Caution: It cannot be said enough how careful you need to be when drilling into a furnace to make pressure testing ports. Avoid drilling through the cabinet and hitting anything such as electrical, condensate traps, or the heat exchanger.
Static pressure is the resistance air faces when moving through a duct system or the balloon pressure outwards towards the walls of the duct.
A high static pressure reading can be related to the obstruction of airflow such as an undersized duct, closed dampers, crushed flex duct, or dirty filters and low static pressure would be the opposite. Missing filter, low fan speed, unattached flex duct, etc.
Maximum Total External Static Pressure (TESP) can be located on the appliance nameplate and is usually within 0.5 to 0.7’’ WC on residential equipment.
A total external static pressure reading is made up of two static pressure measurements added together. One is taken before the air enters the appliance and the other when air exits the appliance. It’s important to note that the return side of the appliance will bring up a negative reading. The negative sign is not to be taken into account. Pretend it doesn’t exist.
The two measurements are then added together. Both individual readings are external (outside) the appliance (furnace or air handler), hence the term total external static pressure.
Some TEST readings may be based on a wet coil. Check with the manufacturer to see if that’s the case with the equipment you’re working on.
The static pressure probes, or pitot tubes, should be placed with the tip facing either away from the airflow or against the airflow.