Manometers have two ports. One port hooks up to a hose and reads the pressure in the duct, and the other port does NOT hook up to a hose; that port reads the room’s pressure to provide a reference point for the other port. The pressure in the duct is usually higher than the pressure in the room. You can also use a manometer to measure the pressure differential between rooms if you put the hose into another room with the door closed and read the pressure of the air in the room you’re occupying.

Many HVAC technicians are used to reading pressures in PSI or inches of water column. However, nowadays, we have been learning how to use Pascals to measure pressures much more finely, thus increasing accuracy and precision in diagnostics.

When measuring the airflow or static pressure on a gas furnace, you want to make sure the system is running at the highest cooling speed and that all the air is going through the main filter (no bypassing). You’ll want to measure the static pressure by placing the probes in the supply and the return so that they can read the pressure drop. Then, the TrueFlow hood can measure the actual airflow in cubic feet per minute. In the first furnace system, we discovered that the static pressure was good but that the CFM was borderline low. We determined that the filter was likely causing the slightly low airflow.

On the second system, the TESP was slightly high (with the supply plenum pressure being VERY high), and the airflow was too low. The TrueFlow app determined that undersized ducts or blockages were likely causes of these poor readings.