Controlled Air Supply and Fuel Feed Rates

Efficient combustion is difficult to accomplish in traditional RWHs especially at low burn rates due to low combustion temperatures, poor air-to-fuel mixing, and variable combustion conditions. If air is mixed with fuel in a highly regulated and turbulent manner, higher temperatures and more efficient combustion can be achieved. Mechanical draft systems, in which air is forced or drawn into the combustion chamber, are used on many wood fired furnaces and all types of small, mid- and large scale commercial and industrial boilers.

A forced draft system moves air into the combustion chamber under positive pressure; i.e., higher pressures exist in the combustion chamber than in the surrounding space. Induced draft, on the other hand, draw exhaust gases out of the firebox, creating negative combustion chamber pressures. Both systems use fans to move the air and gases, hence the term mechanical draft. (Natural draft stoves are more properly "thermally drafted".) The use of a mechanical draft allows much greater control of combustion air flows and patterns, resulting in improved combustion conditions.

Both the induced and forced mechanical draft systems have advantages and disadvantages. Forced draft systems require higher quality construction methods to prevent combustion gases from leaking through doors and other openings into the surrounding space.

Induced draft systems do not require a perfectly sealed system, as most leaks will simply draw air into the combustion chamber or flue. Induced draft fans, however, must move hot and sometime dirty gases, requiring more care in selection and maintenance of fans and motors. In addition, mechanical draft RWHs will not operate without electric power to drive the fans, so these units can not be utilized during power outages.

Among RWH appliances currently available, virtually all mechanical draft stoves are designed to burn pelletized wood fuel. Pellets are typically ¼ inch to ³/₈ inch in diameter and about ½ inch in length. The composition varies among manufacturers, but is primarily sawdust and chips from forest products operations. Some pellets are composed of wood only, while others contain more bark and debris. Most are formed under heat and high pressure, and most use no binder. Heat content of the pellets are typically in the same range as cordwood (i.e., 8750 to 9200 Btu/dry lb), with a moisture content of 6-10 percent. Pelletized fuel has been successfully used as a substitute for coal in many small boiler applications.

The primary advantage of using pellets in residential combustors is the ability to control the amount of fuel involved in combustion at any one time. Air and fuel feed rates are then both controlled, providing optimized combustion conditions. Pellets can be fed at a constant rate into a combustion zone maintained at high temperatures and high turbulence by a forced or induced draft.

The mass of fuel involved in combustion at any time is very small, while oxygen supplies and turbulence are high. Pellet stoves can operate under steady state conditions as a continuous process, rather than the batch process of burning cordwood. The small mass of fuel burning at any given time promotes stable, near steady state conditions, which allows more efficient combustion. Pellet-fired RWHs with appropriate air-to-fuel ratios (i.e., in the rage of 15:1 to 19:1) have efficiencies and emissions comparable or better than catalytic RWHs.

On all existing pellet-fired RWH designs, fuel is stored in a hopper and moved into the combustion chamber/firebox with a motorized auger or cupped-wheel design. The feed rate is controlled by variable speed motors or automatic/electronic time-on switches. Pellets are pushed or dropped into a small cup-shaped tray which is surrounded by combustion air inlet jets, creating a concentrated and intense burn region. Air supply fan speeds can also be varied; some pellet-fired RWHs have combined single-control air and fuel feed rates while others offer combinations of independent fan and fuel feed rates controls, and continuous or intermittent operation. Most pellet-fired RWHs use a refractory lined combustion chamber in which the pellet "burnpot" and air supply ring are located. Gases are then vented from the combustion chamber through heat exchange baffles.