Application Note

3 Fluke Corporation Carbon monoxide: A mechanic’s approach
normal values as steady state
operation is reached. Allow
at least 15 minutes for steady
state operation to be achieved.
Think about what you are
measuring. To find the amount
of excess air (air that has not
been used in the combustion
process), we measure O
2
or
CO
2
. To discover the quality of
combustion, we measure CO.
Since before any of us started
in this trade, the national
standard for maximum CO in
the vent of vented products
has been 400 ppm CO air
free.
(3)
Air free means: if we
remove the excess air from
the sample, what would the
CO be? Or, what would the
CO reading be with stoichio-
metric air: Only enough air for
perfect combustion without
any excess air whatsoever. No
product may legally exceed
this value, but some older
equipment may operate on the
threshold of this value.
Target values for modern
natural gas, LP gas and #2
fuel oil equipment steady state
maximum CO sample levels
should not exceed 100 ppm
CO. Setup values for effi-
ciently operating equipment
is considered to be between
10 ppm CO and 100 ppm
CO. These values are actual
sample levels not corrected to
“air free” levels. This is a good
rule of thumb and is recom-
mended unless specifically
overridden by the equipment
manufacturer.
O
2
and CO
2
are excess air
measurements and vary
widely between products.
O
2
is measured by modern
electronic analyzers and is
basically a direct indicator
of excess air. Six percent O
2
equates to about 40 % excess
air whether the fuel is natural
gas, LP gas, or # 2 fuel oil.
For comparison, 6 % O
2
and
40 % excess air is equivalent
to (using typical ultimate CO
2
values) 8.6 % CO
2
for natural
gas, 9.8 % CO
2
for LP gas, and
11 % CO
2
for #2 fuel oil.
Set up burners and air or
water flow volumes to manu-
facturers specifications. You
may need to consult the prod-
uct service manual or contact
the manufacturer directly for
excess air and CO values.
In the absence of manufactur-
er’s excess air specifications,
the following abbreviated
guidelines may be used as
typical values.
(4)
Excluding the laws of phys-
ics, every rule has exceptions.
Packaged rooftop gas equip-
ment may have unexpectedly
high O
2
(14 % O
2
, or 4 % CO
2
natural gas) in order to control
side vented vent temperatures
(flame temperature is inversely
proportional to excess air
quantities).
(5)
Variable capacity
burners can fire at over 10 %
O
2
at low fire. Older 1725 RPM
burners without flame reten-
tion heads may have a setup
range of 8 % to 10 % CO
2
.
Do not try to force a burner
to operate at typical values if
it does not want to willingly
comply.
As in any diagnostic proce-
dure, no one thing is used as a
stand-alone proof. We use our
senses, instruments, knowl-
edge and common sense to
gather as much information as
we can in order to support our
theory that a process is, or is
not, functioning as it should.
If a natural gas appliance is
installed that requires field
conversion to LP gas, do not
operate the appliance until the
conversion has been com-
pleted. If equipment set up for
operation with natural gas is
operated with LP gas, abun-
dant CO will be produced,
sooting is likely, and more
than one kind of headache
will be created. If the LP con-
version cannot be done at the
time of installation, disable
the appliance so it cannot be
operated until the conversion
has been completed.
The equipment, components
and passageways must be
clean. Components must be in
good mechanical condition,
physically and operationally,
and properly aligned. High CO
in the vent products usually
points to shortcomings of one
or more of those factors.
- Heat exchanger passages
must be clean and clear.
Look for metal flaking, scale
or soot especially at reduced
dimensions around baffles.
- Don’t overlook the impor-
tance of injection (Bunsen
type) burner venturi cleanli-
ness. If the interior of the
burner accumulates lint,
develops rust or scale, or
is coated with anything
6 % to 9 % O
2
11 ppm to 99 ppm
CO for Bunsen type (primary air
and gas mixed injection) burners
- 6.8 % to 8.6 % CO
2
for natural gas
- 7.8 % to 9.8 % CO
2
for LP gas
3 % to 6 % O
2
, 11 ppm to
99 ppm CO for power or
specialty burners (oil or gas)
- 8.6 % to 10.3 % CO
2
for natural gas
- 9.8 % to 11.7 % CO
2
for LP gas
- 11.1 % to 13.4 % CO
2
for #2 fuel
oil with flame retention head
burners