Traditional RTU vs Paragon HVAC - HVAC Calculator Toolbox

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 outdoor airflow on the calculated RTU exceeds 20% of the total supply airflow, supply airflow is increased to maintain a 20% maximum OSA airflow based on RTU equipment limitations. 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 Supply Airflow / OA% / Excess Supply Airflow table, a chart comparing required supply CFM for different discharge temperatures ranging from 58°F to 53°F at the same loads, fan-affinity blower savings, and an applied-capacity check of the specified RTU against the zone loads. An optional Cost of Ownership Analysis adds a second output page comparing annual blower + cooling operating costs between the two units, the belt/bearing maintenance cost avoided by a direct-drive motor, and the estimated duct cost savings from the smaller Paragon airflow.

Use when You need to show how a Paragon HVAC unit's required airflow differs from a traditional RTU's when outside air CFM exceeds 20% of the total supply airflow — both airflows calculated from the zone loads and conditions — and optionally quantify the operating-cost, direct-drive, and duct savings.

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 airflowsTarget Space DB Temp / Humidity Target indoor cooling design dry bulb (°F) and humidity (RH, WB, dew point, etc.) set pointsMin Allowable Discharge DB / Humidity The lowest allowable design discharge supply air dry bulb temperature (°F) and dew point (°F)Specified/Existing RTU Optional — actual supply CFM, OA CFM, and discharge conditions of a specified or existing RTU to override the calculated Traditional RTU columnWeather Station Picks the nearest ASHRAE weather station using the latest available 2025 data — sets the site elevation and the 0.4% outdoor cooling design point used by the density-corrected airflow mathJobsite $/kWh / Operating Hours Blended electricity rate and average daily run hours driving the blower cost calculationsCost Analysis Inputs Optional — rated efficiency (EER/IEER/SEER), blower BHP, and cooling capacity for both units; cooling energy uses expected cooling hours auto-filled from the weather station's cooling degree-days

Airflow Design/Calculation Tool

Outdoor Air CFM

The outdoor airflow requirement

Name

Optional

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

Humidity Input

Relative Humidity (%)

Discharge Conditions

Dry Bulb (°F)

Dry bulb temperature

Humidity Input

Dew Point (°F)

RTU Spec/RTU Replacement Comparison Tool

Specified/Existing RTU CFM

Optional — actual supply 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

Humidity Input

Dew Point (°F)

Cost of Ownership Analysis

Jobsite $/kWh

Blended electricity rate at the jobsite

Operating Hours/Day

Average equipment run hours per day

Calculate Cooling/Blower Operating Costs

Include Direct Drive Maintenance Savings

Calculate Duct Savings

Estimated Blower Savings: Traditional RTU vs Paragon HVAC

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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