About this calculator

This calculator computes a Paragon HVAC unit's required airflow based on the zone loads and set points, then compares it with that of 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 OA airflow based on RTU equipment limitations.

The output shows:

  • A side-by-side Required Supply Airflow / LAT / OA% / Excess Supply Airflow table
  • A table of required supply CFM at discharge temperatures from 53°F to 58°F at the same loads, with the discharge dew point held constant
  • Fan-affinity blower savings, with a chart
  • An applied-capacity check of the Traditional RTU against the zone loads

An optional RTU Spec/RTU Replacement section lets you enter a specified or existing RTU's CFM and outdoor air to override the calculated Traditional RTU airflow; the comparison and applied-capacity check then use that RTU.

An optional Cost of Ownership Analysis adds a second output page comparing annual blower + cooling operating costs between the two units, a Ventilation Load line for the cost of conditioning each unit's outdoor air, the belt replacement/maintenance cost avoided by a direct-drive motor, the equipment + install cost of each unit annualized over its lifespan (12 years for the RTU, 20 for the Paragon), and the estimated duct cost savings from the smaller Paragon airflow.

Use when Comparing the required airflows between a Paragon HVAC unit and a traditional RTU. Airflows are calculated from the zone loads and conditions. Optional: quantifying the operating-cost, maintenance, equipment/install, and duct savings between Paragon HVAC and traditional RTU.
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 Conditions Target indoor cooling design dry bulb (°F) and humidity (RH, WB, dew point, etc.) set pointsDischarge Conditions The Paragon HVAC unit's design discharge supply air dry bulb (°F) and humidity (default 55 °F DB / 53 °F dew point)Specified/Existing RTU Optional — supply CFM and outdoor air CFM of a specified or existing RTU, which override the calculated Traditional RTU airflow and outdoor air. Its discharge conditions (default 57 °F DB / 53.5 °F dew point) set the Traditional RTU's discharge air whether or not a CFM is enteredWeather Station The ASHRAE weather station you choose from a nearest-first list (latest available 2025 data) — sets the site elevation, the 0.4% outdoor cooling design point used by the density-corrected airflow math, the cooling degree-days behind Expected Cooling Hours, and the hourly climate data behind the Ventilation LoadJobsite $/kWh / Operating Hours Blended electricity rate and average daily blower 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-daysAnnual Ownership Costs Optional — equipment cost of each unit and the install cost, annualized over a 12 year RTU and 20 year Paragon lifespan
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 Outdoor 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
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, though never below the outdoor airflow itself. The difference between the two sizings reflects both the outdoor-air cap and the warmer discharge air a traditional RTU supplies. This calculator quantifies that difference for a given set of zone loads and outdoor-air requirement.

Related Calculators

Cooling Process Calculator

Sensible + Latent CFM

Outdoor Air Load Calculator