Residential Combustion Testing Procedures
To
ensure safe and efficient burner operation, all residential, 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 gases and 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 room 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.
When
testing atmospheric, forced air heating
equipment with a clamshell or sectional heat exchanger design, test each of
the exhaust ports at the top of the heat exchanger.
The probe should be inserted back into each of the exhaust ports to
obtain a flue gas sample, before any dilution air is mixed in.
Draft tests should be taken from a hole drilled in the stack downstream from the draft hood.
Combustion
and draft testing fan assist, furnaces/boilers should be done through a hole
drilled in the vent immediately above the inducer fan.
Condensing
furnaces/boilers can be tested through a hole drilled in the plastic vent pipe
(when allowed by the manufacturer or ‘local authority of jurisdiction) or
taken from the exhaust termination.
In
order to obtain an accurate Steady State Efficiency reading, an auxiliary
thermocouple must be inserted in the combustion air intake so that a true net
stack temperature is used in the calculation.
It
is important to remember that the vent system on these units operates under a
positive pressure. As a result, any
holes in the vent need to be sealed.
Domestic
hot water heaters with the ‘bell’ shaped draft diverter on top can be
accurately tested by attaching a section of copper tubing to the probe or using
a flexible probe which is then inserted directly into the top of the fire tube
below the diverter.
Another
common practice is to insert the probe in the hole drilled for the draft test,
direct it down and push it below the level 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.
Acceptable
Combustion Test Results
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)
6% 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)
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)