How do you calculate supply airflow for an HVAC zone?

Use CFM = Q(sensible) / (1.08 x deltaT), where Q is the zone sensible cooling load in BTU/hr, deltaT is the temperature difference between room air and supply air (typically about 20°F), and 1.08 is the constant for air at standard conditions. Use only the sensible load, not the total load, in this equation.

When should you use a VAV system versus packaged RTUs?

Use packaged RTUs for small single-zone spaces such as retail or standalone suites. Use VAV systems for medium-to-large multi-zone buildings where central air handling with terminal boxes provides better zone control and lower fan energy. Large buildings above roughly 100 tons usually move to a central chilled-water plant.

How much outdoor air does ASHRAE 62.1 require?

The breathing-zone outdoor airflow is Vbz = Rp x Pz + Ra x Az, combining a per-person rate (e.g. 5 CFM/person in an office) and a per-area rate (e.g. 0.06 CFM/sq ft). That value is then adjusted by zone air distribution effectiveness and system ventilation efficiency to set the outdoor air the air handler must deliver.

Engineering·14 min read·June 15, 2026

🌡️ How to Design a Commercial HVAC System: A Step-by-Step Guide

A complete engineering walkthrough for designing a commercial HVAC system — cooling load calculation, equipment selection (RTU, VAV, chiller), airflow sizing with CFM = Q / 1.08 x deltaT, duct design, ASHRAE 62.1 ventilation, and controls.

📖Single-Family Residence HVAC Design Drawing Set (Full Access)

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The Commercial HVAC Design Sequence

Designing a commercial HVAC system is a disciplined sequence: calculate the load, select equipment to meet that load, distribute conditioned air at the right flow rate, size the ducts, bring in code-required ventilation, and tie it all together with controls. Skip a step and the system either underperforms, wastes energy, or fails code. This guide walks through each stage in the order a practicing mechanical engineer actually follows.

Topics covered

commercial hvac system designhvac designcooling load calculationduct sizingASHRAE 62.1 ventilationRTUVAVManual N