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kiln ventilation

updated sun 19 oct 03


John Baymore on fri 24 may 96

---------------------------------original message-------------------------------
From: Cynthia Hull
Subject: Kiln Ventalation

I need to know if the folowing system is sufficiently vented. The kiln is a
high fire, downdraft, forced air kiln, 5'sq., insulating brick, with a
chimney extending above the kiln about 3'. It is located in an old
corrigated metal building 60' X 40' with a 25' ceiling that has 4 vents in
the ridge. The chimney is directly below one of the vents. Is this OK or do
I need a fan in the vent or something. What da ya think pyros?


While there are exceptions to most everything in ceramics, and you should never
say never (particularly when you haven't seen the actual thing being discussed)

I'd say that your kiln venting is pretty inadequate. Probably greatly
inadequate. Most likely illegal, to boot, in most any town except somplace
really out in the middle of no-where. Violates your local gas code, probably.
Probably voids any insurance coverage on the building. Likely is a health risk
to anyone working in the building.

If this was a commercial foundry-type building and was designed to house a
melting and pouring operation at one time, it may have met the "technical safety
standards" of the day. But not in the 90's with what we know now.

You are dumping a large voulme of extremely hot gases of somewhat unknown
composition into the room. Most of the gases will be carbon dioxide and water
vapor. There will be some percentage of toxic carbon monoxide, which will
significantly increase if you fire in reduction. The heavier the reduction, the
more CO produced. If you use wax or wax resist on the bottoms of pots, you are
putting some really nasty organic pudding into the mix early in the firing too.
You probably will get some particulate carbon at times. You'll have some
sulphur dioxide from the clays burning off. Then there are the trace volitles
from all of the glazes in the kiln. Most of the metals used in ceramics
volitilize into the kiln gases. So you have a real chemical soup going into the

I wouldn't want to be in there!

In your favor is the high celing! This allows for stratification of the stack
effluent in the room. Basically this means that the hot gunky stuff is staying
"mostly" above the breathing zone of the people below. At least hopefully .
This stratification technique was an old method used in industry to help deal
with all the noxious stuff in places like foundries. They used ridge-line
coupolas with banks of fans and tall buildings, and louvers for make up air at
about floor level. That was long before the idea of local exhaust and slot

If you add fans in the lower reaches of the building, you disturb the
stratification of the hot gases. This mixes the "gunk" into the lower air that
is being breathed by anyone in the room. So DON'T just put fans in down there
blowing in. If you install fans anywhere other than in the ridge, you can
disturb the stratification also.

Also, in your present situation cooler room air is able to mix with the
effluent, cooling it somewhat....... which has probably kept any flamable things
near the peak of the roof from catching on fire immediatly. However, wood as it
is hearted repaetedly chars and the ignition point drops over time. A good
inspection of the peak of the roof is in order........ when the kiln is NOT
firing of course .

You don't mention what the framing is of the building, but if the metal is
strapped onto any wood or fiberglass, you are asking for a fire. If there is
electrical wiring along the top areas, the insulation could melt causing a
short. Paper backing on insulation. Anything flamable is at risk.

If you put a fan in the "vent hole" at the peak, it should be explosion proof,
and it also needs to be rated for high temperatures. Also needs to be rated to
move MORE volume of gases than the kiln is producing. These fans are expensive.
You also need to do some design and testing so that you know the temperature of
the gases the fan handles are low enough so you don't exceed its rating. You'll
also have to provide for an adequate supply of make up air (low on the walls) to
replace what you are exhausting plus provide adequate air for combustion.

I would recommend that you look at putting at least a passive pick up hood
(preferably an active one) over the exit of the chimney and duct this directly
up through the roof line by 3' or so. This will be a high temp vent, and will
require special duct work. This type vent would not be directly connceted to
the existing chimney........ cool room air would also be mixed into the stream.
You should also look at putting a hood over the rest of the kiln to pick up the
leakage off of the chamber and door.

If you extend the chimney in a contigious fashion up through the roof (another
option), you will probably need to install a draft regulator unit, since such a
tall stack will induce excessice draft for such a small kiln.

For the immediiate future, you might get one of the inexpensive battery powered
carbon monoxide detectors at the local hardware store......... Nighthawk has a
nice one with LED ppm readings displayed on the front. They cost about $60.
Put one in the room and monitor it during firings. Try it at different levels
in the room. This might give you some interesting information. Also some

You can get a commercial firm to do air sampling in the room, and you'll really
know what you have got. Costs though! You can get a ventilation engineer to
look at it also.

Been building kilns professionally for about 25 years. I have seen
installations like what you seem to describe in the past. At colleges, no less.
But that was back in the late 60's and early 70's. Unless this is an
installation that was overseen by a qualified engineer, my guess is that it is
not really too good. And that would be by the standards of the 60's and 70's.

Today, the kiln would have a hood on the flue and a hood over the chamber. Or
the flue would lead directly outside and there would still be a hood over the

That, and $0.75 will fill your mug with coffee . Good luck.

John Baymore
River Bend Pottery
Wilton, NH

Edouard Bastarache Inc. on sat 18 oct 03

Hello all,

here is an excerpt from a study conducted in British Columbia, Canada
( for those who know that Canada is not a US state, Hehehehe)

Pottery kiln emissions were measured at 50 sites-10 from each of
5 categories: professional studios, recreation centers, elementary
schools, secondary schools, and colleges.

AUTHOR: Bob Hirtle; Kay Teschke; Chris van Netten; Michael Brauer
SOURCE: American Industrial Hygiene Association Journal v59 no10 p706-14 O '


Ventilation is generally recommended to control emissions from kilns.
Unfortunately, some of the ventilation strategies observed in this study
ineffective. Domestic wall/window fans, for example, appeared to have
minimal impact
on contaminant concentrations. Exhaust slots around kiln lid perimeters were
limited in
their ability to capture rapidly rising contaminants at higher kiln
Certain oversights in design or application likely resulted in compromised
performance at a number of sites. The quantity of replacement air may have
inadequate or the position of the air intake may have resulted in airflow
patterns that
failed to optimize the capture of kiln emissions. Because of their size and
some exhaust vents were not specific for kiln emissions, but acted more as
room exhausts, allowing the mixing of kiln emissions with room air.
Passive ventilation appears at least as effective as four other ventilation
(direct exhaust, overhead exhaust, wall/window fans, and slot exhausts).
This may
reflect conditions specific to those sites where passive ventilation was
observed: low
emissions levels, large dilution volumes, or natural airflow resulting in a
reduced need
for additional ventilation.
Where kilns were old and leaky, lids left ajar, or peep holes open, direct
through the bottom of the kiln was unable to prevent the escape of emissions
into the
kiln room. For negative pressure (direct venting) systems to be effective,
it has been
recommended that the fan should be near the exhaust end.Whether measured
exhaust efficiency of directly venting kilns was compromised by fan
placement of duct
length is not certain.
Despite some variation related to size, design, and placement, the use of
hoods was found to be one of the more effective ventilation strategies.
Because certain
exhaust strategies (e.g., wall exhausts) were not in common use, this study
was limited
in its ability to report their relative effectiveness with certainty.


"Ils sont fous ces quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker