External static pressure, or ESP, is the resistance that the rooftop unit supply fan must overcome in the connected air-distribution system.

It directly affects the actual airflow that the RTU can deliver.

What creates static pressure?

Pressure loss can be created by:

• Main ducts
• Branch ducts
• Elbows and fittings
• Filters
• Fire or balancing dampers
• Diffusers
• Return grilles
• Fresh-air components

Why is ESP important for large rooftop units?

Larger commercial duct networks can have significant airflow resistance. A rooftop unit may have enough nominal cooling capacity but still fail to deliver the required airflow if the available fan pressure is insufficient.

What should buyers ask?

Buyers should request blower or fan-performance data showing airflow at the expected external static pressure.