Combustion
Testing Procedures
To
ensure safe, efficient and reliable burner operation, all commercial and
industrial space and process heating equipment must be properly tested for:
 |
Carbon
monoxide |
|
Smoke
(Fuel oil only) |
|
Excess
air |
|
Stack
temperature |
|
Draft |
|
Possibly
NOx, NO, NO2 |
Oxygen, Carbon Monoxide and
Stack Temperature
The
measurement for flue gases and stack temperature should be taken at the same
point. Typically, this is done by
selecting a sample location ‘upstream’ from the draft diverter/hood,
barometric control or any other opening, which allows ambient air to enter and
dilute flue gases in the stack. In
larger installations it may also be necessary to extract a number of samples
from inside the flue to determine the area of greatest flue gas concentration.
Another common practice is to take the flue gas sample from the ‘Hot
Spot’ or the area with the highest temperature.
Make
sure that the sample point is before any draft diverter/hood or barometric
damper so that the flue gasses are not diluted and the stack temperature has not
been decreased by surrounding air used to balance the draft.
The
sample point should also be as close to the breach area as possible, again, to
obtain an accurate stack temperature. This
may also provide a more accurate O2 reading should air be entering
the flue gas stream through joints in sheet metal vent connectors.
Domestic
hot water heaters with the ‘bell’ shaped draft diverter on top can be
accurately tested by inserting the probe in the hole drilled for the draft test,
then directing it down and pushing it below the level of the draft hood.
Another
common practice is to drill a hole through the top of the draft hood.
When
testing boilers with a draft diverter mounted on the back of the equipment, flue
gas samples should be taken by passing the probe from one side to the other,
again upstream (toward the burner) from the opening into the draft diverter.
Draft
tests should be taken from a hole drilled in the vent connector immediately
above the diverter.
Boilers,
which have a ‘bell’ shaped draft diverter directly on top, should be tested
directly below the diverter through a hole drilled in the vent connector.
Should
draft tests below the diverter measure insufficient draft levels, an additional
test should be performed above the diverter to determine if the reason for
insufficient draft is related to a chimney problem or a draft hood problem.
It
is also a good idea to test any areas with openings that provide a path for
combustion air to be introduced to the flame.
These areas provide a path where flue gases can potentially be exhausted.
With
forced air systems this area is generally limited to immediately in front of the
burners while many styles of boilers allow secondary combustion air to also be
drawn in from all around the base of the cabinet.
Gas
and oil fired power burners should be tested up stream from the barometric, as
close to the breech area as possible.
While
stack draft may be an important measurement, fuel oil and gas fired power
burners require draft control over the fire to maintain a proper and controlled
intake of combustion air.
Comparing
stack and overfire O2 can verify that leakage between boiler
sections, access door, etc is minimal and the combustion test results are
accurate.
Use
caution when taking over fire O2 readings. Do not expose thermocouple
or sampling assembly to excess temperatures longer than necessary.
When
testing (primarily commercial/industrial) equipment with modulating or multiple
firing rates, it is critical that tests are performed throughout the entire
firing range.
Typically,
larger burners begin to fire at a reduced firing rate to insure a safe, reliable
light off. Once ignition has been
proven, air and fuel controls open to the full rated firing capacity of the
boiler. Once the call for heat has
been satisfied, the firing rate is slowly reduced to a minimum position before
the cycle ends and the flame is extinguished.
Failing
to test throughout the entire cycle of burner operation may not identify a
particular point at which O2 readings are outside the
manufacturer’s specifications or excess levels of CO are produced.
It
is very important to consult with the manufacturer or their literature to
determine acceptable ranges of O2,
CO, Stack Temperature, Steady
State Efficiency, Smoke and Draft.
Requirements for NOx and SO2 emissions (if any exist) vary
from local to local.
The
following ranges are generally considered acceptable for
commercial/industrial units; always check with the appliance manufacturer of
specific recommendations, particularly when testing residential systems as these
vary considerably from manufacturer to manufacturer.
Atmospheric
Gas Fired Burners/Fan Assist*
Oxygen
(O2)
7% to 9%
Stack
Temperature (°F)
325°
to 500°
Draft
in Water Column Inches (WC”)
-.02 WC” to -.04 WC” in the stack
Gas Fired
Power Burners*
Oxygen
(O2) Low Fire/Light Off
7% to 9%
Oxygen
(O2) Full Fire
3% to 6%
Stack
Temperature (°F)
275°
to 500°
Draft
in Water Column Inches (WC”)
-.02 WC” Overfire or PMI*
Oil Fired
Power Burners*
Oxygen
(O2)
4% to 7%
Stack
Temperature (°F)
325°
to 600°
Draft
in Water Column Inches (WC”)
-.02 WC” Overfire or PMI*
Carbon
Monoxide in Parts Per Million (ppm)
<100ppm air free
Smoke
Zero or PMI (Bacharach smoke number)
*Always check with the manufacturer of a specific piece of equipment to verify acceptable combustion test results as well as overfire and stack draft readings.