Application Note

4 Fluke Corporation Carbon monoxide: A mechanic’s approach
that creates friction in the
venturi, then less than
normal primary air will be
entrained and a dirty fire
will develop that secondary
air cannot clean up.
- Cold oil from an outdoor
tank will cause increased oil
flow through an oil nozzle
and resists atomization
and proper fuel/air mixing.
Reduce nozzle size and
increase pump pressure to
clean up the fire.
•
CO that steadily increases as
the burner operates usually
indicates combustion air and/
or venting problems.
The furnace heat
exchanger
The culprit is always a leaking
heat exchanger, and leaking
heat exchangers are deadly. The
firefighters said so. But we know
it ain’t necessarily so. We know
that restricted (and non-leaking)
heat exchanger passages draw
in less secondary air and elevate
CO levels. We know that if the
passageways in a non-leaking
heat exchanger are restricted
enough, then floating flames,
increased CO and roll-out
can occur. But unless a heat
exchanger leak interferes with
combustion or draft, the produc-
tion of CO is likely to remain
unchanged.
Even a new heat exchanger
can leak. One of the require-
ments for heat exchanger
manufacture stipulates in ANSI
standard Z21.47 that a new fur-
nace must have a leakage rate
of no more than 2 percent of the
total volume of flue gases. It may
leak when it’s new. Still, heat
exchanger integrity is something
that we try to monitor on an
annual basis. Visual confirma-
tion of a heat exchanger leak is
usually one of our more difficult
tasks. Before breaking out the
visual inspection tools, or tear-
ing down the furnace, try some
old tricks:
•
Close supply registers to
increase static pressure.
If there are no significant
changes before and after the
blower starts, then the heat
exchanger is probably fine.
•
Check O
2
(or CO
2
) before
and after the blower starts.
Increased O
2
(or decreased
CO
2
) would indicate air leak-
age into the heat exchanger.
•
With oil burners, compare
stack O
2
to overfire O
2
. Higher
O
2
(lower CO
2
) in the stack
than overfire points toward a
leaking heat exchanger.
•
With direct-vent products,
tee into the more positive
(“+”) pressure sensing tubes
and see if there is a pres-
sure change when the blower
starts. A change in pressure
differential can indicate a heat
exchanger leak.
What not to rely on:
•
Don’t use a CO reading at a
supply register to determine
heat exchanger integrity. That
only proves that the blower is
operating. But two CO read-
ings that show a difference
between the return at the
furnace (provided there are no
return leaks) and the supply
could point to a leaking heat
exchanger.
•
Don’t rely on a change in CO
readings in the vent when the
blower starts up to determine
a leaking heat exchanger. A
leak in the heat exchanger
might not have any effect on
the quality of combustion and
CO production.
Combustion air
Increasing CO and decreasing
O
2
(or increasing CO
2
) levels in
the vent point to combustion air
problems and possible venting
problems.
We know that adequate
combustion air must always be
available for the operation of
fossil fuel products. Just because
provisions are made that meet
code requirements doesn’t
mean combustion air is indeed
adequate. Even spaces that are
not considered confined (greater
than 50 cubic feet per 1,000
BTU input) still may not have
adequate infiltration for combus-
tion air and venting.
We think nothing of creat-
ing access holes in ductwork
for air measurements (dry and
wet bulb temperature, velocity
readings), but before we allow
fossil fuel products to operate,
shouldn’t we make a hole in the
building envelope to verify that
we are not depressurizing the
interior space?
All we need is a 3/16” or
¼” hole to connect to the “+”
side of our micro-manometer
(Fluke 922 Airflow Meter) for
checking pressure differential
between the equipment room
and outdoors. Start all prod-
ucts that vent to the outdoors:
furnaces, boilers, water heat-
ers, exhaust fans, clothes driers.