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:
Ř
Carbon monoxide
Ř
Smoke (Fuel oil only)
Ř
Excess air
Ř
Stack temperature
Ř
Draft
Ř
Possibly NOx, NO, NO2
and/or SO2
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.
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 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.
Equipment
with an economizer, recouperator, or other similar device requires the
sampling point be downstream from and as close as possible to the device
(assuming they are installed before any draft control) to insure that the net
stack temperature will provide an accurate indication of the effectiveness of
the entire system.
While
combustion analysis is the emphasis here, remember that this is only one
important consideration in the overall scope of hvac system efficiency.
Temperature
rise, duct static pressures and fuel pressures, for example, all contribute to
safe, efficient and reliable heating system operation.
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.
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| Examples of going under the draft hood, into each exhaust port to perform combustion tests on these type units. | |
Combustion
and draft testing fan assist, furnaces/boilers should be done through a hole
drilled in the vent immediately above the inducer fan.
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.
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 |
 |
| Take draft test through hole drilled above draft hood | To obtain combustion test, point probe down and push below the level of the draft hood | Close up of probe going down below the opening in the draft hood |
Also, inspect and test the area in front of the burner access door for any indications of spillage.
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.
'Typical'
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”
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.
'Typical'
Condensing
Gas
Fired Burner
Oxygen (O2)
Our experience has been that test results from these burners vary widely depending on the manufacturer and the particular design of the unit. O2 readings range from 4% to 12% (a two stage unit in low fire).
Stack Temperature (°F)
Again, will vary from unit to unit.
Draft in Water Column Inches (WC”)
May be a high as 1.0 WC", again varies. The manufacturer should be able to provide a range considered acceptable for that particular line of heating equipment.
Carbon Monoxide in Parts Per Million (ppm) <100ppm - air free
Always, check with the manufacturer to determine actual specifications on the system being tested. While more and more manufacturers of condensing units are including this information in the installation manual, many still do not include it. You may have to call the manufacturer and find someone that can provide this information.
One of the things we'd like to include on this web page is a list of acceptable test results for all the different manufacturer's condensing heating system. If you would like to help us out and find the specifications for the make and model furnace/boilers you are installing please pass that info along. We'll post that information as we get it.
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.
'Typical'
Residential Oil/Gas Fired
Power Burners
Oxygen
(O2)
5% to
7%
Stack
Temperature (°F)
325° to 500°
Draft
in Water Column Inches (WC”)
-.02 WC” to -.01 WC”
Zero smoke with acceptable O2 readings (Oil fired).
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.
'Typical'
Commercial/Industrial
Light Oil/Gas Fired Power Burners
Oxygen (O2) Light off or Low Fire 7% to 9%
Oxygen (O2) High Fire 3% to 6% (Gas Fired) 4% to 7% (Oil Fired)
Stack
Temperature (°F)
325° to 500°
Draft
in Water Column Inches (WC”)
-.02 WC” to -.04 WC”
Zero smoke with acceptable O2 readings (Oil fired).
Smoke
Testing
Complete
combustion testing of a fuel oil fired system, #1 - #6, also requires a smoke
test.
When
dealing with fuel oil fired heating equipment, also perform a smoke test to
help identify incomplete combustion. A
common misconception is that before an oil-fired appliance will produce CO, it
will smoke so badly that it will be immediately evident a problem is
occurring.
While
it is generally true that a smoky oil flame will produce CO,
years of testing experience with electronic instruments has established that
the reverse is not always the case.
An
oil-fired unit not producing a measurable amount of smoke is very capable of
CO production. This is often seen
when too much combustion air is introduced into the flame which results in a
greater volume of flue gases being produced which acts to dilute the smoke to
the point where it may not be picked up by the smoke pump filter paper.
Smoke
tests are taken from the same sample location as the combustion tests.
A clean piece of filter paper is inserted into the tip of the smoke
tester and 10 strokes of the pump are taken.
The
filter paper is removed and the dot compared to the Smoke Spot Chart.
Generally,
modern residential flame retention burners should be set up for a zero smoke with O2
readings within manufacturer’s specifications, while an older conventional
style burner may be allowed between a #1 and #2 smoke.
A “yellow” dot is an indication of unburned, raw fuel that is escaping the flame pattern and being exhausted with the flue gases.