The second improvement is a metal channel adjacent to the firebox that heats secondary combustion air and feeds this preheated air into the top of the fire, providing fresh oxygen without reducing the fire’s temperature. This allows for volatile gases and particulates to be further combusted in the top of the firebox.
The third innovation is the catalytic converter, a metal honeycombed chamber that allows the wood smoke to combust at about half the temperature—about 500°F. The catalyst itself often reaches temperatures of 1,500°F. Catalytic converters work great when new (as they are when tested) but the catalytic element degrades with use. Ironically, by the time the catalyst is due for replacement, emissions are a lot higher than 7.5 grams per hour limit set for noncatalytic wood heaters, according to WoodHeat.org. An additional technological improvement, coming out of Northern Europe and especially Denmark, are stoves that regulate secondary air intake using oxygen sensors, which results in a reliably hot, and thus more efficient and low-emission, fire. The sensors work in conjunction with regulating the airflow. If the oxygen is low, more air is admitted, and vice versa.
The firebox size and the heater’s efficiency determine how often it will need to be stoked, known as its burn time. Because they are low-mass, these types of wood-heating appliances have a high heating efficiency—they rapidly transfer heat produced by the fire to the room, creating a very hot space around them. They also cool fairly quickly when not in use. Depending on your heating needs, this may be an advantage or disadvantage. It may be difficult to regulate the heat output so that it is comfortable, but restricting the air supply to regulate heat output reduces combustion efficiency, creating more emissions.
Some metal stoves may have enough horizontal surface to serve as a cooking surface. If the power goes out, you’ll still have space heating—and a place to cook meals.
Pellet heaters. Instead of using logs, pellet heaters burn little compressed nuggets of sawdust or wood shavings. Because their emissions are lower, wood pellet heaters may be a better choice in places where population densities are higher, such as in suburban areas, which average between 1,000 and 3,000 people per square mile. Because pellets burn hotter than cordwood and more completely, they produce fewer particulate emissions. The regularity of the shape of the pellets and their high surface area compared to cordwood lends to their burning more evenly.
Pellets are first loaded into a hopper and an electrically powered auger conveys them into the firebox, where they are burned. One fan moves air through the firebox to combust the pellets, and another fan blows heated air into the room. Compared to a typical wood heater, a pellet heater is easier to operate. It also burns longer without attention and burns more efficiently.
While pellet heaters cost more, this expense can often be recouped in lower installation costs since these heaters can often be vented through a sidewall instead of through the roof, although this is subject to model type and local building codes. Pellet heaters require annual maintenance, which tends to be more costly than wood heater maintenance because of their increased complexity, but require less-frequent ash removal during the heating season. If you want to have a secure source of heat during power outages, you’ll need a generator or battery backup for your pellet heater, since it relies on an electric screw conveyor and blower.
In many suburban areas, pellets are readily available. While obtaining pellets is usually not difficult in forested areas where there is a wood products industry, the sustainability of these products can be somewhat questionable, since they take energy to manufacture and transport. In some areas, Forest Stewardship Council-certified wood pellets may be available. If not, inquire from the retailer about their origin.