Air Flow Diagnostics w/ Joseph C Henderson

Recently, the industry updated its testing standards (SEER2, EER2, etc.) to account for the static pressure of ductwork; previous testing protocols didn’t consider the effects of static pressure in the ducts.

ACCA Manual D is the industry standard of duct design, and Manual Zr deals with zoning (which depends on the integrity of the ductwork). Systems designed with Manual D tend to have better performance and airflow than systems that aren’t designed with Manual D in mind. When measuring and optimizing airflow, we can only expect better results if we use the proper test instrumentation, not just the “handometer.”

In the ductwork, we have two main sources of pressure: velocity pressure and static pressure. Air is always trying to expand, so static pressure is exerted on the surfaces of the duct and creates resistance because ducts can’t expand. When the static pressure is too high, the blower motor can perform poorly and even fail prematurely. We want just enough static pressure to ensure that we have a solid throw of supply air and smooth, quiet return air.

We can measure static pressure with manometers. Manometers measure pressure differentials, and we can use them with static pressure tips (pointed into the flow stream) to measure static pressure. A static pressure tip does not have a hole in it and is NOT the same thing as a pitot tube. Alternatively, you can insert a straight piece of tubing into the duct and point at a 45-degree angle WITH the airflow.

You want to account for all sources of resistance when measuring static pressure, so you’ll want to account for the filter and the coil. The airflow should be straight, also known as laminar airflow. You will need 3–5 feet of straight duct to ensure that you achieve that laminar flow.

You can also see how the motor is doing by taking an amp draw reading. Lower amp draws indicate that a PSC motor isn’t moving the full amount of air. Full load amps will indicate that the full amount of air is being moved, but it will not tell you if the static pressure is balanced. (Ideally, the return static should be lower than the supply static.)

Variable-speed and X13 motors tend to be more efficient, but their airflow capabilities are similar to PSC motors, and they still have their static pressure limitations. Constant-torque motors will have a slight increase in amp draw before dropping. Constant-CFM motors will pull far more amps than X13 and PSC motors.

PSC and X13 motors may only deliver air up to 0.5” of external static pressure, so you’ll want to make sure you start off with a maximum of 0.3” (0.1” on the return and 0.2” on the supply) so that the blower can handle additional resistance from coils and filters as they get dirty.

Constant-CFM motors may maintain their set airflow up to 0.8”–1” of static pressure, and you can typically start off with 0.35”–0.5” of static pressure; when the motor has to ramp up too much to maintain a constant CFM, the efficiency takes a hit. Constant-CFM motors are appealing because they tend to be quiet and efficient, but they can get louder and more inefficient under higher static pressure conditions.

To keep the static pressure down in the ductwork, you’ll want to make the trunk line as straight as possible. You’ll also want to keep flex ducts straight and tight to prevent compression, which adds resistance. Mitering the inside turns is also best practice to cut static restrictions (unless you have turning vanes). Be mindful of duct fittings, as they can significantly affect the static pressure restrictions.