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Oxygen,
Carbon Dioxide 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.
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