About this calculator This calculator computes a Paragon HVAC unit's required airflow from the zone loads and conditions, then compares it with a traditional RTU. When the outdoor-air fraction of the calculated RTU airflow exceeds 20%, the traditional RTU's airflow is overridden to five times the outdoor-air requirement to keep the OA fraction at the cap. A second section lets you enter a specified or existing RTU's CFM, outdoor air, and discharge conditions to override the calculated Traditional RTU column. The result is a side-by-side Required Airflow / OA% / Excess Airflow table, with a discharge-temperature sweep below it showing how required CFM varies between 58°F and 53°F at the same loads.
Use when You need to show how a Paragon HVAC unit's required airflow differs from a traditional RTU's when the outdoor-air fraction exceeds 20% of the cooling design CFM — both airflows calculated from the zone loads and conditions.
Variables
Outdoor Air CFM The outdoor airflow requirement (greater of code ventilation and any user-specified OA)Internal Zone Loads Space-side internal + infiltration sensible and latent loads (BTU/hr) used to calculate the airflowsSpace DB / Humidity Indoor cooling design dry bulb and a known humidity property (RH, WB, dew point, etc.)Discharge DB / Humidity Design supply air dry bulb and a known humidity property (WB, dew point, or humidity ratio)Specified/Existing RTU Optional — actual CFM, OA CFM, and discharge conditions of a specified or existing RTU to override the calculated Traditional RTU columnElevation Site elevation in feet — autofills from the selected weather station
Airflow Design/Calculation Tool
Outdoor Air CFM
The outdoor airflow requirement
Internal Zone Loads
Sensible Load (Btu/hr)
Space-side internal + infiltration sensible load
Latent Load (Btu/hr)
Space-side internal + infiltration latent load
Space Conditions
Dry Bulb (°F)
Dry bulb temperature
Relative Humidity (%)
Known Property
Relative Humidity (%)
Discharge Conditions
Dry Bulb (°F)
Dry bulb temperature
Dew Point (°F)
Known Property
Dew Point (°F)
RTU Spec/RTU Replacement Comparison Tool
Specified/Existing RTU CFM
Optional — actual airflow of the specified or existing unit
Specified/Existing RTU Outside Air CFM
Optional — outdoor air of the specified or existing unit
Specified/Existing RTU Discharge Conditions
Dry Bulb (°F)
Dry bulb temperature
Dew Point (°F)
Known Property
Dew Point (°F)
Estimated Blower Savings: Modern HVAC vs. Traditional RTU
Pick a weather station to enable required-CFM analysis.

Why does the 20% outdoor-air cap matter?

Traditional packaged rooftop units are typically sized so that outdoor air does not exceed 20% of the supply airflow. When the building's ventilation requirement exceeds that fraction, the unit must be enlarged so the outdoor-air ratio falls back to 20% — driving total airflow, fan power, and noise up disproportionately. A Paragon HVAC unit sizes the supply airflow to the actual zone loads regardless of the outdoor-air fraction, so the difference between the two sizings is a direct measure of the over-airflow imposed by the cap. This calculator quantifies that difference for a given set of zone loads and outdoor-air requirement.

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