Why the New Unit Doesn’t Always Move Air Like the Old One Did

A replacement RTU can match the old unit in tonnage, footprint, and even curb dimensions and still perform differently once it’s running.

One factor that can contribute to that difference is static pressure.

Static pressure issues are easy to overlook during a retrofit. The equipment fits. The curb adapter lines up. The unit gets set, wired, and started up, but the airflow numbers still don’t match what the project called for.

In many cases, the clues were available before installation in the equipment data, airflow requirements, or existing field conditions.

This guide explains why static pressure can change during an RTU retrofit, what to review before startup, and how better planning can help reduce airflow-related surprises.

Relax… It’s Curbco.

Figure 1: How static pressure is measured across a rooftop unit’s supply and return path.

A Quick Refresher on Static Pressure

Static pressure represents resistance to airflow within an HVAC system.

Ductwork, filters, coils, dampers, transitions, diffusers, and other components can all contribute to pressure losses along the airflow path.

For retrofit planning, one important value to review is the external static pressure (ESP) the replacement RTU is designed to operate against.

The exact components included in an ESP rating can vary by equipment configuration and manufacturer, so always reference the replacement unit’s published fan data, installation instructions, and project requirements.

A replacement RTU may be designed to deliver the required airflow under different operating conditions than the unit it replaces.

That’s why matching tonnage alone does not necessarily mean the new unit will behave exactly like the old one.

Why Static Pressure Can Change During a Retrofit

Several conditions can change the airflow profile even when much of the existing ductwork remains in place.

1. Fan and Motor Differences

Newer RTUs may use different fan designs, motor sizes, ECMs, or variable-speed drives than older equipment.

Two units may be selected for the same airflow requirement but operate differently at a given static pressure.

The replacement unit’s fan curve should always be reviewed against the actual airflow requirement for the project.

2. Higher-Efficiency Filtration

Replacement units may use different filter configurations or higher-efficiency filters than the equipment they replace.

Depending on the filter type and operating condition, this can change pressure drop through the air path.

Filter specifications should be reviewed as part of the overall system evaluation.

3. Coil Configuration

Replacement equipment may also use different coil configurations than the original RTU.

Changes in coil size, depth, face area, or construction can affect the resistance encountered by the airflow path.

Again, the manufacturer’s published performance data should be used rather than assuming the new unit will behave exactly like the old one.

4. Curb Adapter and Transition Geometry

The replacement RTU is not the only consideration.

The transition between the unit, curb adapter, and existing duct system also becomes part of the airflow path.

Offsets, abrupt changes in direction, restricted openings, and transition geometry can contribute additional pressure loss.

That’s why a curb adapter should account for more than whether the new unit physically fits the existing curb. Supply and return geometry matter too.

➡️ [Explore Curbco Custom Curb Adapters]

5. Dampers, Economizers, and Outside Air Components

Retrofit projects may add or change economizers, motorized dampers, or outside air sections.

These components can affect airflow resistance and should be considered when reviewing the new equipment configuration.

What to Check Before Startup

Figure 2: A pre-startup static pressure checklist for RTU retrofit projects.

Before energizing the new unit, review:

• The replacement unit’s published ESP and fan data
• Project design airflow requirements
• Filter type, configuration, and published pressure drop
• Coil performance data
• Supply and return opening geometry
• Duct transition sizing
• Newly added economizers, dampers, or outside air components
• Fan performance at the airflow the project actually requires

Comparing these items against the project’s design conditions can help identify potential issues before they become startup or comfort problems.

Note: Always follow the RTU manufacturer’s published fan data, installation instructions, and project engineering requirements. Final airflow verification and system balancing should be performed by qualified HVAC professionals.

Field Signs Worth Investigating

If an airflow or static pressure issue is present after startup, it may show up as:

• Airflow below the project requirement
• Uneven conditioning across zones
• Unexpected supply or discharge air conditions
• Fan noise or operation that seems different from expected
• High-static alarms or safety trips on equipped systems

These symptoms can have several possible causes.

Before assuming the issue is controls-related or a balancing problem, compare the replacement unit’s fan performance and operating conditions with the system it is connected to.

How Curb Adapter Design Fits Into the Airflow Path

Static pressure problems are not always caused by the RTU itself.

The transition between the unit, curb adapter, and existing ductwork is also part of the airflow path.

A curb adapter engineered around the replacement unit’s actual supply and return geometry can help avoid unnecessary restrictions at the transition.

That does not replace system-level airflow design or balancing, but it does help ensure the adapter itself is designed around the specific equipment configuration rather than treated as a generic transition.

➡️ [Explore Curbco Custom Curb Adapter Solutions]

How Curbco Helps

Static pressure and airflow performance involve the entire HVAC system, but thoughtful curb adapter design can help avoid introducing unnecessary restrictions at the equipment transition.

For custom curb adapter projects, Curbco considers the equipment information and supply and return opening geometry provided for the application so the transition is designed around the specific RTU configuration.

Contractors and distributors can also turn to Curbco for:

• [Custom Curb Adapters] designed for specific equipment combinations
• [Curb Measurement Service] for projects where existing conditions need to be documented
• [KurbHub] for a simpler quoting and project workflow
• [FastPass] for qualifying projects with compressed schedules

Our role is to help simplify the transition between existing rooftop conditions and replacement equipment so your team can move forward with greater confidence.

Relax… It’s Curbco.

Planning a Retrofit With Airflow-Critical Equipment?

Before finalizing your equipment order, gather:

• Original system design ESP, if available
• Replacement unit fan data and published ESP information
• Filter and coil specifications
• Existing supply and return opening dimensions
• Details on economizers, dampers, or outside air components
• Project drawings and equipment submittals

📁 [Submit Your Drawings] so our team can review the curb adapter application and transition geometry for your project.

Need it sooner?

⚡ [Ask about FastPass] for expedited production on qualifying projects.

Related Resources

Continue building your retrofit knowledge with these guides and tools:

• [Complete Guide to RTU Replacement Projects]
• [How Contractors Measure Existing Curbs Before RTU Replacement]
• [10 Common Rooftop HVAC Retrofit Mistakes (And How to Avoid Them)]
• [How to Avoid Costly Change Orders During RTU Replacement]
• [Custom Curb Adapters]
• [Curb Measurement Service]
• [KurbHub]

Relax… It’s Curbco.