Monthly Archives: September 2013

In Part One of our discussion on total external static pressure testing of HVAC systems, we discussed checking the manufacturer’s installation information to determine what the TESP should be in a given system. As our chart showed, there isn’t just one number that applies to all systems regardless of the tonnage and the amount of air being moved by the indoor air handler.

However, one general application rule we can consider when evaluating any system in regard to the static pressure in the duct system is that most manufacturers of residential and light commercial equipment don’t want a pressure difference of more than .50″ W.C.  if a standard PSC, multiple-speed motor is employed by the air handling system. If the system employs a variable-speed, ECM-type motor, the TESP can be higher that that of PSC systems.

And, when it comes to evaluating an HVAC air flow system and going beyond the aspect of  isolating a given component in the duct system, such as the filter as we showed in the last segment, exact positioning the probes in the proper place varies, depending on the specific design of the system. For example, if you were evaluating a split system that employed a gas furnace as an air handler with an indoor coil positioned in the upflow position, Figure One shows you where you would effectively test the TESP of the blower itself in this particular type of system.

Figure One

Figure One

In this illustration, you’ll note that we have positioned the probe on the return side of the system after the filter so we can be sure we’re getting information directly at the inlet of the squirrel cage assembly of the blower. And when it comes to the probe on the supply side of the system, it’s positioned so that it is ahead of the indoor coil. Testing at these two points will tell us if the air handling system is operating properly in regard to the furnace itself, providing proper air flow through the duct system and the heat exchanger in the furnace. You’ll also note that we are just slightly above the general ‘.50 or less’ rule we mentioned for PSC blower assemblies. As we mentioned in the last segment on this subject, accomplishing static pressure tests provides us with the information we need to pursue a complete evaluation of the equipment operation and correct any potential problems affecting the total air flow through the duct system. If this system was a PSC rather than a variable-speed motor, we would need to expand our testing in order to bring the TESP across this blower down, and ensure the proper operation of this equipment.

Learn From Yesterday….Live For Today….Look Forward To Tomorrow


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.

Learn From Yesterday….Live For Today….Look Forward To Tomorrow



Educators and trainers may
qualify for discounts on selected
training products through
the Esco Group.

HVACR students click here
for student pricing on tools
and training materials

Appliance Repair Training Information


Technical Training Associates
PO Box 2259
Green Valley, AZ 85622-2259
Phone: 520-625-6847


Shopping Cart

Items in Your Cart

Your cart is empty


Click to View a
Training Video Sample


Appliance Repair Training Information
Link To Us

Increase your website traffic with