External static pressure, commonly abbreviated as ESP, represents the airflow resistance that an air handler must overcome outside the unit.

This resistance is created by the actual air-distribution system connected to the equipment.

What creates external static pressure?

Supply ducts, return ducts, elbows, transitions, dampers, filters, grilles, diffusers and other air-side accessories all create pressure loss.

Why is ESP important when selecting an air handler?

An air handler may be capable of delivering a certain airflow at low resistance, but the available airflow can decrease as external static pressure increases.

Therefore, simply knowing that an air handler is rated for 1,600 or 2,000 CFM does not guarantee that it will deliver that airflow in the actual duct system.

How should an HVAC system be selected?

The required airflow and estimated system pressure loss should be compared with the manufacturer's blower-performance data.