Air-Tight, Air-Limited, and Fuel-Limited

All open fireplaces and many early woodstoves can be classified as "Fuel limited." This means the size of the fire and the heat output are completely determined by the fuel supplied. If you wanted more heat you put more wood on, particularly small (split) pieces that burn rapidly because there is a lot of surface of wood on fire. The actual fire-heat output is directly proportional to the area of wood burning. Since the amount of air available is unlimited, little carbon monoxide and little creosote is produced, but much wasted house air goes up the chimney causing an overall efficiency (total heat gain divided by heat content of the wood) of 7% for large fireplaces to 15% for smaller ones. This is optimum; often open fireplaces have a negative efficiency! More house heat is lost than the wood heat gain.

Modern competing woodstoves are "air-tight." These are not "Fuel-limited" but rather "Air-limited." There are several advantages that result. Minimal house air is lost. There is good control of fire size and therefore heat output. Therefore thin pieces of wood and kindling can be loaded and when it starts to roar, the air control can be closed back (manually or thermostatically) to hold back the fire to make it last longer and be more even in output. These stoves have some intrinsic drawbacks. Wood usually needs to be cut fairly small. The partial lack of air causes substantial production of carbon monoxide and creosote, which wastes much energy.

Creosote is very volatile material that wastes some of the heat-energy initially in the wood. The carbon monoxide is a greater loss though (even neglecting safety considerations). The carbon in wood combines with the oxygen in the air and forms carbon monoxide, giving off 10,000 Btu/lb of carbon. Now if conditions allowed, the carbon monoxide could combine with more oxygen to form carbon dioxide, the safe final product of normal combustion, and give off an additional 4400 Btu/lb.

The conditions mentioned are: an adequate supply of oxygen and a local temperature of over 1211°F. In an open or "Fuel limited" fire these conditions generally allow a full combustion with little creosote formation. Airtight stoves or "Air limited" fires do not allow enough air for efficient combustion.

Some woodstoves try to introduce "secondary air" above the fire in order to complete combustion of the volatile gases, particularly carbon monoxide. But many of these stoves are poorly designed; the way of introducing secondary air often has two flaws. In some, the flue gases are generally below 1211°F at the point of introduction of secondary air, and no reaction can occur. In others, cold secondary air immediately cools off the volatile gases below the critical temperature also resulting in no reaction. There are a few well-designed stoves that preheat secondary air and introduce it at the correct place. Under some conditions secondary combustion may occur, but much of the time it still does not. Their normal USABLE efficiency is 40-50%. Sometimes natural convection (no blower) woodstoves claim overall efficiencies of 60-80%. This is possible under ideal laboratory test conditions, but normal use by normal people will limit it to about 50%.

JUCA units can be used as either "air limited" or "fuel limited." We intend use as "fuel limited." The extensive forced air heat exchanger is capable of removing such a large fraction of the heat produced that a minimal fire is all that is needed. You can use large diameter (6" to 9") unsplit logs after getting a fire going. For example, the fire won't roar up when cooking in the unit with the door open. The heat output is so constant that an automatic draft control is unnecessary. A little practice will allow keeping the house always in the range of 70 to 72°F. In this mode (Fuel limited) creosote formation is minimized. And the slow fire allows plenty of opportunity for the carbon monoxide to complete combustion to carbon dioxide. Therefore secondary air is not needed.

The forced-air heat exchangers help create an overall efficiency of about 80% as "fuel-limited." Note that a fuel limited fire does not need to be air tight. Indeed, the JUCA design uses a slight "looseness" to minimize carbon monoxide formation so a chimney flaw wouldn't fill the house with deadly gases. Users can get the small fire required by using kindling and small split wood. Then they control the fire size by limiting air supply with the draft control ("Air limited"). Fires will last several hours and some creosote and carbon monoxide will be produced. Efficiency is a little lower (about 70%) due to the poorer burning characteristics of 'air limited' operation.

JUCAs were originally designed completely for efficiency -- appearance considerations came later. Airflows in the firebox (combustion air) and in the warm air chambers (house air) were carefully designed from aerodynamic considerations. We intentionally introduce some 'controlled turbulence' in order to make greatest advantage of heat transfer coefficients. Airflow can occur in several ways - laminar, critical or turbulent. On first appearance, laminar would seem most efficient since it is very smooth and orderly. However, a thin layer of air is heated but then the greater portion of the available air is insulated from being heated. So, under most conditions, a more turbulent flow is desirable; where as soon as the air is heated it is moved away so other air can be heated.

We arranged a special sequence of house air flow through the JUCA. Cool air is preheated in the first (rear) heat exchanger and gets boosted to higher temperatures by several different succeeding heat exchangers so that the output house air is quite warm. Each area of heat exchanger is designed for its own specific purpose, so that overall it is an effective arrangement. Once heat is produced by the fire the smoke has to go past the tubular heat exchangers before leaving the unit. We found that round shaped exchangers cause the proper turbulence so that the maximum amount of hot smoke is near the heat exchangers, so the greatest possible heat output is generated as pure warm air on the other side of the heat exchanger surface.

Our most efficient units have "staggered" exchangers to get even better use of the effects we created. They also have a tapered firebox shape. As heat is removed from the smoke, it cools and takes up less space. The JUCA taper is designed around this effect by creating an "isobaric equilibrium" situation. The above considerations show that we capture a lot of heat from the smoke before it goes up the chimney - this is convective heat from the fire. Fire also gives off radiative heat. JUCA models are constructed so that virtually all this heat is captured either in the upper heat exchanger system, or by direct radiation through the glass and from the metal and the base.

No other products have so many design considerations toward efficiency built into them.

The JUCA Home Page is at: juca