Carbon
Monoxide and Combustion Testing Procedures
It is vitally important to discover and work within the local Authority
of Jurisdictions standards for CO concentration limits in flue gas. All
concentrations are referenced to a steady-state or stabilized condition of the
systems operation with combustion gas sampling points before the draft hood of
an appliance or other entries of dilution air.
Test procedures and conditions are examined in the following sections of
this manual.
<100 ppm CO air free
Gas furnaces, space and water
heaters are usually considered safe and left in operation. Annual tests.
It is reported to be as low as 25 PPM in some weatherization programs.
The more testing performed, the more the technician understands what is
achievable and reasonable.
<150 ppm CO air free
Common ceiling concentration
for unvented gas oven emission. Repair
recommended if over this amount.
100 - 400 ppm air free
Gas furnaces, space, water
heaters and boilers require further testing and correction.
Not necessarily immediately lethal concentrations of CO but conditions
generally found to be correctable within parameters of normal service work.
Systems are generally left in operation with set time limits for
correction to have been enacted.
400+ ppm air free
Frequently, gas systems are shut off and/or corrected when concentrations exceed
this concentration.
Carbon
Monoxide Sample Locations
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
The
sample point should also be as close to the breach area as possible.
This may also provide a more accurate CO reading should air be entering
the flue gas stream through joints in sheet metal vent connectors.
Oil
Burners
Locate the sampling hole at least six inches upstream from the breech
side of the barometric control and as close to the boiler breeching as possible.
In addition, the sample hole should be located twice the diameter of the pipe
away from any elbows.
Gas
Burners Locate the
sampling hole on power burner fired boilers/forced air units at least six inches
upstream from the breech side of any double acting barometric control and as
close to the boiler breeching as possible. Again, try to stay away from elbows. When testing atmospheric
equipment with a draft diverter/hood, the flue gas sample should be taken inside
the port(s) where flue gases exhaust the heat exchanger.
CO
testing should also be conducted around the burner area.
Draft
tests should be taken from a hole drilled in the stack downstream from the draft
hood.
Combustion
and draft testing fan assist (80%), furnaces/boilers should be done through a
hole drilled in the vent immediately above the inducer fan.
CO
testing should also be conducted around the burner area.
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.
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.
If
the furnace/boiler is not a ‘sealed system’ (which draws combustion air
through a pipe from the outside) and the burners are open, CO testing should
also be conducted around the burner area.
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.
CO
testing should also be conducted around the burner area.
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.
CO
testing should also be conducted around the burner area.
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Boilers/forced
air units, 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.
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.
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 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
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)
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)
*Per Manufacturer’s Instructions
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