Buying a Wood Furnace
Updated Oct 26, 2016
By Gary Sprague
An outdoor wood furnace is a furnace that burns wood or pellets to heat water (or air) for use in hot water (or warm-air) heating systems. For people with a ready supply of firewood a wood-fired furnace can be a safe and economical way to reduce home energy costs. Because of new EPA regulations, most of today's wood furnaces are hydronic gasification furnaces, which require less fuel while producing fewer emissions.
How Wood Furnaces Work
Outdoor wood furnaces installed before the EPA changes are made up of a cabinet, a sealed firebox, a blower and access doors for stoking the fire and cleaning. Firewood (or other fuels) are burned in the firebox, and that fire heats the water. The hot water is then piped to the building you want to heat though insulated underground pipes.
The next step depends upon the existing mechanism for distributing heat in your home or office. If you have a warm-air furnace and ductwork, heat is removed from the water via a water-to-air heat exchanger and used to heat the air circulating through the ductwork. If you have a forced hot water system, the water can be piped directly into that system. Once the heat has been removed, the cold water is circulated back out the wood furnace and heated again.
Many furnaces have a blower fan that can be used to enlarge the fire, and create more heat. There are also combination wood furnaces that have a gas or oil burner or electric elements as backup. Wood furnaces may also have accessories like electronic air cleaners, humidifiers, and an evaporator coil for central air, or a coil for heating domestic hot water.
Gasification furnaces, in contrast, have two chambers. When wood (or other fuel) is put into the feed or primary chamber, it begins to emit gasses which mix with oxygen and flow into the secondary reaction chamber. The gases are then heated at temperatures around 2000 degrees Fahrenheit, creating an extremely clean burn. The heat is then transferred to the water jacket or heat exchanger to be used to heat a home or other building. The gasification process produces far less emissions while producing longer burn times and greatly reducing wood usage. Many of these furnaces are also designed to burn coal and biomass.
Pros And Cons Of Wood Furnaces
The most commonly mentioned advantage of a wood furnace is that they can dramatically cut your energy bills. This is especially true if you need a heating system that heats several buildings, such as your home, a workshop, and perhaps an outdoor sauna.
Before recent EPA changes, emissions were a real problem. The Northeast States for Coordinated Air Use Management (NESCAUM) released a report in March 2006 that was very critical of the emissions produced by outdoor wood furnaces. In particular they found that average real-world emissions were "twenty times higher than the average in-use emissions of an EPA certified wood stove." The report speculated that these emissions could cause an increase in respiratory and lung problems for people living near outdoor wood furnaces.
In 2015, the U.S. Environmental Protection Agency (EPA) established the first-ever federal air standards for outdoor wood boilers. These standards, which apply only to new heaters and not those already installed and in use, will phase in emission limits in two phases, Phase 1 (already in effect) and Phase 2 (2020).
Phase 1 requires that all hydronic heaters sold after December 31, 2015, meet an emissions limit of 0.32 pounds per million Btu heat output. Phase 2 requires that by 2020, hydronic heaters meet the lower emissions limit of 0.10 pounds per million Btu heat output. It is believed that these standards will help reduce emissions by up to 80%.
To be EPA-certified, an appliance has to meet regulatory emission requirements established by the EPA by May 15, 2015 (existing inventory was allowed to be sold through December 31, 2015). These appliances included wood and pellet stoves, fireplace inserts, hydronic heaters (outdoor wood boilers), and forced air furnaces (not in effect until May 15, 2016 for small units and May 15, 2017 for large units).
EPA-certified is not the same as EPA-qualified. Products including fireplaces and outdoor coal furnaces are not currently regulated or certified by the EPA, but manufacturers may choose to meet voluntary EPA standards. By meeting these standards, appliances are considered EPA-qualified and burn more cleanly than unqualified units.
The EPA estimates the health benefits of the new requirements for residential wood heaters at $3.4 to $7.6 billion annually. The health benefits include avoiding asthma attacks, emergency room visits and lost work days, among others.
While emissions and the resulting smoke and smell that goes with it are greatly reduced, there are still disadvantages to owning one. An outdoor gasification wood furnace is not practical for use in an urban area. Also, there is the problem of loading the furnace. However, tThere's no magic to this -- you need to have a ready supply of fuel and a willingness to feed that fuel into the furnace on a regular basis – usually twice a day, every 12 hours.
Installing an Outdoor Wood Boiler
Typically the manufacturer of the furnace will arrange for it to be delivered to your home or business. They will place the furnace somewhere between 20 - 200 feet from other buildings. The installation procedures require that a trench be dug from the furnace to the point at which you will connect the furnace to your existing heating system. Insulated pipes are then buried in the trench to circulate the hot water. There is generally some technical work needed to connect the furnace to your heating system. If you don't know a contractor that can install a wood boiler for you, HomeAdvisor can help you find one.