Combustion Technology (Pty) Ltd
|Combustion||Perfect Combustion||Incomplete Combustion|
|Efficiency Types||Testing||O2 versus CO2|
|Three T's of Combustion||NOx||Draft|
|Flue Gas Test Procedures||Acceptable Test Results||Accurate Testing|
|Savings Potential||Training Centre|
Combustion Testing Procedures
Ø Carbon monoxide
Ø Smoke (Fuel oil only)
Ø Excess air
Ø Stack temperature
Ø 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.
Combustion and draft
testing fan assist, furnaces/boilers should be done through a hole drilled in the vent
immediately above the inducer fan.
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
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
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
Gas and oil fired power
burners should be tested up stream from the barometric, as close to the breech area as
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.
caution when taking over fire O2 readings. Do not expose thermocouple or
sampling assembly to excess temperatures longer than necessary.
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 manufacturers specifications or excess
levels of CO are produced.
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.
residential flame retention burners should be set up for a zero smoke with O2 readings within manufacturers 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.