Comparing Combustion Equipment
With all the different heating technologies available today, it can be difficult to make a decision about what system best fulfills your personal heating needs. Below you will find a discussion of the benefits and disadvantages of the main types of systems along with the range of heat outputs for each system, allowable emissions, and a general price range of a typical unit. Additional information can be found by visiting the links and resources on the left of the page. Here, we focus on wood stoves, pellet stoves, fireplaces , masonry heaters, and boilers. When considering what type of equipment is best for you, the most important considerations are to select the cleanest-burning equipment in its class and to size it and site it appropriately for your heating needs.
For those of you interested in combustion characteristics of the most efficient, cleanest-burning combustion equipment currently being made anywhere in the world, in April 2010, the New York State Energy Research and Development Authority (NYSERDA) published a report entitled European Wood-Heating Technology Survey: An Overview of Combustion Principles and the Energy and Emissions Performance Characteristics of Commercially Available Systems in Austria, Germany, Denmark, Norway, and Sweden. (The report is too large to link to here, but contact us if you'd like a copy.)
Another report that may be of interest to some is one summarizing biomass combustion in Europe, which was published in 2008 and was also commissioned by NYSERDA. This file is also quite large, so we can't link you to it here, but it is available by contacting us directly.
Note: We have included photos to illustrate the different types of combustion equipment, but the models shown should not be construed as a recommendation or endorsement of any specific manufacturer's product.
The amount of particulate matter released by your wood burning appliance is an important consideration when purchasing a new or used stove. In 1990, the EPA came out with regulations governing wood stove emissions that all wood stove manufacturers must comply with. These so-called Phase II-certified stoves release 15-30 fewer grams of smoke per hour than older, uncertified stoves. So it's important to replace older units and not buy any older used units you may come across through Craig's List or at a yard sale.
It's important to note that wood stoves are basically space heaters. They work best in buildings with an open floor plan, where the heat can circulate relatively easily. That said, if properly sized, they can very effectively heat rooms of just about any size. Most of the new wood stoves currently on the market have heat outputs ranging from 35,000-100,000 BTUs/hr with catalytic stoves tending to produce slightly higher heat outputs than non-catalytic stoves of the same size. Wood stoves range in price from about $1,500 to $5,000, including installation.
Catalytic wood stoves use a coated ceramic honeycomb to lower the temperature at which stove gases will burn. Because of the way they are designed and the fact that their efficiency depends heavily upon proper maintenance, catalytic stoves have a steeper learning curve for new wood burners. However, by passing the smoke through the catalytic honeycomb, your stove's efficiency sees a significant boost and its emissions are lessened as harmful particulate matter is burnt away. The EPA requires that a catalytic wood stove have a maximum emissions level of 4.1 grams of smoke per hour, which is lower than the emissions cap placed upon non-catalytic stoves.
The vast majority of the wood stoves that you will encounter will be non-catalytic. These stoves are regulated by the EPA to a maximum emissions level of 7.5 grams of particulate emissions per hour. As suggested by their name, they do not use a catalyst to burn the smoke produced through combustion. Instead, they utilize firebox insulation and baffles to recirculate combustion air to achieve more complete combustion, which increases your stove's efficiency and produces a cleaner burn.
Of the materials commonly used to construct stoves, soapstone is unique in that it stores and radiates heat for a longer period of time and at a more even temperature, despite fluctuations in burn intensity. Even as your fire dies down, the soapstone releases heat that it has stored throughout the burn, making it possible to warm your house at night without leaving a fire burning. Stoves made of cast iron or any other types of metal are unable to store warmth as effectively, are hotter to the touch, and are prone to giving off blasts of intense heat. However, be prepared to pay a higher price for the soapstone stove and wait longer after starting a fire for the unit to reach its peak heat output.
While manufacturers continuously make minor improvements to their equipment, for the most part, there have not been any major changes in stove design since the 1990s. One exception is a two-stage gasifying wood stove made in Germany by Specht (and imported to the U.S. by the New York firm Wittus-Fire by Design). The "xeoos TwinFire" heats up wood in one compartment in the relative absence of oxygen, which releases combustible gases into a separate compartment, where they are blasted with fresh air (no fans--and therefore no electricity--required). This two-stage gasification, normally seen in much larger wood boilers (see below) makes for a very complete combustion, resulting in very low emissions and high efficiency.
Of the biomass technologies currently available, pellet stoves may be the easiest to use. To operate a pellet stove, all you really have to do is load pellets into the hopper (and you don't even have to do this every time you use the stove!). The rest of the burn process is usually mechanized; in fact, most pellet stoves are self-lighting so you needn't even worry about starting the fire. The heat outputs of these units can range from 8,000- 90,000 BTUs/hr and you can expect to pay anywhere from $1,700- $3,000 for the stove alone. When installation is factored in, pellet stoves are generally more expensive than wood stoves.
You're paying for more than ease though. Pellet stoves do have significantly lower emissions than wood stoves. Although pellet stoves aren't currently regulated by the EPA, their emissions tend to be considerably lower than those of wood stoves (the EPA is working on new source point standards for all combustion equipment that will most likely include pellet stoves).
As with wood stoves, pellet stoves are basically space heaters, so they work best in an open floor plan. Also like wood stoves, there are two types of pellet stoves. The difference in this case rests in the pellet delivery system. Top-fed pellet stoves protect your pellet supply by decreasing the probability that the fire will burn back into the hopper. However, the trade off of the top-fed delivery system is that the combustion chamber becomes more likely to fill with ash and other debris. If you purchase a top-fed stove, it is wise to buy premium pellets that produce less ash. The other option is a bottom-fed pellet stove. Bottom-fed stoves create a lot of ash that collects regularly in the ash pan (meaning you'll have to empty this pan often) but there is no real need to buy premium pellets for a bottom-fed unit.
Pellet stoves also accept a variety of fuel types, sometimes in the same unit (called multifuel units). Most burn either wood or corn, but some also accept other types of pelleted fuels, including hulled wheat, cherry pits, waste paper pellets, and grass pellets. Don't try to burn a fuel your stove is not approved for, though, as some fuels can physically damage the stove.
While pellet stove systems are very easy to use and low on emissions, they do require electricity to run (~100 kWh per month) which means that power outages could interrupt your heat supply if you have not hooked up a generator or battery supply. Another possible negative of this system is the fan that draws air from your room into the stove and then returns the air after heating it. Some people find the noise from the fan to be bothersome, so be sure to test the stove out before buying it.
For a comprehensive discussion of pellet stoves, take a look at this article that appeared in Mother Earth News in 2009: www.motherearthnews.com/Renewable-Energy/Heat-Your-Home-With-A-Pellet-Stove.aspx
Open fireplaces are, as a class, the least efficient way to heat your home of the common ones discussed here. Yet they are not currently regulated for efficiency or emissions. Fireplace inserts can be considerably better, but, at this time (December 2010), the only standards for them are set by the industry. This will change when the EPA rewrites their source performance standards for all combustion equipment (which is expected to be approved in 2011 for implementation in 2012). For the time being, however, one should avoid using an open fireplace and, at the least, consider installing an insert that is as efficient and low in emissions as possible.
Masonry heaters have two features that set them apart from other types of combustion equipment. The first is apparent on the outside: they are covered by a large amount of stone or bricks. The second is concealed on the inside: they are designed with a system of tunnels through which the combustion air travels before going up the chimney or stovepipe. Together, these design features make for very efficient, clean burning heaters. Masonry heaters still have a firebox, where wood is burned. But unlike wood stoves, it takes a very long time for the heater to heat up. The large thermal mass (the bricks or stone cover a frame of firebrick) may take a full day to heat up but it will also hold that heat for a very long time (and is warm-to-hot to the touch, but nowhere near as hot as a wood stove). One or two small, hot fires per day will keep the heater hot with comfortable, even heat that radiates in all directions. Masonry heaters also often have built-in cooking compartments ideal for baking bread or making stews.
The downside is their sticker price. Commissioning someone to build a custom masonry heater can run $15,000 to $30,000. Kits are available for the guts of the heater (starting around $5,000) that allow you to build one yourself.
BOILERS (wood and pellet)
There are basically two kinds of wood-fired boilers, the standard hydronic heater, commonly known as an outdoor wood boiler, and two-stage gasification cord wood or pellet boilers. However, even within each category, there can be a large amount of variation in terms of emissions, efficiency, and cost. (In 2010 NYSERDA published the results of a study of the performance of four common wood boilers.) In general, boilers have heat outputs that range from 50,000- 200,000 BTUs/hr. Boilers that burn cord wood range in price from about $3,000 to $12,000. Some boilers are designed to burn wood chips or pellets. These range in price from about $4,500 to $25,000 for the chip boilers and about $13,000 to $14,000 for a pellet boiler.
Boilers can be used to heat water or glycol (antifreeze), which circulates around a building to heat it or, through heat exchangers, it can be used in a forced-air system (though this is less efficient). Here is a good overview of how to install a wood or pellet boiler in a home that is using a water- or glycol-based system.
Outdoor Wood Boilers
Outdoor wood boilers (OWBs) usually suffer from design characteristics that make them notorious for having very high emissions and low efficiencies: they are based on a large firebox, which means large fires that smolder often; they heat a water jacket, which is often not well insulated; and they usually have low exhaust stacks, which allows the exhaust to hover close to the ground. Currently, there are no nationally established legal emissions limits for OWBs. The hourly emissions of one (OWB) can be twenty times as high as the hourly emissions of an EPA-certified wood stove. While the EPA has initiated a voluntary regulatory program in an effort to encourage manufacturers to produce cleaner, more efficient units there are still many unqualified, unhealthy boilers on the market. So be cautious when purchasing and installing one of these systems.
In early 2011, the New York State Department of Environmental Conservation (DEC) instituted regulations governing OWBs. These limit the types of OWBs that can be sold in the state, and were designed to offer guidelines for the optimal use of these units (in order to minimize emissions, mainly). Provisions for phasing out existing units did not make it into the final legislation.
Two-stage gasification combustion systems are much more efficient and cleaner-burning than single-stage systems (for a more thorough discussion of the benefits of two-stage gasification combustion, NYSERDA researchers published an article in early 2011 on the topic). All combustion is gasification: the wood (or other material) is heated to the point where volatile gases are released, and it is these gases that burn. In a single-stage system, many of the gases are released up the chimney before they're fully combusted (which means increased emissions and decreased efficiency). With two-stage combustion, the combustible material (cord wood or wood pellets, typically) is heated in one chamber in the relative absence of oxygen, and the gases are released into another chamber where fresh air is injected, which puts the combustion into overdrive and burns the gases almost completely, at much higher temperatures than can be reached in a wood stove, pellet stove, or traditional outdoor wood boiler (most gasifying boilers release <1 gram of smoke per hour).
Europeans lead the world in two-stage gasification boilers, but many of the units are now available as imports. In addition, several companies are now making very clean-burning, highly efficient units domestically, including in New York State. These include Econoburn from Alternative Fuel Boilers, and Hestia from Bluestone Boiler.
Many boilers employ thermal storage to improve efficiency by minimizing the amount of on-off cycling. This is usually a large, insulated metal tank (600 to 1500 gallons) that holds the heated water before circulating it through the house or building. Without thermal storage, any time the house calls for heat, the heater turns on, which means the heater cycles on and off fairly often. High efficiencies are only attained when the fire is burning hot, so every time the equipment cycles on and off, efficiency drops and emissions increase. With thermal storage, the boiler fires for a long period of time in order to heat the large amount of water in storage, and then it shuts down for a long period of time while the house draws the hot water off storage. This greatly increases efficiency and reduces emissions. But there is a cost: thermal storage can add about $1,500 to $3,000 to a biomass boiler system.