Catalytic Combustors
Here is how your catalytic combustor works.
The catalytic combustor is a ceramic honeycomb with hundreds of cells. If you looked
at the inside of each cell with a microscope, you would see that the walls are uneven and
filled with minute nooks and crannies. Precious metals, such as platinum, are sprayed
on the inside of these cells to coat all of the nooks and crannies. This creates the largest
possible surface area to interact with the wood smoke. The catalytic combustor in your
stove is very similar to the one in the exhaust system of your automobile and works to
achieve the same results - high efficiency and clean air!
When you first start a fire, you should bypass your catalytic combustor and let the
smoke go directly up the chimney. Once wood smoke reaches 500º F (250º F on the
stovetop thermometer), it is hot enough to ignite the catalytic combustor. As the wood
smoke passes through the cells in the combustor, the smoke reacts with the precious
metals which line the inside of the honeycomb and both combustible gases and particles
in the smoke ignite and burn. This “secondary burn” reduces emissions and also
increases heat output from the stove.
Without a catalytic combustor, between 5% - 40% of the chemical energy contained in
wood simply escapes up the chimney when wood is burned. Energy laden gases are
exhausted up the chimney, or they are condensed on the inside of the chimney flue as
creosote. The slower the burn, the greater the amount of energy that is not used. A long
smoldering fire is the least efficient use of energy in wood, yet it produces lots of smoke,
which is the fuel supply for the catalytic combustor.
The catalytic combustor is made up of
two squares, assembled with white, feltlike gasket to make one rectangular unit.
Most of the chemical compounds in wood smoke are combustible. High temperatures
(in excess of 1100-12000 Fahrenheit ) can loosen the bonds of these chemical compounds
and “burn” wood smoke. A stove that “burns” these compounds and uses smoke as
additional fuel will burn more efficiently and produce more heat, while reducing
creosote and air pollution at the same time. However, most stoves cannot consistently
produce temperatures in excess of 1100-1200º, particularly during long burning times hence the need for a catalytic combustor.
Your catalytic combustor can get the most efficiency out of every piece of wood if it has
three things: temperature, turbulence, and time.
1. Temperature. The catalytic combustor can only start burning the gases in the wood
smoke after the smoke has reached at least 500 degrees F. Before the smoke reaches
that temperature, it will simply “gum-up” the cells of your combustor.
2. Turbulence. The wood smoke can interact best with the precious metals inside the
honeycomb cells if there is some variation in the air flow. Increased turbulence
enables more of the wood smoke to come into contact with more of the nooks and
crannies in the honeycomb cells. The expanded metal screen that sits in front of your
catalytic combustor helps to create turbulence. It also protects the catalytic
combustor from direct flame contact.
3. Time. Once the temperature and turbulence are achieved, the catalytic combustor
just needs to have enough time to burn all the gases in the wood smoke. For this
reason, it is best to minimize the amount of air you allow into the firebox once the
combustor is ignited. Allowing too much air into the firebox speeds up the rate at
which the fire burns and allows more wood smoke to pass through the combustor
than it can handle at one time. Too much air allows unburned wood smoke (and
heat) to go up the chimney. The ideal air setting is one that allows enough air to
keep the wood burning and producing smoke, but not so much that the smoke is
racing through the combustor without being burned. Most people find this point
close to #1 (either just above, or just below) on the primary air control.
With proper care, a new catalytic combustor will give years of fuel savings and lowered
emissions. By following some simple guidelines you can ensure maximum combustor
Cut-away drawing of the airflow pattern
in a Palladian and Keystone. Air enters
the firebox by washing down over the
window and exits after going through
the combustor.
performance and longevity. Your catalytic combustor is designed to last for
12,000 -14,000 hours of use. You can ensure yourself of getting the maximum life from
your combustor by following these simple guidelines:
1) Burn only natural, well-seasoned wood.
2) Wait until the stove top thermometer reaches 250º (500º inside the firebox) to
engage combustor.
3) Bypass the combustor before reloading and leave the bypass open for 10 - 15
minutes after reloading to burn off the moisture in the wood.
4) Don’t overfire the stove. See page 14.
5) Clean the combustor regularly. See below.
You can also obtain a lot of useful information by visiting the web-site of the maker of
our catalytic combustors ( Other very useful web sites on all
aspects of wood burning are (, ( and
( CSIA is the Chimney Safety Institute of America.
Inspection & Cleaning
You should inspect and clean both sides of your catalytic combustor after every 4-6
weeks of operation, and remove any fly ash that has accumulated on it. An
accumulation of fly ash can cause back puffing, sluggish burning, and inefficient heating
performance. Because the catalyst is made from a ceramic material, it may crack slightly
with normal heating/cooling cycles. If you notice cracks or small pieces gone, it’s OK.
The combustor still has plenty of surface area with which to burn smoke. If severe
crumbling is occurring, this is an indication of overfiring, thermal shock, or moisture
exposure, and the stove operation should be reviewed.
To clean the catalyst you will need a 7/16”
and a 1/2” wrench (or an adjustable wrench)
You should wear gloves and safety glasses.
You can clean the actual catalyst with a vacuum cleaner.
Tools needed: (1) work gloves and safety glasses; (2) 7/16” and 1/2” socket or box
wrench, or adjustable wrench; (3) soft bristled paint brush or vacuum cleaner (preferably
one designed for ash removal).
There are two ways to clean your combustor.
thorough method of cleaning your combustor.
1. Be sure the fire is out and the stove is cold. Even though the stove may be cold, there
may be some lingering embers in the ashes.
Step Number 4: Remove the stainless
steel heat shield before you take out the
combustor box and combustor.
2. Your Palladian/Keystone has an interchangeable cover plate and flue collar that
enable you to vent the stove from the top or back. Leave the flue collar and pipe in
place and remove the cover plate.
3. Using the Allen wrench provided with the stove, remove the four screws.
4. Reach in through the opening, remove the single head bolt that holds the stainless
steel heat shield in place, using an adjustable wrench or 7/16” open-end wrench, and
remove the heat shield (see illustration).
5. If you haven’t already done so, remove the two bolts that hold the combustor box in
place using the 1/2” open-end wrench. These bolts are located at either side of the
combustor box and have flat washers under the bolt heads. These are shipping bolts
so the combustor doesn’t move while your stove is in transit. You can either discard
them now or hold onto them if you ever plan to move your stove again. They will
not have to be re-installed (see illustration at right.)
Step Number 5: Two bolts hold the combustor box in place. Remove them using
a 1/2” wrench.
6. Tip the front of the combustor box up, turn it 90 degrees, and remove it through the
rear opening. If you’re removing it through the top of the stove, you will have to
slide one end of the combustor into the flue opening, and tip it up so that it will
come out through the top opening.
7. Once out, you can use a paint brush (or other soft bristled brush) or a vacuum
cleaner to remove any fly ash from both sides of the combustor. Never use anything
abrasive to clean the combustor. A vacuum cleaner may be used, but never use high
pressured (or compressed) air to blow the cells free of build-up. It could strip the
precious metal coating from the inside of the cells. Any cell blockage can be removed
with a pipe cleaner or a cotton swab. Vacuum the area where the combustor is
installed inside the stove.
8. Once cleaned, put the combustor back into place inside the stove.
You should now clean the expanded metal screen located inside of the firebox.
Step Number 6: Use the tab on the bottom
of the combustor box to tip it up, turn the
combustor box 90º, and slide it out through
the back opening.
9. Open the side door. The top of the firebox (inside the stove) consists of a flat cast iron
baffle plate (W-249 Baffle Plate) with an attached expanded metal screen. This screen
prevents direct flame from hitting the combustor and creates turbulence in the
exhaust stream. The baffle plate is held in place with two cast iron brackets and one
1/2” bolt located on the bracket closest to the loading door. The other end of the
baffle plate is set in between the arms of the bracket on the other end of the firebox
but is not bolted in. The baffle plate is also supported in the slot under the bypass
damper toward the back of the stove. Wear gloves. The screen has very sharp edges.
10. Remove the 1/2” bolt on the right baffle support that holds the baffle in place. Once
the bolt is out, gently pull the baffle and screen towards the door. This will remove
it from the left bracket slot.
11. Twist the baffle 1/4 turn counterclockwise, so that it will fit through the door
opening, and remove it from the stove.
12. You can now use a brush or vacuum to remove any ash that may have accumulated.
13. To reassemble, put the baffle/screen assembly back in place, taking care that the
back edge of the baffle is wedged in the slot under the bypass damper and that the
far end is supported in the slot within the left bracket support. Fasten the bolt on
the bracket closest to the door (do not over tighten). You are finished.
a quicker way to give your combustor and screen a good cleaning when you don’t have
the time to do the more thorough method described above.
1. Follow the steps above to remove the baffle plate and screen (#9-#11, above).
2. Brush them off with a paint brush or vacuum.
How to clean the combustor and expanded
metal screen from inside the firebox.
Step 1: Unbolt the
baffle plate and
Step 2: Remove
the baffle plate
and screen from
the stove.
3. While the baffle is removed, reach into the firebox with the brush or vacuum and
remove the ash from the “upstream” of the combustor while it sits in place. Fly ash
will accumulate on this side more than the backside. This will keep the cells open for
the smoke to flow through.
4. Follow the instructions for re-installing the baffle back into place. (#13, above).
Replacing A Catalytic Combustor
You will need the following tools: (1) 5/32” allen wrench that came with stove, (2)
1/2” open end wrench, (3) 2- 7/16” wrenches, (4) vacuum — preferably one designed for
ash removal, (5) work gloves, and (6) safety glasses.
Step 3: Brush soot
and fly ash off the
Step 4: Also
brush the exposed
side of the combustor.
1. Your Palladian/Keystone has an interchangeable cover plate and flue collar which
enables you to vent the stove from either the top or back. Leave the flue collar in
place and the pipe connected to it. Remove the cover plate, using the enclosed 5/32”
allen wrench to loosen the 4 socket set screws in the corners.
2. Reach in through the 7” hexagonal opening, and remove the single hex-head bolt that
holds the stainless steel combustor shield in place with an adjustable wrench or
7/16” open-end wrench to loosen the bolt. Remove the stainless steel shield.
3. If you haven’t already, remove the two bolts that hold the combustor box in place
using the 1/2” open-end wrench. These two bolts are located at either side of the
combustor box and have flat washers under the bolt heads. These are shipping bolts
so the combustor doesn’t move while the stove is being shipped. To make future
maintenance easier, do not re-install them.
4. Tip the front of the combustor box up, turn the box 90 degrees, and remove it
through the rear opening. If you are removing the combustor box through the top of
the stove, you will have to slide one end of the combustor into the flue collar
opening, and tip it up so that it will come out through the top opening.
5. Separate the top and bottom of the combustor box by removing the 2 bolts with two
7/16” wrenches. Clean off any old gasket residue. Put the new combustor with its
new gasket into the combustor box. It may fit loosely at first but when the stove is
fired, the gasket will expand to fill the space. Reassemble the combustor box. Place
the box back into the stove so it is seated on the gasket. Bolt the stainless shield back
in place.
6. Attach the cover plate again, tightening the 4 socket set screws with the enclosed
Allen wrench.
7. Place the magnetic thermometer on the top, middle piece of soapstone. This is where
you will get the most accurate reading.
Frequently Asked Questions
Q. How does the catalytic combustor work?
A. The catalytic combustor loosens the bond that holds the chemicals together in wood
smoke. The result of this is that these chemicals begin to burn at temperatures of
about 500 degrees (the normal range of exhaust gas temperatures). Without the
catalytic combustor, wood smoke would have to be brought up to a temperature of
1000 to 1200 degrees in order to start to burn. Having a stove with a catalytic
combustor will generate 25% more heat from each piece of wood, thus reducing the
amount of fuel used during the year.
There are three advantages to burning the smoke created by burning wood. First,
the smoke becomes another source of fuel, giving you more heat from the same
amount of wood. Second, creosote components will be burned up instead of being
deposited in your chimney and possibly causing a chimney fire. Third, air pollution
will be drastically reduced.
Q. How can I tell if the catalytic combustor is working?
A. First - The best way to tell if the catalytic combustor is working is by observing the
smoke coming out of your chimney. If there is only a small amount of smoke,
and/or it’s white in color, the catalytic combustor is working. You will see
significantly more smoke when the combustor is being bypassed than when the
smoke is being burned by the combustor.
Second - One very noticeable effect of a well functioning combustor is the high
efficiency of your stove. Catalytic combustors produce lots of heat, which the stove
radiates into your home. If you notice that the stove is producing less heat, or that a
load of wood doesn’t provide as much warmth, under similar circumstances, as it
did before, it’s time to check the combustor. If and when the catalyst ceases to
function properly, the stove will produce noticeably less heat.
Third - Inspection of the chimney should show that there is much less soot and
creosote than when the combustor is not used. Soot is also different when a
combustor is used; it is brown and powdery, instead of black and sticky.
Q. How do I maintain my catalytic combustor?
A. Combustors should be checked and cleaned if necessary every 4-6 weeks during the
heating season. When the stove is cool, the combustor can be cleaned by vacuuming
or brushing both sides of the combustor.
There is a stainless steel screen in front of the catalytic combustor. If this screen
becomes partially plugged with fly ash, you can remove the ash by gently tapping on
the front of the screen (between the screen and front glass) until the fly ash becomes
If the stove does not draft well when the catalytic combustor is engaged and the
stainless steel screen is clean, then the combustor cells themselves might be partially
plugged with fly ash. If this is the case, you can clean the combustor by (a) removing
the combustor through the back of the stove and cleaning the cells (see page 22), or
(b) removing the baffle and stainless steel screen assembly from inside the firebox
and cleaning the combustor cells from inside the firebox (see page 23).
Q. How will I know if the combustor is “worn out”?
A. There are three symptoms that will indicate that the catalyst in your stove may not
be working: First, your stove will generate noticeably less heat than it will when the
catalyst is working. Second, you will notice a dramatic increase in the amount of
soot and/or creosote in your stovepipe or chimney. Third, the color of the smoke
produced by the stove will change. Smoke will appear black or brownish, instead of
clear, white smoke (almost steam) from a catalytic stove.
If you suspect that your catalyst is not working, let the stove cool down and clean
the combustor and screen and try it again.
Q. Is it all right to burn my stove hot daily to clean any build up in my chimney
A. It is not necessary to burn your stove hot daily to burn off any creosote build up in
the chimney. This function is performed by the catalytic combustor. It is there to
reduce the particulate matter in the chimney.
View many more useful frequently asked questions at the web-site of our catalytic
combustor supplier, Other very useful web-sites on all
aspects of wood burning are,, and CSIA is the Chimney Safety Institute of America.
1) The combustor uses wood smoke as fuel. Most smoke is created in the early stages of the
“burn”. When a bed of coals is all that remains of your wood, there is little smoke left to
fuel the combustor, and it will no longer create substantial amounts of heat. Hence, the
temperatures on the surface thermometer tend to fall toward the end of the burn, even
though the firebox is full of hot coals. This does not mean that you have to reload the
stove. Let the hot coals burn down to ashes.
2) Since the combustor blocks the path of exiting smoke, it can reduce the draft in your
stove. When draft is reduced by warm or rainy weather, open the bypass damper longer
when starting the stove to create more draft.
Model #202 Palladian and Model #204 Keystone have a 3-5/8” x 9-1.2” rectangular
catalytic combustor, which consists of two sections. The combustor is wrapped with flat
interam gasket (see illustration, page 21). These combustors are manufactured by Applied
All catalytic combustors in Woodstock Soapstone Stoves have a three year unconditional
warranty, with an additional three years of pro-rated warranty (copy enclosed). Warranty
claims should be addressed to:
Applied Ceramics
PO Box 29664
Atlanta, GA 30359
Phone 1-404-448-6888
Woodstock Soapstone Co., Inc.
66 Airpark Road
West Lebanon, NH 03784
Phone 1-800-866-4344
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