Understanding External Static Pressure in Variable Refrigerant Flow (VRF) Systems

Understanding External Static Pressure in Variable Refrigerant Flow (VRF) Systems

External Static Pressure (ESP) in Variable Refrigerant Flow (VRF) systems refers to the resistance to airflow that a fan must overcome to deliver conditioned air through ducts and other components (e.g., filters, grilles, diffusers).

It's a critical parameter in ducted VRF indoor units and plays a major role in ensuring efficient and balanced air distribution.

CONTENTS

How External Static Pressure Works in VRF Systems

• How Ducts, Filters, and Diffusers Affect ESP

Why External Static Pressure Matters

• Thermal Comfort and Airflow Balance
• Energy Efficiency
• Noise and System Reliability

Typical ESP Requirements in VRF Applications

• Low vs. High ESP Use Cases
• When Higher ESP-Rating Is Needed

Key Takeaway

How We Approach ESP in Our VRF Systems at Johnson Controls Arabia

How External Static Pressure Works in VRF Systems

A ducted VRF indoor unit operates on the same airflow principles as any air handling unit. The amount of airflow delivered depends on where the fan performance curve intersects the duct system resistance curve.

External Static Pressure is critical for proper system design. It affects:

1. Fan selection: Indoor units come with built-in blowers rated for specific ESP ranges. Selecting the correct ESP rating is essential based on the ductwork design.

2. System sizing: ESP determines how much airflow the unit can deliver through the duct system. If the static pressure is higher than what the unit is designed to handle, airflow will drop. If the fan is oversized for the actual pressure, it may cause noise and waste energy.

3. Flexibility in layout: Medium and high-ESP models allow for longer ducts and more complex layouts, but the pressure must still remain within acceptable limits.

How Ducts, Filters, and Diffusers Affect ESP

Each air distribution component contributes to the overall external static pressure:

Ductwork

• Undersized ducts increase air velocity and friction losses, raising ESP.
• Long runs, bends, and sharp transitions introduce pressure drops.
• Well-designed duct systems with proper sizing and layout reduce friction and maintain ESP within range.

Filters

• Filters increase resistance to airflow, especially when:
• They're clogged or dirty, increasing resistance beyond the design limit.
• Their surface area is too small, forcing air through a tighter path.
• Many ducted VRF units come with built-in filters. If additional filters are added (e.g., for odor control), the ESP must be recalculated.

Diffusers

• The number, size, and type of diffusers determine how easily air is discharged.
• Small or restrictive diffusers increase pressure.
• Balancing dampers, if partially closed, raise ESP to control flow distribution.
• Choosing aerodynamically efficient diffusers and placing them correctly helps maintain low ESP.

Why External Static Pressure Matters

External static pressure (ESP) directly affects how well a VRF system performs.

Thermal Comfort and Airflow Balance

ESP directly affects how well air is delivered to each space. If the system faces more resistance than it's designed for, airflow drops. This leads to uneven temperatures between rooms, weak air throw at supply outlets, and inconsistent humidity and ventilation performance. Thus, maintaining proper ESP ensures that every room gets the air it needs for stable, comfortable conditions.

Energy Efficiency

When ESP is too high, fans and blowers must work harder to overcome the added resistance. This means:

• Higher energy consumption, as the system increases the fan speed to push air through restrictive ducts and components.
• Shorter equipment lifespan due to strain on motors.
• Reduced system efficiency, even if all other components are correctly sized.
• Optimizing duct design, filter selection, and diffuser placement can lower ESP and reduce the overall operating costs.

Noise and System Reliability

When the external static pressure (ESP) exceeds the system's design limits, it can lead to significant acoustic issues. This includes:

• Whistling or humming sounds from ducts and grilles.
• Fan noise from overworked blowers.
• Rattling or vibration in fittings and access panels.

Typical ESP Requirements in VRF Applications

VRF systems offer a range of ducted indoor units, each with different ESP ratings to suit various installation needs. Choosing the right ESP rating is essential to maintain proper airflow and system efficiency.

Low vs. High ESP Use Cases

• Low ESP Units (e.g., 0–30 Pa): Ideal for short duct runs, open ceiling layouts, or single-room applications. These units are quieter and more efficient in compact spaces but can't handle much resistance. Common in hotel rooms, small offices, or apartments with minimal ductwork.
• Medium to High ESP Units (e.g., 30 Pa-100 Pa): Designed for larger areas, multi-room zones, or where ductwork must travel long distances. These units use more powerful fans to overcome higher-pressure losses, making them suitable for villas or offices with drop ceilings and concealed installations.

When Higher ESP-Rating Is Needed

• Long duct runs or layouts with multiple bends, elbows, or fittings, which make it harder for air to flow smoothly.
• Return air is ducted instead of using open return, adding to the total pressure the fan must overcome.
• Several diffusers or grilles are connected to a single unit, increasing the overall demand.
• Extra components like high-efficiency filters or sound attenuators are added, restricting airflow further.
• Zoned systems with balancing dampers, which partially block the airflow to control distribution.

Key Takeaway

In ducted VRF systems, external static pressure (ESP) directly determines whether the indoor unit can deliver airflow through duct layouts evenly and efficiently.

If total system resistance, caused components like ducts, filters, and diffusers, stays within the unit's ESP capability, airflow remains stable, comfort is maintained, and energy efficiency is preserved.

But if the ESP is not properly calculated, airflow drops, compromising temperature control, increasing fan energy use, raising noise levels, and risking equipment strain.

How We Approach ESP in Our VRF Systems at Johnson Controls Arabia

At Johnson Controls Arabia, we understand how crucial external static pressure is when it comes to getting the most out of any VRF system. That's why our YORK® Mini VRF and Hitachi VRF AC Systems solutions are built with real-world duct conditions in mind, from compact residential setups to light commercial installations with longer duct runs.

We offer units with low, medium, and high-ESP ratings to fit different applications. Our engineering teams will work closely with you during the design phase to ensure that ESP is properly accounted for based on your building's requirements.

Contact us today to learn more about the VRF systems that works better for you.