Overview of Outdoor Wood Furnace System
An outdoor wood furnace system consists of the furnace unit, underground insulated piping, and the heat exchanger:
- Furnace Unit – This is located outdoors, with a firebox surrounded by a water jacket. Wood is burned in the firebox, heating the water in the jacket.
- Underground Piping – Insulated PVC or HDPE pipes carry the heated water underground to the home. This piping runs 3-4 feet below the frost line to prevent freezing.
- Heat Exchanger – Located in the basement, this device transfers the heat from the hot water into the home’s existing heating system.
How an Outdoor Wood Furnace Works
Outdoor wood furnaces work via a straightforward process:
- Wood is loaded into the firebox and ignited. Hardwoods like oak and maple are ideal fuels.
- As the fire burns, it heats the water reservoir surrounding the firebox to 160-180°F.
- This heated water circulates through the underground insulated pipes, traveling to the indoor heat exchanger.
- The heat exchanger transfers the heat from the hot water into the existing heating system of the home.
- Cooled water flows back to the outdoor furnace unit, where it is reheated.
- The cycle continues uninterrupted, 24/7, during the heating season.
Key Components and Operation
Outdoor wood furnaces have several key components that enable them to generate and transfer heat efficiently:
Firebox and Combustion Chamber
This is where the wood fuel is loaded, ignited, and burned. Fireboxes are lined with firebrick orsteel to withstand high temperatures. Baffles guide airflow and optimize combustion.
Water Reservoir
Surrounding the firebox is a water jacket or reservoir that holds 30-80 gallons of water. As the fire burns, this water is continuously heated to 160-180°F.
Insulated Underground Piping
Heated water is pumped through insulated underground PVC or HDPE pipes to the home. These pipes are buried 3-4 feet below the frost line.
Draft Inducer Fan
This fan improves combustion by optimizing airflow into the firebox. It allows the furnace to “breathe” better.
Heat Exchanger
Located indoors, this device has a coil containing the heated water from the furnace. It transfers thermal energy into the existing heating system.
Controls
Sensors monitor indoor/outdoor temperatures and water temperature. The thermostat controls the water circulation pumps when heat is needed.
Efficiency and Heat Output
The efficiency and heat output of an outdoor wood furnace depend on:
- Furnace design – Newer models have enhanced insulation and air circulation.
- Wood moisture content – Driest wood burns most efficiently. Below 20% moisture is ideal.
- Fuel loaded – Greater mass of wood fuel burns longer and hotter.
- Heat loss of home – The furnace must offset all the heat loss via insulation, air leaks, etc.
Many newer outdoor wood furnaces are 65-75% efficient. With ideal conditions, they can output 60,000-90,000 BTUs per hour and heat 2,000-3,000 square feet.
Benefits of Outdoor Wood Furnaces
Outdoor wood furnaces have several advantages that make them a popular heating choice:
- Lower heating costs – Burning wood is much cheaper than propane or fuel oil. Reduces dependency on conventional fuels.
- Simple installation – Furnace unit requires minimal site prep and connects to existing heating system.
- Renewable fuel source – Burning wood is carbon neutral since trees absorb CO2 as they grow.
- Self-sufficient heating – Lets homeowners utilize wood from their own property to heat their home for free.
- Safe operation – Located outdoors away from the home, risks of fire or fumes are minimized.
- Long-lasting – Stainless steel furnace units withstand decades of use if properly maintained.
Considerations Before Purchasing
There are several factors to weigh before investing in an outdoor wood furnace:
- Upfront cost – The furnace unit, installation, and piping can cost $6,000-$12,000 total.
- Wood consumption – Requires burning 4-8 full cords of wood to heat all winter. Need storage space.
- Electricity required – Most units run on household power to operate pumps and fans.
- Labor for operation – Daily loading/unloading of wood and removing ash. Not fully automated.
- Emissions – Older models release more smoke and particulates. New EPA-certified models are cleaner.
Take these considerations into account before deciding if an outdoor wood furnace is the right heating solution for your household.
Frequently Asked Questions
How much wood does an outdoor furnace use?
A typical outdoor wood furnace burns 4-8 full cords of wood over a heating season, depending on climate, home size, and insulation. One cord equals a stack of split firewood 4 ft x 4 ft x 8 ft.
What size furnace do I need?
Choose a furnace sized for your climate and square footage. Most homes require a unit with at least 80,000 BTU heating capacity. An installer can help match a furnace to your heating requirements.
How do I connect it to my home?
The furnace hooks into your existing heating system, usually through a heat exchanger with a pump that circulates water. This heats air or radiators in the normal distribution system.
What maintenance is required?
You’ll need to remove ash 1-2 times per week. Annually inspect components and seals. Drain and flush the system. Retest your underground pipes for leaks.
How long will an outdoor wood furnace last?
With proper maintenance and protection from elements, the furnace unit can have a service life of 25+ years. The underground piping lasts 15-25 years typically.
Conclusion
Outdoor wood furnaces are a reliable, cost-effective way to heat a home using renewable wood as fuel. They burn wood safely outside, transferring heat via hot water circulated through underground pipes. Newer EPA-certified models are extremely efficient. While requiring an initial investment and ongoing labor for fueling, many homeowners appreciate heating self-sufficiently using their own wood supply. Considering your climate, heating needs, and site constraints will determine if an outdoor wood furnace is the right solution for your household.