HVAC Static Pressure - How It Affects Efficiency and Performance

Static pressure in regards to HVAC is one of the most critical measurements when looking at the performance of a system. You may have heard the term static pressure if you recently had maintenance or a check-up on your HVAC system, and your HVAC pro might have told you that your HVAC system has static pressure that is too high or too low.


Static pressure that is too low or too high can cause problems with an HVAC system and is one of the main reasons for common air conditioning problems, and can also cause problems with the heating systems.


Static pressure can be challenging to understand and may sound confusing because we often relate the term static to static electricity shocks. Nonetheless, our goal is to ensure you clearly understand what static pressure is, how to test static pressure, how it can affect your HVAC system, and how to fix high or low static pressure problems within an HVAC system.


Jump to a Section

  1. What is static pressure

  2. How to do a static pressure test

  3. Effects of high or low static pressure

  4. What causes high or low static pressure

  5. How to improve static pressure

  6. Conclusion


What is static pressure


Static pressure is a measurement that measures airflow within the ducting and air handling components of an HVAC system. It measures the static (air that does not move) pressure force created when air is forced through the system. When a system has more restrictions, the pressure will be higher, and when there are fewer restrictions, the pressure will be lower.


The scientific explanation of static pressure in HVAC is a force or pressure that is exerted in all directions evenly.


Below is a diagram of how static pressure looks within an air duct to help visualize the concept.



Static pressure diagram


The circle is a cut-out of an air duct, so you can imagine you are looking into the air duct. The outer ring is the perimeter of the duct, and everything inside of the blue perimeter ring is for visual representation but is hollow in reality.


In the diagram, the solid blue circle represents the “static pressure,” which in reality is just air. The arrows represent the pressure exerted in all directions, and the dotted lines represent the flow zone, which is the area where the air can flow through the ducting.


When static pressure is high, the area of the static pressure increases and less air can flow throughout the system, and when static pressure is lower, more air can flow throughout the system.


Static pressure can be affected by the conditions of the HVAC system, such as the internal components of the heat exchanger and evaporator coil, or other factors such as air filters, the size and number of air ducts, or the type and position of registers (vents).


When static pressure is high, less air flows through the system, but the air flows at a high velocity (speed). And when static pressure is lower, the velocity decreases. Keep in mind that high velocity does not mean high air volume.


How to do a static pressure test


Every HVAC system has a maximum static pressure to work within manufacturer specifications. The maximums static pressure is usually stated on the air handling unit.


Static pressure is measured in inches of water column and should be done by an HVAC professional because the system must work in the correct testing conditions and because it requires expertise to know how to check it correctly. Static pressure is checked along specific points within the HVAC system, which can help identify the root of a static pressure problem.


When checking static pressure, the reading is taken at four points: before the filter, after the filter but before the blower wheel and heat exchanger, after the blower wheel and heat exchanger but before the evaporator coil, and after the evaporator coil.


The first two points checked are before the blower wheel. Here the reading is under negative pressure because there is a lack of air (imagine a vacuum). And after it passes the blower wheel, the reading is in positive pressure (imaging blowing into a balloon).


The static pressure reading at each point helps the HVAC technician understand how the system performs along different points in the HVAC system. This allows the HVAC technician to determine the root cause of the problem.


Common reasons for high or low static pressure include poorly sized ducts, low-quality or dirty air filters, closed registers (vents), disconnected ducts, or a dirty evaporator coil.


The effects of high or low static pressure


When static pressure is too low or too high, it negatively affects the HVAC system and home comfort. Static pressure too high will cause the blower motor to work harder when forcing the air through the system, which can cause it to wear faster and consume more energy. In some cases, the strain on the motors can lead to a faulty AC capacitor.


Furthermore, less air flows through the system when the static pressure is high, and the air flows out at a faster velocity. Think of it like a water hose. If you close off the end of it with your thumb, the water will flow faster and with more velocity, but less water is flowing through it.


This is precisely the case with high static pressure and air ducts. Think of the air duct as the hose and the air as the water. Less total air is flowing through the system, and at the same time, it is coming out the supply (air going into the home) vents at a high velocity which can lead to an uncomfortable environment in the home and cause problems with the HVAC system.


Because high static pressure means less airflow through the system, it can affect both the heating and cooling cycles. The HVAC system uses refrigerant to cool (and heat for heat pumps) the home with an indoor and outdoor coil and a compressor in a refrigeration cycle.


Lack of airflow will prevent the refrigerant in the indoor coil from evaporating or condensing (heat pumps) into a gas or liquid (heat pump) correctly. When air passes through the indoor coil, it facilitates a heat transfer and changes the state of the refrigerant.


In the cooling (and heating for heat pumps) stages, when the refrigeration cycle doesn’t happen correctly, it can cause the air conditioner to freeze and damage the AC compressor. And in the heating stages, it can cause the system to trigger safety switches and cause the heat exchanger to crack over time.


Although rare, static pressure that is too low can also limit the performance of an HVAC system. The main reason for low static pressure is leaking, disconnected, or air ducts that are too big.


A leaking or disconnected air duct on the return side can cause the internal components of the HVAC system to get dirty and negatively affect the home's indoor air quality. A leaking or disconnected air duct on the supply side (air going into the home) wastes energy because the conditioned air is not making its way into the home. And if the ducts are too large, the low static pressure will cause the velocity of the air flowing out of the registers (vents) to be very low, which will limit the conditioned air from flowing in and around the home.


What causes high or low static pressure?


There can be many reasons why static pressure is high or low. Ducting can be bad because it is installed incorrectly, is sized incorrectly, or is old.


One of the most common mistakes and reasons for high static pressure is installing a new HVAC system and not replacing the ducting. Because each system is unique and designed to work under different design specifications, old ducting is usually insufficient for newer HVAC systems and can cause high static pressure.


When ducting is not appropriately sized, the HVAC system lacks the airflow needed to work correctly and can cause the system to work inefficiently. New ducts are better insulated, preventing heat loss when the conditioned air travels through the ducts, and when properly sized, the blower fan motor won’t need to work as hard, consuming less energy.


Great care and calculation are needed when sizing and installing ducts. The ducting design in an HVAC system needs to meet the system requirements. An HVAC pro should account for the length, diameter, material, bends, and registers (vents) of the system to ensure the HVAC system won’t exceed the maximum rated static pressure. And when installing the ducts, the HVAC pro should follow the design specifications of the ducting and ensure no sharp bends are made, or any objects such as wood in an attic will deform the duct's shape, prohibiting the airflow.


Other reasons for high static pressure include a dirty indoor (evaporator coil), a clogged filter, or closed-off registers (vents). Again, the same concept applies. When there are restrictions within the HVAC system's ducting and air handling components, static pressure will be higher.


Low static pressure in an HVAC system can be caused by ducts that are too large or disconnected.


How to improve static pressure?


When looking to lower or increase the static pressure of the HVAC system, you first need to understand the root cause of why the system has high or low static pressure. Therefore we always recommend getting the static pressure tested before.


Decreasing static pressure can be done by increasing the size or number of ducts, upgrading filters, cleaning the indoor (evaporator) coil, ensuring all registers (vents) are open, and possibly zoning*. However, a static pressure test is needed to determine the location of the restrictions within the air handling and ducting systems.


If the solution is to increase the number or size of ducts, a test will determine if it should be on the supply (air going into the home) or the HVAC system's return side (air going into the system).


Increasing static pressure can be done by ensuring all ducts are appropriately connected, decreasing the size or number of ducts, upgrading filters, and possibly zoning*.


It is important to note that HVAC systems often have high static pressure rather than low.


*Zoning may be considered an option to increase or decrease static pressure; however, it may not always be possible because of system limitations.


Conclusion


Static pressure in regards to HVAC is one of the most critical measurements when looking at the performance of a system. When static pressure is too low or too high, it can cause problems with an HVAC system and decrease system efficiency.


Now that you know what static pressure is, what causes high or low static pressure, how to decrease or increase static pressure, and how an HVAC pro will test static pressure. You can confidently approach any HVAC-related static pressure problems.


If a dirty indoor (evaporator) coil is the cause of high static pressure, you can learn more about HVAC system cleanings. And if you suspect it is poor ducting, you can learn about Air Design or contact an Air Pro.


Contact us or comment below if you have questions.