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External and Secondary Combustion Air

Many authorities tell how important it is to introduce secondary air or use of a catalytic combustor for an efficient woodstove. They also say to use external combustion air for fireplaces. For normal woodstoves and fireplaces they are completely correct. But, we don't think either are usually necessary in the operation of a JUCA unit, even though our C and F models can use external combustion air. Let's look carefully at why our units are different.

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EXTERNAL COMBUSTION AIR

Open fireplaces allow a lot of air to go up the chimney. Often 90% of that air had nothing to do with the fire; it is just "going along for the ride." Much house heat can be lost this way. Under some conditions, more house heat can be lost than the fireplace puts into the house - the efficiency is actually less than zero!

Glass fireplace doors eliminates this unused air, and thereby increases the overall efficiency. (Note that a similar effect occurs with Franklin-type stoves. Their efficiency closed is about 25%; open about 15%.)

Normal designs of fireplaces and woodstoves have such a low efficiency that a roaring fire is generally needed, which consumes large amounts of oxygen (air). For all the air used here (removed from the house), an equal amount of cold outside air must seep into the house. That creates cold drafts and increases heating bills. This is often a large enough effect that there is an advantage in NOT using up house air for the fire, but rather bring in outside air for the fire through a duct.

Our JUCA units commonly operate with a "slow fire" that uses oxygen pretty slowly. We have discovered that in many cases it is not worth the considerable expense to install it even in our C and F models. The cases where it should be considered are: EXTREMELY tight houses; or where it is particularly convenient to install. In the extremely tight house case, any flame heater (gas or oil furnace included) should also have external combustion air intake since they use oxygen too.

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SECONDARY COMBUSTION AIR

In the normal operation of a regular woodstove, the roaring fire causes a great draft - the air goes past the fire very quickly. This has the effect of rapidly carrying heat up the chimney (causing low efficiency) but also does something else. In burning, some of the chemical reactions occur in two (or more) steps. Sometimes the second step can not occur, which reduces efficiency and increases creosote deposition in the chimney.

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As an example, a main reaction is:
  1. Carbon plus oxygen gives carbon monoxide; and
  2. Carbon monoxide plus oxygen gives carbon dioxide.
This second step will occur only if enough oxygen is present AND the temperature is over 1211°F.

An airtight woodstove's restricted air supply causes unburnt gases to get out of the 1200°F zone, so they have to introduce secondary air and re-heat the gases to complete the burning. They are using secondary air to try to cure a problem caused by the airtight operation itself. In 1979 and 1980, independent studies have shown that most secondary air systems don't work anyway due to inadequate design. The JUCAs normally operate with a "slower" fire that allows the second reaction to occur while still in the 1200°F zone due to slow draft flow and adequate combustion air supply. Our research indicates rather complete combustion and introducing secondary air is unnecessary in our units.

The JUCA Home Page is at: juca