Heating With Wood As oil supplies dwindle and environmentalists and homeowners alike are becoming more and more aware of the need for new sustainable heating options, wood is re-entering the picture as a feasible renewable fuel alternative. But many wonder how clean and renewable it really is. Wood heating has evolved greatly from the days of wood cooking stoves and fireplaces as major heating sources. New gasification technology has made wood heating environmentally friendly by eliminating 90% of emissions released by conventional wood burning appliances. But with new technology and standards emerging constantly in the field, it is hard to make the right choices for yourself and for the environment.

The following article briefly outlines the major types of wood burning appliances and the major regulations addressing them.

Wood Burning Stoves A wood burning stove is an appliance which burns solid fuel to produce heat for a room or building through direct transfer. This includes decorative wood burning devices, such as fireplaces, which directly transfer heat from the fire to the adjacent room. A wood burning stove can also be used to heat an entire building by directing heated air through a home's duct work. Figure 1 shows the configuration of a wood burning stove with an electric backup. Notice, the heated air is transferred directly into the home's heating system.

Regulations: With the increasing use of wood burning stoves for both decorative and heating purposes, the United States Environmental Protection Agency (EPA) has developed emission standards and testing for wood burning stoves. The EPA certifies three types of wood burning stoves under the New Source Performance Standards (NSPS): wood stoves with catalytic converters, pellet stoves, and non-catalytic wood stoves with other emission reducing technology. A catalytic converter is a honeycomb shaped device, which burns gases in the smoke to reduce emissions. The EPA requires certified wood burning stoves with catalytic converters to maintain emission levels lower than 4.1 g/hr. Non-catalytic stoves are limited to 7.5 g/hr emissions.

EPA certified wood burning stoves are 80% cleaner and 30% more efficient than typical non-certified wood stoves.

Wood Burning Hydronic Heaters A wood burning hydronic furnace uses solid fuel combustion to produce indirect heat for residences or other buildings. A hydronic heater – also called a boiler – heats water, which is piped to the building. This heated water is used to heat the building through a water-to-air heat exchanger or through radiant in-floor heating. Heated air from combustion is cooled by the water before it is released through a chimney. Hydronic boilers are often located outside the building being heated. Figure 2 shows a classic setup for an outdoor wood burning boiler. Notice that heat reaches the home only by means of piped water, not heated air, as in Figure 1. A wood burning boiler is generally installed as the primary heat source for a building and are rarely used for purely aesthetic purposes.

Regulations: Wood burning boilers have gained popularity lately for their safety, cleanliness, and more dependable space and water heating. Due to their higher heating capacity and unique design, the EPA has applied a more stringent efficiency regulation to hydronic heaters: the White Tag and Orange Tag standard.

Because hydronic heaters produce more heat and are generally used as primary heating souces, regulations on hydronic heaters are based on emissions per unit of heat produced rather than on emissions per hour of operation. Phase 1 of this 2-Phase standard was implemented in 2008, requiring boiler manufacturers to maintain emissions under 0.60 lb/mmBTU heat input. Phase 2, the 2010 standard, forced emissions down even further to 0.32 lb/mmBTU heat output. The EPA rates Orange Tag (Phase 1) approved furnaces as 70% cleaner than conventional wood boilers and White Tag (Phase 2) approved furnaces as 90% cleaner than conventional wood furnaces.

Comparing Wood Burning Stoves and Hydronic Heaters The variety of wood burning appliances and regulations are difficult not only for homeowners, but also for legislators developing local wood furnace regulations. When choosing a wood burning appliance or developing regulations, three factors should be carefully considered: heat output, emissions and efficiency.

Heat Output Wood burning appliances are used for two main purposes: heating and aesthetics. When choosing a wood burning appliance as a primary heat source for your home or other buildings, it is essential to choose a furnace that produces sufficient heat. Hydronic heaters are generally intended for this purpose. EPA White Tag approved hydronic heaters range from 66,000 to 212,000 BTU/hr heat output. This range meets the heating requirement of the majority of North American homes.

Wood burning appliances installed for aesthetic purposes or for a combination of aesthetic and space heating purposes are generally used as secondary heat sources. EPA certified wood burning stoves have heat outputs ranging from 10,000 to 35,000 BTU/hr. As a heating source, these units rarely serve as primary heaters, but rather provide space heating for one or several rooms of a building heated by a separate primary heating source (often natural gas, oil or electric).

Emissions Wood combustion always releases gases. A large fire with excess oxygen and fuel, like a campfire, releases significant emissions into the air in its smoke. EPA emission regulations are based on the amount of dangerous gases present in the smoke when it leaves a wood burning device. Stove and boiler manufacturers today have developed new technologies to reduce emissions in the smoke. Secondary burners – catalytic converters in wood stoves and gasification chambers in hydronic heaters – burn gases from the smoke before releasing it to the atmosphere.

The EPA regulates safe emission levels for both types of appliances. On wood stoves, emissions are measured by grams per hour of burning. On hydronic heaters, emissions are regulated by grams per hour of operation and pounds per million BTU's heat output. While emission standards for wood stoves may appear to be much lower than those for hydronic heaters, they actually produce significantly higher emissions per unit of heat produced.

When calculating total emissions for a household, regulators should also consider the emissions produced by the primary heating source in a home operating a wood burning stove.

Efficiency New technology has made wood burning of all types a cleaner and more efficient process. When considering a wood burning appliance for heating purposes, keep in mind that higher efficiency means that more of the fuel's heating potential is reaching your home. Higher efficiency generally also means the fuel has gone through complete combustion, resulting in lower emissions and lower fuel costs.

Average efficiency and heat output levels based on published EPA data for NSPS Certified Wood Burning Stoves (2009) and White Tag Approved Wood Burning Hydronic Heaters (2009). **Maximum value for Wood Burning Stoves based on average heat output and maximum emissions. Sources: US Environmental Protection Agency. Natural Resources Canada.