Total external static pressure (TESP) is the sum of the supply-side and return-side static readings across an air handler. Most residential systems are rated for about 0.5 in. w.c.; much higher means restricted airflow.
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Frequently asked questions
What is a good static pressure?
Many residential systems target around 0.5 in. w.c. total; readings above ~0.8 usually indicate restriction.
Where do I measure?
On the supply side after the coil and on the return side before the blower, then add the absolute values.