What is Static Pressure in HVAC?


Static pressure is a common term used in HVAC. Not only homeowners but junior engineers often don't understand what static pressure means. As static pressure is critical to the performance of HVAC systems, I thought a full explanation is needed.

In HVAC, static pressure is the strength of a fan needed to deliver the required airflow. Ductwork components such as flexible ducts, return air grilles and air filters have static losses. Hence, a fan must have sufficient static pressure to overcome the static loss in order to deliver the required airflow.

Many people often have low airflow problems in their HVAC system. Sometimes, it is due to insufficient fan static pressure. Hence, it is important to understand the static pressure in HVAC.

HVAC Static Pressure Basics

Air conditioners, blowers, exhaust fans and other types of fans have a certain amount of static pressure depending on their design and airflow capacity.

How much airflow can be delivered by a fan is directly proportional to its static pressure. The higher the static pressure, the greater the airflow subjected to the maximum capacity of the fan.

Conversely, if the fan static pressure is not enough, the fan may not even be able to supply the airflow as promised on its specification sheet or nameplate.

The airflow rate of a fan is always coupled with a static pressure value. For instance, at medium fan speed setting, the below fan model has an airflow rate of 875 cfm when the external static pressure is at 0.3 in.wc.

Sample Gas Furnace Specification

But, if the external static pressure raises to 0.8 in.wc, the same gas furnace is now left with only 670 cfm of airflow. That's more than 20% reduction in airflow!

Therefore, it is important to select the correct fan static pressure based on the required airflow.

The unit of measurement for static pressure is commonly expressed as follow:

UnitDescriptionStandard
in.wcinch of water columnImperial Unit
in.wginch of water gauge
(same as in.wc)
Imperial Unit
PaPascalSI Unit

For unit conversion, 0.1 in.wg is equivalent to 25 Pa.

Static pressure is sometimes used interchangeably with external static pressure. External static pressure (ESP) is a more accurate term for the strength of the fan for actual airflow delivery.

There are two types of static pressures; a) internal static pressure and b) external static pressure.

Internal static pressure is the static pressure of the fan required to overcome the airflow resistance in the fan itself while external static pressure is the static pressure of the fan required to overcome the airflow resistance in the associated ductwork.

Ductwork Static Loss

As mentioned earlier, HVAC ductwork components have friction loss or static loss.

Similarly, friction loss and static loss are commonly used interchangeably. Static loss is also known as pressure loss or static pressure loss.

Below is the static loss of a few common ductwork components:

ComponentsCriteriaStatic Loss
90° Duct Elbow20×10″ @ 1200 cfm0.01 in.wg
45° Duct Elbow20×10″ @ 1800 cfm0.01 in.wg
Volume Control Damper8″ dia. @ 230 cfm0.02 in.wg
Flexible Duct16″ dia. @ 1200 cfm, 10 ft length0.05 in.wg
Round Duct (Rigid)16″ dia. @ 1200 cfm, 10 ft length0.01 in.wg
Rectangular Duct (Rigid)20×10″ @ 1200 cfm, 10 ft length0.01 in.wg
Return Air Grille10×10″ @ 180 cfm0.09 in.wg
4-Way Ceiling Diffuser2×2 ft; 8″ inlet @ 175 cfm0.04 in.wg
Linear Diffuser2-slot; 2 ft @ 175 cfm0.05 in.wg
Air FilterMERV 80.14 in.wg
Ductwork Component Static Loss

From the above table, we can see that different components have different static losses. The amount of static loss for an HVAC component is depending on the criteria such as size, airflow and length.

Needless to say, the “smoother” the component, the lesser the airflow resistance and thus, the lower the static loss. For instance, 45° duct elbow is smoother than 90° duct elbow and thus, the former has a lower static loss value. The above table shows the same static loss value despite the 45° duct elbow is having a higher airflow than the 90° duct elbow.

For VCDs or volume control dampers, the position of the damper blade affects the static loss value. The more you close the damper, the higher the airflow resistance and thus, the greater the static loss.

Flexible ducts generally have a higher static loss value than rigid ducts because they tend to bend in the middle. Otherwise, if a flexible duct is stretched perfectly straight, it has the same static loss as a straight round duct.

Undersized return air grilles and supply air diffusers not only produce more noise but they also cause high static losses which in turn reduce the airflow.

Lastly, the better the air filtration performance, the greater the static loss.

Therefore, we must pay attention to certain ductwork components that have exceptionally high static losses such as flexible ducts, return air grilles, supply air diffusers and air filters when calculating the required fan static pressure to ensure the required airflow can be delivered.

Fan Static Pressure

Static pressure is important when selecting a fan. In order for a fan to delivery the required airflow, it must have enough static pressure to overcome the total static loss which is the sum of the static loss of all ductwork components.

The amount of airflow supplied by a fan is affected by the total static loss. Underestimating the total static loss will result in low airflow supply.

Below is a diagram showing the characteristic of a typical fix-speed axial fan:

Optimal Performance

From the above diagram, the fan can operate optimally if the required airflow is 200 cfm and the total static loss is 0.4 in.wg. However, if we underestimated the total static loss, we could end up with something like below:

High Static Loss

If we expect the static loss to be at 0.4 in.wg but the actual static loss is 0.6 in.wg. The fan can't deliver the required airflow at 200 cfm. Rather, the best it can do is about 180 cfm at 0.5 in.wg. If the static pressure were to pull up to 0.6 in.wg, the resulted airflow could be around 160 cfm only.

Now, let's see what happen to the fan if we overestimated the total static loss.

Low Static Loss

As you might have guessed already, the fan will supply more airflow than it is required. If we selected the fan based on 0.4 in.wg but the actual static pressure required is 0.2 in.wg only, the resulted airflow will be around 240 cfm.

Nonetheless, the total static loss calculation is almost impossible to be 100% accurate. Many elements involve in the calculation are mere assumptions.

Therefore, we should expect the airflow to be at a range. In other words, we should give a safety factor in the static loss calculation. Usually, more airflow is better than not enough airflow.

However, more airflow or higher static pressure will lead to a more expensive fan.

Otherwise, we can select a variable speed fan to enable a wider range of operation thereby reducing the risk of airflow oversupply or undersupply.

One example of a variable speed fan is the performance datasheet shown earlier. The gas furnace has 5 different airflow rates at a single static pressure condition thanks to its multiple fan speed settings.

Conclusion

Static pressure is the pressure of a fan required to overcome the static losses in ductwork in order to deliver the required airflow. High static pressure fans may secure the performance of the HVAC system but they increase the cost of the HVAC system.

Some HVAC ductwork components have exceptionally high static loss value. Hence, it is important to be aware of them and increase the fan static pressure as necessary.

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Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

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