Wood Stoves

To meet mandatory  EPA regulations established in 1988 with which all new wood stoves, fireplace inserts, and some factory-built fireplaces sold in the U.S. must comply, the internal design of new wood stoves had changed radically.  

EPA's smoke emission limit for wood stoves is 7.5 grams of particulates per hour (g/h) for non-catalytic stoves, and 4.1 g/h for catalytic stoves. Stove manufacturers have improved their combustion technologies to the point at which some newer stoves have certified emissions in the 1 to 4 g/h range. EPA-certified stoves have an efficiency rating of approximately 60%-80%. Non-certified stoves are typically 45%-60%.

Generally, improved emmisions levels are reached by improving the combustion preocess so that more fuel and exhaust gases are burned. The two  approaches to meeting the EPA smoke emission limits are non-catalytic, and catalytic combustion. Both approaches have proven effective, but have performance differences.

Although most stoves on the market are non-cats, some of the most popular high-end stoves use catalytic combustion. Both options have their pros and cons. Good-quality catalytic stoves can perform exceptionally well, but are somewhat more complicated to operate and so are more suitable for people who are prepared to maintain them properly so that they continue to operate at peak performance.

Non-catalytic Stoves

Non-catalytic stoves don't use a catalyst, but have three internal characteristics that create a good environment for complete combustion.

1.  firebox insulation (typically brick or fiberboard)
2. a large baffle to produce a longer, hotter gas flow path
3. pre-heated combustion air introduced through small holes above the fuel in the firebox.

Non-cats typ0ically heat less evenly than catalytic stoves. The baffle and other internal parts of a non-catalytic stove will need replacement from time to time as they deteriorate with the high heat of efficient combustion.

Modern non-catalytic stove use a secondary burner which looks like one or more tubes with holes in them mounted on the roof of the burn chamber.

Left: The tubes of the secondary burner are visible at the roof of the firebox.             

Right: Silver-colored gasket lines the door.

Catalytic Stoves

Source 

                                         Catalytic, with by-pass damper                                   Non-catalytic with baffle and large combustion air supply

In catalytic combustion the smoky exhaust passes through a coated, ceramic, honeycomb-like structure inside the stove where additional combustion of exhaust gases and particles takes place.

All catalytic stoves have a lever-operated catalyst bypass damper, which is opened for starting and refueling. The catalytic honeycomb deteriorates over time and eventually must be replaced, but correct use can extend its lifespan to more than six seasons. Over-firing the stove, burning garbage or poor maintenance practices may shorten its lifespan to as little as 2 years.

(EPA note: Garbage should never be burned in a wood stove or fireplace.)

Note: The descriptions of combustion options and high-efficiency stoves below are courtesy of John Gulland of The Wood Heat Organization. , a Canadian non-profit which promotes the use of wood for heating. 

TEN CHARACTERISTICS of SUCCESSFUL STOVE and FIREPLACE SYSTEMS

A perfect woodburning system builds strong draft quickly so kindling a new fire is always easy, not frustrating. Smoke never spills into the room, and cold smelly air never comes down the chimney when no fire is burning. In short, a perfect system is a pleasure to use. It is the kind of system you want in your house.

The following list of design characteristics looks simple enough, but underlying these ten elements is twenty years and over a million dollars in research effort. I have spent part of my thirty-year career in the hearth industry with research scientists trying to understand the theoretical basis for successful venting and with chimney sweeps and retailers reaping their insights from thousands of hours of observations in the real world. Only by synthesizing the science and the practical was it possible to devise such a simple ten-point list.

When choosing the design of your hearth system, consider the extent to which it strays from perfection. That is, assign a demerit point for each characteristic that does not conform to perfection. Think of each item on the list as a "driving" characteristic that induces the flow of air and gas up the chimney rather than down the chimney into the house. Think of each flaw as an "adverse" characteristic that compromises the system's ability to perform successfully. Here is the list of characteristics that defines perfection.

ONE The chimney runs inside the building envelope (inside the heated space) so air and flue gases stay at least as warm as the air in the house until they are expelled outside.

TWO The chimney penetrates the highest part of the building envelope so the chimney always functions better as a chimney than the house does, even when there is no fire burning.

THREE The chimney is tall enough and its top is clear of obstacles to wind flow so it can produce stable draft and it has a chimney (rain) cap because without one any chimney is vulnerable to adverse wind pressures.

FOUR The chimney flue is insulated and is the correct size for the appliance so flue gases are kept warm and flow quickly through the system.

FIVE The flue pipe (if there is one) runs straight up from the appliance to the chimney and the chimney has no offsets because each change in direction presents resistance to flow.

SIX The appliance and venting system are reasonably well-sealed because leaks introduce cool air and big leaks make the system more vulnerable to adverse pressures.

SEVEN The stove or fireplace is EPA/CSA certified for low smoke emissions or has equivalent characteristics (like masonry heaters do) so it is unlikely to smolder because smoldering appliances are much more likely to spill smoke.

EIGHT The system is installed in a house that may be tightly sealed but has a balanced ventilation system. The alternative, exhaust-only ventilation, cause houses to be constantly depressurized and is disastrous for chimney vented systems.

NINE There is no large exhaust fan (like a downdraft kitchen range exhaust), or if one is present, it is electrically interlocked to a fan-forced make-up air system to prevent the house from becoming excessively depressurized when it operates.

TEN The appliance is operated by an informed user because the best of designs can be disabled by improper operation and a lack of maintenance.

You may have noticed that an outside air supply does not appear on the list. There is a good reason it was not included. While there is some anecdotal evidence that providing combustion air from outdoors can help in certain situations, there is no solid scientific evidence to suggest that outdoor aired systems are any less likely to spill smoke than are appliances that take their combustion air from the room. In developing the list, I have only included characteristics that consistently and reliably contribute to successful venting, so outside air does not qualify.

With new houses becoming increasingly well sealed, it is more important than ever that hearth systems consistently produce strong draft; so that, even when not operating, air will flow up the chimney rather than down. As you plan your appliance and chimney system, strive to reach perfection by meeting all ten design objectives. Perfection is neither easy nor common, so it is an achievement worth celebration. In fact, you will probably celebrate its perfection each time you use it because your woodburning system will work flawlessly.

Aiming for perfection is a worthy goal because it results in systems that do not spill smoke when operating and do not spill odors when not running. If every system were perfect, people would love their hearths even more than they do now.

JG

Why advanced stoves are worth the extra cost

1.       On average, advanced EPA-certified stoves are about one-third more efficient than the old box, pot belly, or step stoves, and almost all of the currently available wood-burning central heating furnaces and boilers. That’s one-third less cost if you buy firewood, or one-third less cutting, hauling and stacking if you cut your own. Although this higher efficiency is a by-product of mandatory emissions limits, it has made the EPA rules a winner for both the environment and stove users. The extra cost of advanced technology is about $200 per stove. Over just two seasons of wood burning the greater efficiency of the stove will more than compensate for the higher initial cost.

2.      Advanced stoves produce about 90 percent less particulate matter - smoke - than older stoves. After a fire is ignited, you should see no visible smoke from the chimney, so neighbors won't complain and the foul smell and thick smoke won't blanket your yard.

3.      Fires ignite more easily and burn more completely in these new stoves. The result is a far more convenient and pleasurable wood burning experience.

4.      Virtually all the new stoves have a glass panel in their door and an air-wash system to keep it clear. This not only means being able to monitor the fire and adjust it periodically to get a perfect burn, but the fire itself is spectacular to watch. No fire in a conventional stove or fireplace can compare with the beauty of an efficient wood fire. 

5.      Ninety percent less smoke means 90 percent less creosote. This gives two important benefits. First, the chance of chimney fire is virtually eliminated as long as the stove is operated correctly and reasonable maintenance is done. And second, the flue pipe and chimney will need cleaning much less frequently, which is another way the new technology stoves save time and money.

 

Inspectors may wish to research states or local jurisdictions emission requirements.

Sources for More Information 

Wood Heat Organization Inc., a nonprofit, nongovernmental agency dedicated to the responsible use of wood as a home heating fuel.