can safely deliver the heat from burning wood into existing heating and hot water systems inside the home. Proper installation and operation are critical, however, as we will explore next.
Installation and Operation
Proper installation and operation help ensure an outdoor wood furnace performs safely, efficiently, and trouble-free:
Siting
The furnace should be located at least 100 feet from any residences and have an insulated chimney tall enough to avoid smoke drifting toward the building. Follow all codes for clearances. Avoid low lying areas prone to cold air drainage.
Foundation
A reinforced concrete pad helps keep the furnace level and allows it to be secured to the foundation.
Piping
The piping size, layout, depth below frost, insulation rating, and installation technique must be carefully engineered.
Electrical
A qualified electrician handles the electrical hookup and the pumping system. The pumps should be on a dedicated circuit sized for the loads.
Existing Systems
A heating contractor ensures the existing heating and hot water systems are compatible and integrated properly with the new furnace.
Condensation Drain
Condensate during startup must be able to drain freely without freezing. This requires an adequately sloped and protected drain line.
Dry Wood
For cleanest burning, only well seasoned wood with a moisture content below 20% should be burned. An external wood shelter aids drying.
Loading
Filling the firebox full allows for longer, complete burns. Larger pieces stacked tightly prevent excess air from wasting heat up the chimney.
Draft Controls
The dampers must be precisely set to balance the fire’s air intake and exhaust draft through the firebox.
Maintenance
Ash, creosote, gaskets, pumps, and the chimney require regular inspection and cleaning for long term performance.
Operator Training
The owner/operator should thoroughly study the manufacturer’s operating and maintenance instructions to ensure excellent results from their investment.
With proper installation and knowledge to operate the furnace optimally, an outdoor wood furnace can provide years of reliable, low cost heat.
Types of Outdoor Wood Furnaces
If you are considering an outdoor wood furnace, you will find a variety of designs and features available:
Traditional Models
The simplest, most affordable outdoor wood furnaces have a single combustion chamber surrounded by the water jacket. These require more frequent loading with wood.
High Capacity Models
Large furnaces hold bigger loads of wood in an oversized firebox for less loading frequency. Advanced secondary combustion chambers also prolong burn times.
Indoor/Outdoor Models
Some furnaces offer both an indoor firebox for quicker heating and optional outdoor firebox for milder weather. A ducted system switches heating between the units.
Pellet-Fired Models
Instead of wood, some units burn compact wood pellets using an automatically fed burner system. Pellets have low moisture and high density for maximum heat.
Multi-Fuel Models
The most versatile models allow burning wood, wood pellets, corn, biomass pellets, and other fuels. This flexibility lets you utilize whatever fuel is most affordable and available.
EPA Certified
The highest emission standards are met by EPA certified furnaces passing strict testing for particulate and smoke emissions, although these models are more expensive.
By evaluating these key differences, you can select the optimal outdoor wood furnace design for your heating needs, property, climate, and budget.
Tips for Safe, Efficient Operation
Here are some top tips and best practices for operating your outdoor wood furnace safely, efficiently, and economically:
- Only burn dry, seasoned hardwood for clean combustion and maximum heat value. Softwoods burn too quickly. Green wood smolders inefficiently.
- Ensure air intakes are clear and chimneys are clean for proper airflow.
- Load the furnace fully each time up to the recommended capacity to allow for complete combustion.
- Stack wood tightly, organized front to back and side to side for most efficient heating.
- Monitor water temperatures and do not exceed 200°F. Higher heat wastes fuel.
- In mild weather, burn smaller, hotter fires to prevent excessive idling and smoldering.
- Utilize larger diameter piping and higher performance insulation to minimize heat loss in underground lines.
- Keep furnace doors tightly sealed and lock down latches to prevent air leakage and maximize heat transfer into the water jacket.
- Follow a regular maintenance schedule for removing ash, cleaning flues, testing door gaskets, maintaining water chemistry, etc.
- Take advantage of federal tax credits up to 26% of the furnace cost through 2023 for high efficiency models.
Using these operational best practices allows an outdoor wood furnace to burn wood cleanly and productively while reducing fuel consumption.
Cost Considerations
Outdoor wood furnaces represent a significant investment, but one with attractive potential payback:
Initial Costs:
- Furnace unit: $5,000 – $15,000+
- Installation: $4,000 – $10,000 for foundation, piping, pumps, electrical
Operational Costs:
- Wood: Varies by region, typically $100 – $300 per cord split and delivered
- Electricity: For running pumps, minimal usage
Savings:
- Up to 80% reduction in existing heating bills
- Payback period: as little as 2-5 years depending on energy costs
Additional Expenses:
- Wood storage structure
- Chimney installation
- Annual maintenance
Although the upfront cost is steep, an outdoor wood furnace becomes extremely affordable after the payback period when considering the minimal operating expenses using wood as fuel. Hardwood from sustainably managed forests or woodlots also makes heating with wood a renewable, self-sufficient option.
Environmental Considerations
Outdoor wood furnaces have sometimes received criticism over their environmental impact, namely:
- Smoke and particulates – Older units produced excess smoke and particle pollution. Modern EPA certified furnaces have extremely low particulate emissions under 0.32 lb/MMBtu heat output.
- Carbon and soot – Improper combustion can produce more carbon dioxide, carbon monoxide, and soot due to smoldering fires and unseasoned wood. Properly operated furnaces minimize these emissions.
- Wood sources – Harvesting wood in an unsustainable manner may deplete forests over the long term. Using managed woodlots, sawmill residues, urban wood waste, and fast growing energy crops as fuel sources prevents environmental harm.
- Location – If located too close to homes, smoke and fumes become nuisance issues. Adequate setback distances alleviate this concern.
Overall, when operated properly using dry wood and warm burn cycles, outdoor wood furnaces can provide a sustainable, renewable heating option with minimal environmental impact. Regular maintenance and conscientious wood sourcing are key.
Pros and Cons of Outdoor Wood Furnaces
Advantages of Outdoor Wood Furnaces:
- Provide space heating and hot water at a fraction of the cost of fossil fuel systems
- Allow self-sufficient heating with a renewable, affordable wood fuel source
- Free up indoor space previously used for a boiler or wood stove
- Eliminate mess and hazard of an indoor wood fire
- Integrate with existing heating systems and thermostats
- Long lifespan exceeding 25 years for many models
**Potential Drawbacks: **
- High initial purchase and installation cost
- Need to have sufficient dry wood supply and storage
- Fueling and ash removal is labor intensive
- Not approved for use in some municipalities
- Improper operation can produce excess smoke and emissions
- Requires electrical power for circulation pumps
- Risk of leaks from underground piping, if not installed correctly
If the benefits align with your heating requirements, outdoor wood furnaces offer an economical way to heat with wood while keeping the fire safely outdoors. But they require a substantial initial investment and responsibility operate and maintain properly.
Comparison to Other Heating Systems
Key differences compared to indoor wood stoves:
- No need to bring wood inside
- Connects to central heating system and switches automatically
- Fire risk is eliminated inside the home
- More advanced combustion technology
- Greater firebox capacity for longer burn times
Versus indoor wood boilers:
- Locates fire hazard outside
- Frees up indoor space
- Keeps smoke and mess out of living areas
- Convenient access for loading and maintenance
- Simpler installation without venting
Compared to fossil fuel furnaces and boilers:
- Lowers heating costs significantly
- Uses renewable fuel source unaffected by market volatility
- Avoids reliance on natural gas or fuel oil
- Comparable automatically controlled central heat
When selecting the optimal system, be sure to evaluate the pros and cons for your specific home heating requirements. Outdoor wood furnaces offer a unique combination of advantages over other heating options that make them an attractive choice for many homeowners.
Conclusion
Outdoor wood furnaces harness one of mankind’s most ancient heating fuels in a remarkably simple and sustainable system—burning wood outdoors provides clean, affordable heat and hot water indoors. With a basic understanding how outdoor wood furnaces work, you can decide if utilizing this renewable heating source makes sense for your situation. If so, be sure to work with qualified experts in sizing, siting, installing, and operating your furnace safely and efficiently. With proper care and maintenance, an outdoor wood furnace is a smart investment that returns heating savings and self-sufficiency for decades.