In HVAC, static pressure is one of the most critical measurements. Static pressure affects the performance of a system. And if it is too high or too low, it signals that something is wrong with the system.
You may have heard the term static pressure if you recently had maintenance or a check-up on your HVAC systems. Your HVAC pro might have told you that your HVAC system has static pressure that is too high or too low.
HVAC or air conditioning (AC) static pressure that is too low or too high can cause problems with an HVAC system. And one of the main reasons for common air conditioner problems. And can also cause problems with the heating systems.
Static pressure can be challenging to understand and may sound confusing. That is because we often relate the term static to static electricity shocks. Nonetheless, our goal is to ensure you understand what static pressure is. How to test the AC static pressure. How it can affect your HVAC systems. And how to fix high or low static pressure problems within an HVAC system.
Jump to a Section
What is static pressure?
The definition of static pressure in HVAC is a measurement of airflow in an HVAC system. That includes the ducting and air handling components. It measures the static (air that does not move) pressure force. The pressure is created from resistance in airflow 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 definition of static pressure in HVAC is that it is a force or pressure that is exerted in all directions evenly.
Below is a diagram of how static pressure in air conditioning looks within a duct system to help visualize the concept.
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 the blue perimeter ring is for visual representation. But it is hollow in reality.
In the diagram, the solid blue circle represents the “static pressure,” which is just air. The arrows represent the pressure exerted in all directions. And the dotted lines represent the flow zone (dynamic pressure), which is the area where the air can flow through the ducting.
When static pressure is high, the area of the static pressure increases. Which means 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 internal components of the HVAC systems. Like the heat exchanger and evaporator coil. Or other factors include the air filters, the size and the number of HVAC 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 it is lower, the velocity decreases. Keep in mind that high velocity does not mean high air volume.
What is good static pressure? That depends on the system. Each system has a maximum rated external static pressure. The ideal static pressure is good if the HVAC system doesn't exceed that rating and stays within an ideal range. Nonetheless, if it is too low, it can indicate other problems.
How to do a static pressure test
Every HVAC system has a maximum rated static pressure to work within manufacturer specifications. The maximum static pressure is usually stated on HVAC equipment—usually the furnace or air handling unit.
Static pressure is measured in inches of water column. The testing should be done by an HVAC professional. That is because the HVAC system must work in the correct testing conditions. And requires expertise to know how to check it correctly. The furnace or air handler static pressure is checked along specific points within the heating and cooling systems with a tool called a manometer. Doing so can help identify the root of a pressure problem.
When doing static pressure testing, the reading is taken at four points. Before the air filter. After the air filter but before the blower fan motor, wheel, and heat exchanger. After the blower motor, wheel, and heat exchanger, but before the evaporator coil. And after the evaporator coil.
The first two points checked are before the blower fan motor and wheel. Here the reading is under negative pressure because there is a lack of air (imagine a vacuum). And after it passes the blower motor and wheel. The reading is in positive pressure (imaging blowing into a balloon).
Each section of the heating and cooling systems has an ideal percentage of the total static pressure. Measuring static pressure at different points lets the HVAC technician determine the cause of the problem. And helps the HVAC pro to determine the static pressure measurement.
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 fan motor to work harder. As it forces the air through the system, it can cause it to wear faster and consume more energy.
In some cases, the strain on the motors can lead to a faulty air conditioner 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 with your thumb, the water will flow faster. And with more velocity but less water volume flowing through it.
This is precisely the case with high static pressure and HVAC air ducts.
Think of the air duct as the hose and the air as the water when static pressure is high. There is less air volume flowing through the system. And at the same time, it is coming out of the supply (air going into the home) vents at a high velocity. This can lead to an uncomfortable environment in the home. And cause problems with the HVAC system.
Noticeable symptoms of high static pressure include air coming out of the vents at a high velocity. The system fan is excessively noisy, and noisy rattling air vents.
Because high static pressure means less airflow through the system, it can affect both the heating and cooling systems. The HVAC system uses refrigerant to cool (and heat for heat pumps) the home. Along 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). When air passes through the indoor coil, it facilitates 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 air conditioner compressor. 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 a heating and cooling system. The main reason for it being low is leaking HVAC air ducts, disconnected HVAC air ducts, or HVAC 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 air duct system is too large, the low 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, 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. Older homes with older systems have HVAC ducting that is unsuitable for the new system pressure ratings.
And because each system is unique and designed to work under different design specifications. Old ducting is usually insufficient for newer heating and cooling systems. And can cause the air handler or furnace to have static pressure that is too high.
When ducting is not appropriately sized, the HVAC system lacks the airflow needed. Or can cause the system to work inefficiently. New air ducts are better insulated, which prevents 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. When designing a system, the HVAC pro should account for the system's ducting length, diameter, material, bends, and registers (vents). This ensures the HVAC system won’t exceed the maximum rated static pressure.
And when installing the ducts, the HVAC pro should follow the design specifications. And ensure there are no sharp bends. Or any objects such as wood in an attic will deform the duct's shape, prohibiting the airflow.
Another reason for high static pressure is a dirty indoor (evaporator coil). A clogged air filter or closed-off registers (vents). Or dirty HVAC air ducts. Again, the same concept applies. When there are restrictions within the HVAC system's ducting and air handling components, static pressure will be higher.
If you suspect your system is dirty, consider cleaning the evaporator coil. Or get an HVAC air duct cleaning. Your system will operate more efficiently, and you will breathe cleaner air.
Low static pressure in an HVAC system can be caused by HVAC air ducts that are too large or are disconnected.
How to improve static pressure?
When trying to lower or increase the static pressure of the HVAC system. You first need to understand the root cause of why is too high or low. Therefore we always recommend getting the static pressure tested before.
Wondering how to decrease static pressure? Reducing 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 zoning*. However, a static pressure test is essential. 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 return side (air going into the system).
Wondering how to increase static pressure? Increasing static pressure can be done by ensuring all ducts are appropriately connected. Decreasing the size (diameter) or the number of ducts, upgrading filters, and 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.
Static pressure vs. Dynamic pressure (What's the difference?)
To give you an idea, imagine you are standing outside, and the wind is blowing on your face. You can feel the wind pushing against your face with a certain amount of force. This force is what we call "pressure."
In HVAC, there are two types of pressure that we need to consider: static pressure and dynamic pressure.
Static pressure is the pressure of air that is not moving. For example, if you put your hand in front of a vent and feel air blowing on your hand, you feel static pressure. This pressure is important because it tells us how much resistance the air encounters as it flows through ductwork or HVAC equipment.
On the other hand, dynamic pressure is the pressure of air in motion. When air moves, it has energy that we call "velocity." This energy can be harnessed to do work, such as moving a turbine or propelling an airplane. In air conditioning, we use dynamic pressure to measure air velocity as it flows through ductwork or HVAC equipment.
A heating and cooling system with a dynamic pressure that is too low may not be able to move the air around the home properly—causing hot and cold spots.
So, in summary, static pressure is the pressure of air that is not moving, while dynamic pressure is the pressure of air that is in motion. Both types of pressure are important in HVAC because they help us understand how air flows through a system and how much energy it has.
Conclusion
Static pressure is one of the most critical measurements when looking at the performance of an HVAC system. When it is too low or too high, it can cause problems with the HVAC systems. Or decrease the HVAC systems efficiency.
It is important to ensure your system has good static pressure to operate efficiently and help reduce hot and cold spots in the home.
Now that you know what static pressure is. What causes it to be high or low? How to decrease or increase it. 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. Plus, you can benefit from breathing cleaner air.
And if you suspect it is poor ducting, you can learn about Air Design or contact an Air Pro.
Do not confuse static and dynamic pressures. Although one affects the other, they are not the same.
Contact us or comment below if you have questions.