Wood Stoves

April 21, 2010 at 4:43 am (Wood Stoves, Woods)

Wood Stoves

Looking to purchase a wood stove that is right for your home? Some of the biggest and best-known hearth manufacturers make some good wood stoves. There are many factors to consider and lots of products to choose from. Finding a high quality clean burning wood stove that meets your needs may require some digging. The specifications and terms used by the manufacturers and the EPA are technical and generally confusing. Understanding the specifications and ratings (and how they are determined) will help you make a better buying decision.

On top of confusing ratings and specifications there generally are not independent third-party reviews such as Consumer Reports to rely on. Underwriters Laboratory (UL) can evaluate gas fired solid-fuel fired hearth appliances, including fireplace stoves and fireplace inserts, to applicable U.S., Canadian and global requirements. The UL mark will appear on hearth products that have been evaluated. The largest trade group in the industry, Hearth, Patio & Barbeque Association (HPBA), provides general product information and guidelines pertaining to buying, installing and operating hearth products (i.e., fireplace inserts, gas fireplaces, gas logs) but does not recommend hearth products.

Wood stoves are not part of the energy star program, so it’s not as easy to know which are the most efficient stoves (aside from the wood stove efficiency rating which is discussed below). However, as of this year, wood stoves that are 75% efficient or more will be designated (see sticker on back of stove) as such in order to show that they are eligible for the 30% Biomass Federal Tax Credit that is (up to $1,500 federal tax credit) available in 2009 and 2010.

In order to properly evaluate wood stoves and fireplace stove inserts the best place to start is a basic understanding of the more significant ratings and specifications that accompany wood stoves and fireplace stove inserts.

Catalytic versus Non-Catalytic

A catalytic combustor is a device used on some wood stoves to increase combustion efficiency of wood stoves by lowering flue gas ignition temperatures of wood stoves.

The two general approaches to meeting EPA smoke emission limits are catalytic and non-catalytic combustion. Both approaches have proved relatively effective, but there are performance differences. In catalytic combustion the smoky exhaust is passed through a coated ceramic honeycomb inside the wood stove where the smoke gases and particles ignite and burn. Catalytic stoves are capable of producing a long, even heat output. All catalytic stoves have a lever-operated catalyst bypass damper which is opened for starting and reloading. The catalytic honeycomb degrades over time and must be replaced, but its durability is largely in the hands of the stove user. The catalyst can last more than six seasons if the stove is used properly, but if the stove is over-fired, garbage is burned and regular cleaning and maintenance are not done, the catalyst may break down in as little as two years.

EPA certified wood stoves have a particulate emissions limit of 7.5 grams per hour for non catalytic wood stoves and 4.1 grams per hour for catalytic wood stoves. All wood heating appliances subject to the New Source Performance Standard for Residential Wood Heaters under the Clean Air Act offered for sale in the United States are required to meet these emission limits.

Firebox Size

Size of the chamber where the firewood burns. Usually referenced in cubic feet and firewood capacity of the chamber in weight. Big fireboxes can be nice. They are easier to load, and can often accommodate those extra-long pieces of firewood that somehow find their way into the woodpile. When choosing your woodstove, however, keep in mind that stoves with large fireboxes tend to produce higher heat output, and easy fueling is a dear price to pay for being cooked out of the house.

Maximum Log Size

Largest log length that will fit into firebox. The standard firewood length for wood stoves and fireplace stove inserts is 16″, mostly because it is the most practical length for handling. Knowing maximum log length is useful because for convenient loading, the firebox should be about three inches bigger than your average piece of firewood.

Heating Efficiency

Measure of how much of the heat value contained in the firewood is extracted and delivered into the living space. This is the equivalent of the MPG rating of your car or truck. Remember the quality of the firewood will impact actual results.

woodstove

The heating efficiency rating is determined by the stove manufacturer by testing full loads of seasoned cordwood. When testing for heating efficiency, two criteria are examined: extraction efficiency; the firewood load is weighed going in, and the particulate emissions and ashes are weighed after the fire to determine how effectively a given firebox design breaks down the fuel to extract the available heat and heat transfer efficiency; this testing is performed in calorimeter rooms equipped with temperature sensors. Similar temperature sensors are installed in the exhaust flue. The degree changes in the room and flue are monitored for the duration of the test fires to determine how much of the heat extracted by the fire is delivered into the room, as compared to the heat lost up the flue.

Emissions

Measurement of particulate matter emissions in grams per hour. Particulate Matter is a fancy term for air pollution and means small pieces of matter such as dust and soot that are suspended in the air.

Emissions testing is performed in EPA-approved test labs using the EPA’s prescribed protocol. When testing for emissions, a nailed-together “charge” of kiln-dried Pine is burned, and the particulate matter in the exhaust is measured throughout the duration of several fires at various draft control settings. In this way, an average grams/hour particulate emissions rating is derived. Heating efficiency is not measured during EPA emissions testing.

The internal design of wood stoves has changed entirely since 1990, as the result of the EPA regulation established in the late 1980’s. The EPA’s mandatory smoke emission limit for wood stoves is currently 7.5 grams of smoke per hour. Today, all wood stoves and fireplace inserts, and some factory-built fireplaces sold in the U.S. must meet this limit. Stove manufacturers have improved their combustion technologies over the years, and many newer wood stoves have certified emissions in the 1 to 4 g/h range. The EPA certified emission rate is a reliable number that can be compared from one model to the next, but a one or two gram per hour difference in smoke emissions does not mean much in day-to-day use.

Heat Output

Usually represented as maximum heat output (you sometimes see a heat output range) of the wood stove expressed in BTU’s per hour. The British Thermal Unit (BTU) is the primary heat measurement unit used by the hearth industry to indicate heat output. It is the amount of energy required to raise the temperature of 1 lb. of water by 1 degree F. Typically 10,000 BTU can heat approximately 500 square feet. All wood stoves and wood burning fireplace inserts are rated by BTU output.

The heat output ratings can be misleading. In determining a maximum heat output rating, test labs used by manufacturers (usually using hardwood fuel) cram the firebox full of firewood and crank the draft control wide open. This raging, short-duration fire is just the opposite of how people burn their wood stoves, and can be misleading: if the only thing you look at is the maximum heat output rating, a small wood stove with a really big air intake can seem just as powerful as the largest wood stoves. Some manufacturers use the heat output rating from EPA testing, which uses softwood fuel. Another way these figures can be misleading is that non-catalytic wood stoves tend to produce a higher peak heat output, but that alone doesn’t mean they’ll produce more heat over an eight hour burn cycle, which is a more relevant performance indicator. The result is that you can’t compare the heat output of stoves because the ratings are not standardized.

Heating Capacity

The estimated square feet of space the wood stove will heat. Many manufacturers display very wide ranges like 1,000 to 2,000 square feet or suggest the maximum area the unit will heat. The reason for the big ranges and vague estimates is that a particular wood stove might heat 1,000 sq. ft. in Maryland, but only a 500 sq. ft. house in New Hampshire due to the climate difference. In addition, an old house might have twice the heat loss of a new house of the same size in the same climate zone. Also, the layout of the house could materially impact capacity. For example, if your house is divided into many small rooms, you probably won’t be able to move the heat around the rest of the house, so the square footage rating is useless to you. And lastly, a stove burning softwood will put out much less heat per firebox load than it will burning a hardwood. Heating capacity ratings based on square footage are unreliable.

Burn Time

Maximum estimated wood stove burn time. Burn time depends on wood species and moisture content, and on how much heat is needed during the burn. How long will a given stove burn on a single load of wood? The only reasonable answer is: It depends. One advantage of catalytic wood stoves is that the good ones can deliver a lower burn rate over a longer period than non-catalytic wood stoves and yet still burn clean. But the disadvantage of these long burn times is that the door glass tends to get dirty at very low firing rates. In other words, a stove that has a claimed burn time of ten hours may not be better or more convenient to use than one that delivers an eight hour burn.

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