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.
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.
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.