Measuring a system’s static pressure is a bellwether to HVAC system performance, similar to measuring your blood pressure to determine overall health. Static pressure is the resistance against airflow in an HVAC system that must be overcome to deliver warm or cool air to a conditioned space. Manufacturers of air handlers and furnaces design their systems for optimal performance at or below a specified Total External Static Pressure (TESP). And measuring TESP with a digital manometer is just one way to help techs diagnose and troubleshoot HVAC systems.
How to Measure TESP
TESP is measured using a dual port manometer with probes inserted at the return side, typically after the filter, and the supply side of the furnace or air handling unit. Test ports will likely need to be created, so it’s important to be cautious and drill outside of the manufactured box in order to not puncture the coil, blower or heat exchanger. While the optimal TESP will vary based on the equipment, a typical system will run most efficiently at or below 0.5” water column (w.c.). What that means is that you don’t want to see a static pressure buildup in the return and supply that is higher than the specified TESP because it’s harder for the blower to work as designed and effectively move air.
Zero and Align Probes
Before inserting the hoses or probes into the test ports, hit the “zero” button on the manometer to make both port 1 and port 2 equal relative to each other and to the atmosphere. Insert the manometer hoses or probes so that the openings are perpendicular to the direction of airflow. More repeatable and accurate results can be achieved with static pressure probes instead of hoses. The probes with a magnetic base and arrow marking that aligns the probe tip into the airflow provide the highest guarantee of successful placement.
Taking Measurements
Once the probes are inserted and the system is operating at the highest airflow speed, wait approximately three minutes for the system to stabilize before taking measurements. Note that after a few minutes field manometers will drift, so it’s also important to reset with the “zero” button before each use.
Add the absolute value of the port 1 and port 2 measurements together to determine the TESP and compare to the manufacturer’s specified TESP. Some manometers will only provide the differential value, rather than a supply and return value. Fieldpiece manometers, such as the SDMN6 and Job Link® JL3KM2 Probes have independent sensors for each port so that if the TESP is not in compliance, you can troubleshoot more quickly by targeting the side of the system with the largest variance.
Diagnostic Tips
If the TESP exceeds the manufacturers specification, some common causes include dirty filters, blocked ducts, closed dampers, an unbalanced system, undersized ductwork, kinked flex duct or too much airflow. Under these conditions, fixed speed and variable speed motors will not operate optimally and experience premature failure.
With a much lower than specified TESP, common causes include leaky ductwork, separated duct connections, missing filters and low fan speeds. Resulting poor system performance typically means that a space is not being cooled or heated appropriately and the customer is not comfortable.
Using a digital manometer, techs can see the results of irregular maintenance and poor installation practices reflected in an out-of-spec TESP measurement. Whether you’re servicing a furnace, air conditioner or heat pump, airflow analysis and measuring total external static pressure is a critical indicator of a system’s health.
