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Static-Pressure Probes: Measuring Combustion Draft

January/February 2013
This article originally appeared in the January/February 2013 issue of Home Energy Magazine.
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January 02, 2013
Looking for a draft in a flue by measuring just the pressure in the flue—with reference to the combustion appliance zone, or CAZ—is a bit like discussing the speed of a moving vehicle without referencing the distance travelled or the time taken to get there. It’s interesting, but it doesn’t tell the whole story. The velocity of the airflow in a flue can be calculated by knowing the velocity pressure and the density of the air. The velocity pressure is the difference between the total pressure and the static pressure (see “Pitot Tube”). The total pressure is the impact pressure of the flow. The static pressure is exerted uniformly throughout the gas or fluid. The static pressure pushes out against the walls of the flue or duct. Despite the fact that the static pressure is exerted uniformly, to get an accurate draft reading, the probe used must be oriented to eliminate the impact of the total pressure on the static pressure.

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