Understanding Airflow

Why Proper Airflow Is Critical To HVAC Performance

Back to HVAC 101

TL;DR - What You'll Learn
✔ Proper airflow is essential for your HVAC system to heat and cool efficiently.
✔ High static pressure (TESP) forces your system to work harder and can shorten equipment life.
✔ Many comfort, efficiency, and indoor air quality problems can be traced back to airflow restrictions. 

​Airflow is one of the most important—and most overlooked—aspects of HVAC performance. The force that drives airflow is called static pressure. Understanding how airflow and static pressure work together can help explain many common comfort, efficiency, and ductwork problems.
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We know that air filtration is important. The American Lung Association emphasizes consistent filter maintenance as critical to a healthy home. But airflow is an important partner to filtration. NIOSH (National Institute for Occupational Safety and Health) emphasizes that filtration effectiveness depends on proper airflow conditions.
 
We covered in another article how your home is a system. Your HVAC system is designed to move a certain volume of air through the ductwork and into your rooms. The air must pass through:

• The air filter and return ducts,
• The blower (indoor fan) and indoor coil,
• The supply ducts,
• And finally into your rooms.

Introducing Total External Static Pressure or TESP

​To move air, the blower motor creates pressure inside the duct system. This resistance to airflow is called static pressure. An easy way to think about static pressure is like blood pressure for your HVAC system.

• Too low → the system isn’t moving enough air
• Too high → the system has to work much harder to move air

When static pressure is in the right range, your system can move air efficiently and operate the way it was designed.

Most residential HVAC systems are designed to operate with a target Total External Static Pressure (TESP) of around 0.50 inches of water column ("w.c."). Some specialty systems are designed to run at higher or lower pressures, but those are less common in residential homes. If you're unsure what the target TESP should be for your system, your HVAC technician can compare it to the manufacturer's specifications.

What Happens When Static Pressure Is Too High?

Typically in Austin, TX, we see more systems that have problems with high static pressure than low static pressure, so we’re going to focus on high static pressure issues here. The most common reasons we see for high static pressure are:

• Dirty filter.
• Dirty evaporator coil or blower wheel.
• Fan speed was never optimized and is too high.
• Ductwork is undersized.
• Not enough return area or a filter that is too restrictive for the existing return area.

 
High static pressure can lead to:

• More wear and tear on your system. Motors work harder and wear down faster.
• Lower energy efficiency. Your system uses more electricity to move air.
• More duct leakage.  If your supply ducts are leaky, more conditioned air will be pushed into your attic where it doesn’t help cool your home.
• Potentially worse indoor air quality. If your return ducts leak, dusty attic air may get pulled into the system without ever going through the filter.

If you're experiencing uneven temperatures, poor airflow, or comfort problems, Bluebonnet A/C Services can evaluate your HVAC system and identify what's restricting performance. Call 512-470-6240 or book online to schedule an evaluation.