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External Static Pressure In HVAC Systems Part One

When it comes to the improper proper operation of an HVAC system, incorrect air flow is the number one cause of the malfunction. And one way for technicians to understand air flow in an HVAC system is to understand the concept of external static pressure.

A major factor regarding the performance of the air handling system is the static pressure that will exist between the negative side of the system (the return) and the positive side of the system (the supply). From a practical standpoint, a manufacturer’s blower performance chart (see Figure One) dictates what the TESP…Total External Static Pressure… is allowed to be in a given system.

Figure One
Figure One

In our illustration you can see that this chart is specific to two particular equipment models. When considering the high speed mode of blower operation on the chart, then following the CFM listings to the right until you find the block that shows 1311 CFM, you can then follow up to the top of the chart to the E.S.P listings to see that in this particular case, the manufacturer’s specifications state that the maximum TESP allowed is 0.4” WC (Water Column Inches). What this specifies is that when all items that would contribute to pressure drop in the air handling system are considered, including the filter, the ductwork itself, an indoor coil, and the air supply registers and return grilles, this is the maximum TESP that should be measured. A measurement beyond that number indicates that the performance of the air handling system, and therefore the performance of the refrigeration system and its ability to transfer heat, is being affected, preventing the equipment from operating at peak efficiency.

In the event that a test of a system accomplished with either a Magnehelic or a digital manometer showed a higher-than normal static pressure, the next step would be to perform tests on individual components to find out what steps could be taken to bring the equipment up to its intended operating standards. For example, isolating the filter and performing a static pressure test there, could reveal a simple solution to an overall static pressure that exceeds manufacturer’s specifications. (See Figure Two)

Figure Two
Figure Two

In this case, you’ll note that we have a static test probe positioned on either side of the filter in the return duct, and our Magnehelic is showing a pressure of less than .20” WC. The factor to consider here is that if the measurement at this or any other point in the system gave us enough room to be within the manufacturer’s standards, the equipment would be on track to perform properly. However, if this number was excessive, it would indicate that action must be taken, and there is one fundamental factor that a we could consider here.

That would be whether or not the filter was of the proper design for this system. While a high-grade filter works in one particular system, the design of another system may dictate that filter with lower resistance would be required. And, if a consumer, incorrectly assuming that the more dense filter would be the right one use, installed one that was of improper design, this simple action could affect the overall performance of the air handling system.

Just one of the simple things we need to understand about troubleshooting an HVAC system.

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