Static-Pressure Probes: Measuring Combustion Draft

January/February 2013
A version of this article appears in the January/February 2013 issue of Home Energy Magazine.
Click here to read more articles about Column
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.

To read complete online articles, you need to sign up for an Online Subscription.

Once an order has been placed there is an automatic $10 processing fee that will be deducted with any cancellation.

The Home Energy Online articles are for personal use only and may not be printed for distribution. For permission to reprint, please send an e-mail to

Email Newsletter

Home Energy E-Newsletter

Sign up for our free monthly