search  current discussion  categories  kilns & firing - misc 

gas kiln help? (was: firing with oil help?)

updated wed 13 jun 01


John Baymore on tue 12 jun 01


This problem is probably not any one big single factor but a combination =
little things adding up. Negative synergy . I think you are getting
nibbled to death by gnats .

Burners are four of your basic "raku" burners 75,000 btu. orifice very
close to 1/8"

When I fired with propane I used to peak out at 1 3/4 to 2 lb pressure a=
got very fast firings if I wanted.

Gas pressure at the meter is 4" I believe and goes fifty feet by 1"
flexible tubing to the kiln. The manifold is all 1" iron pipe.

Update on the information I have. My plumber tells me I have 1/2lb
pressure at the meter or 7wc inches. =

First of all........ if your gas plumber is telling you that 7" WC per Sq=
In. G. is EQUAL to 1/2 Lb. Sq. In. G., then you might want to find anothe=
gas plumber fast . Roughly speaking, 14 Inches Water Column is equal=

to 1/2 Pound per Square Inch. 7" WC is roughly equal to only about 1/4
Pound per Square Inch. BIG DIFFERENCE! Half what you were told. The
conversion factor from In. WC to Lb/ Sq In is to multiply In. WC by .0360=

When all is said and done....... one REAL problem with your kiln
installation may be your gas plumber!!!!! It is not all that uncommo=

What was your orfice size when you were on propane? How many gallons of
propane did it take for you to fire to what endpoint cone?

Four burners with roughly 1/8" orifice at 7" WC natural have roughly 100
percent excess capacity for a 15 CF total space kiln. Overkill at over
300,000 BTU's of input, but 4 burners were probably necessary for heat
source distribution in your particular design ideas. Are they "hand
drilled" orfices, or commercial orifices? Hand drilled can actually redu=
the theoretical output a little due to the roughness of the drilling job.=

Reduces the flow. More importantly, if the drilling is a little "off
axis", it can affect the air entrainment of the venturi....since the gas
does not squirt out tangental to the throat of the venturi. Technical cr=

When you say "flexible tubing" do you mean that kinda' bumpy stuff that
looks sort of like armored Romex cable ....or do you mean soft seamless
copper tubing? Or some sort of plastic / rubber pipe? By 1 y=
mean 1 inch OD (outside diameter) or 1 inch ID (inside diameter).....make=
a difference.

The flow capacity through that bumpy stuff is less than for smooth pipe. =
inch OD passes less gas then 1 inch ID.

If you actually have only 4" WC at the meter and then a 50-60 foot run of=
inch OD pipe and a few elbows, valves, and the like and anything ELSE is
hooked to the installation downstream of the meter (like a gas dryer or
stove), then my guess is that you don't have the capacity reaching the ki=
that you think you do, and it also will vary depending on if you are dryi=
clothes or not.

Now....... those "classic raku" burners you mentioned......... which I am=

ASSuming to be the cheapie 50 buck-ish MR750 or MR100, are the absolute
bottom of the barrel for burners. When running at a range of 1 to 2 Poun=
of pressure or more they are capable of entraining a reasonable (but low)=

amount of primary air. They'll work fine for the pretty un-demanding rak=
application, and sort of minimally OK for a regular kiln when on high
pressure. At 4-7" Water Column however.......... they will entrain VERY
LITTLE primary air. So you are completely tied to secondary air for
combustion issues. Not the place you really want to be if you want to ha=
any real control over a kiln. =

Prinary Air is a major controlling factor in firing a kiln. Preferably y=
want to have the option of having 105-110 percent primary air capacity
(excess air) at peak BTU output as a basic design factor. At that point
you are divorced completely from Seconday Air if you WANT to be. That, o=
course, requires power burners. So if you don't go the forced air burner=

route...... you want the best ventury type burner you can get, in order t=
maximize your firing control options.

While those MR burners are typically rated at roughly about 75,000 BTU's =
7" Water Column with Natural Gas, that is ASSUMING that the gas is actual=
burned. That is the BTU value of the gas that flow out of the orfice....=
not the real heat output. You only get the heat output if the gas is
burned in the kiln.

Are the burners mounted so that the face of the retention head (such as i=
is ) is about 1/2 to 3/4 inch back away from the plane of the exterior=

kiln wall? This allows Secondary Air to flow into the burner port, lets
the retention head operate properly, and also extends the life of the
burner. (Also see port size comments below.)

I tested and found at least 168 CFH getting to the kiln. so this should =
enough gas at least. =

How did you test this? Install a flow meter on the kiln end of the line?=

Or just look at the main gas meter readings while you ran all the burners=

at full throttle for a while and extrapolate?

Did you also check the PRESSURE at the burners under those full flow
conditions? Was there a pressure drop off of your nominal 7 Inches WC? =

Have you MEASURED the pressure at the burner manifold as 7 inches WC wi=
all the burners running full tilt boogie? Important question!!!!!!

168 CFH of Natural is about 168,000 BTU's per hour of POTENTIAL heat valu=
assuming your local version of Natural has a BTU/ Cubic Foot value of 100=
Probably close enough for government work . I still don't know if yo=
kiln is 15 cubic feet TOTAL space....or 15 cubic feet stacking space from=

the details you sent. The "rule of thumb" for heat input for an IFB kiln=

for high fire work is about 10,000 BTU's per Cubic Foot of total volume (=
use 9000 BTU's). So that says if your kiln is 15 need about
150,000 BTU's of heat input to operate well. So that puts your 168,000 B=
available pretty close to the mark. If the kiln is 15 CF stacking....the=
you might be just a little light on your potential heat input.

Again, that is assuming you are REALIZING that heat value. So you are JU=
in the approprate range for gas flow...... not much room for error there.=

Now..... couple this "right on the cusp" available fuel with the particul=
burners you have .....and you might have some problems. Ask your supplie=
what the primary air entrainment figure is on your "classic raku" burners=

at 7" WC pressure. You'll find that it is likely very low. The BEST
venturi burners operating at high pressures only entrain roughly about
75-80 percent of the primary air needed for combustion. These high quali=
venturi mixing units are quite expensive.....and worth every penny due to=

the P Air flexibility they provide to the operator.

If you are squirting fuel out of the burner orifices.... but are not
burning it IN THE KILN..... then you don't have 168,000 BTU's of heat
input. You have something less.

Now, add in the factor that utilizing secondary air for combustion requir=
SPACE and TIME (Carl Sagan, eat your heart out ). The secondary has t=
mix with the unburned fuel, and has to have time to do this. The amount =
turbulence in the kiln is a factor in this. At low pressure there is
little turbulence induced by the burners. This makes firing adjustments
critical to success with this type of kiln installation. This is a very
small kiln..... not a lot of room or time before the still mixing flame
front hits the exit flue. Then it doesn't matter . So it is possible=

that you are putting a lot of unburned fuel up the chimney. Have you
tested the exit flue conditions with a flue gas analyzer (ie Fyrite)? Ha=
your home furnace guy (who has one for sure) get a CO2 reading off your

Are there flames in the base of your chimney prior to putting the kiln in=

reduction? There shouldn't be. Even in want the flam
front JUST burning off in the base of the chimney as it exits the chamber=
. =


The fact that you are "stalling" SO low has me wondering if there is a
significant kiln control factor at work here also resulting in the heat n=
being realized in the kiln. Please describe the adjustments to the kiln
you make (gas pressure, P air shutters, damper settings, etc) during a
firing to get to where it stalls. What do the flames look like? At the
burner tips..... in the kiln, in the base of the chimney, etc. =

Also.... possibly in this somewhere is the 4 burner ports of "4 inches by=
inches" you mention. That is not usual brick module size. Are they
actually 4 1/2 inches wide by 5 inches tall? That would be much more
typical sizes to find, since it is brick module sizes. In either
case...... these are kinda large for such a small kiln and for the burner=
nozzle diameter that I THINK you are using. Overly large inlet ports
reduce turbulance in the firebox area due to the decreased velocity of th=
gases through the large hole. This isn't helping the mixing. These migh=
be able to be cut down a bit, and improve mixing a little. You only want=

about 1/2 inch of clearance around the nozzle.

A kiln that depends on significant secondary air is incredibly dependent =
stacking, weather, and exact control of the damper. Makes them really
tricky to fire. They tend to have "one" firing cycle....and that is it. =

Pain in the butt.

Damper is a kilnshelf in the chimney. fairly loose fit...1/4 inch to
When damper is all the way out it drops down to close off crack by the fa=
of being on an angle.

Here is a place that you are losing secondary air entrainment, which
probably isn't helping you. Air leaking past the damper into the chimney=

is air that is noit being drawn through the kiln. Try to close this
"crack" up when you are firing and get all the "draw" that you can.

Should I enlarge my jets? Raise my Chimney? Shrink my jets? What?

No...the orifices don't need to be bigger...... way more than enough
capacity with four burners. They should actually be reduced in size a bi=
which might improve the Primary Air entrainment of those burners SLIGHTLY=
. =

Theoretically you should size the orifices so that the 4 of them output =
little above the design BTU requirements of the kiln. However, that migh=
bring the orifice size down too low for the minimum size that allows prop=
venturi operation (decreasing P. Air entrainment) for the particular
burner. So check that minimum orifice size with your burner supplier.

Chimney might be a bit short (reducing some Sec. Air entrainment) but no
real info to go on....send the interior dimensions of the chamber.

Check the actual flow capacity of the "flexible" gas line in CFH at the
burner manifold....this is certainly possibly a factor here. =

Personally, no matter what, I'd look at possibly changing the burners if
they are the "MR" thing-ies. At least to a better venturi unit.

Hope this is helpful. And sorry to be asking so many questions. =

Diagnosing this stuff is exactly like being a medical doctor......... if
you miss the one critical question, you miss the key that points you in t=
right direction. Many diseases have the similar symptoms do
kiln issues. =



John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA

603-654-2752 (s)
800-900-1110 (s)

"Earth, Water, and Fire Noborigama Woodfiring Workshop August 17-26,
2001" (Full- now on waiting list)