HVAC Airflow, Static Pressure & Delta T Study Guide

CFM (Cubic Feet per Minute):

• Cooling: 400 CFM per ton
• Heating: varies by furnace BTU and rise

Static Pressure:

• Total external static pressure: 0.5" WC max
• Measure with manometer or magnehelic gauge
• Supply + Return = Total ESP

• Ducts are sized using a friction rate and a duct slide rule or software (like Manual D), based on required CFM.
• Undersized trunk or branch ducts create excess static pressure and reduce delivered airflow.
• Flex duct should be pulled tight — sagging or coiled flex duct dramatically increases resistance.
• Return air must be sized generously; undersized returns are one of the most common airflow problems in the field.

• Sensible heat: heat that changes a substance's temperature (what a thermometer reads), with no change in physical state.
• Latent heat: the heat required to convert a liquid into vapor, without change of temperature (and, in reverse, the heat released when a vapor condenses back into a liquid). In an HVAC system this shows up as the heat absorbed when water vapor in the air condenses into liquid on the cold evaporator coil — the water changes state and releases its latent heat into the coil, which is how moisture is removed from the air. Because temperature doesn't change during this process, latent heat doesn't register on a thermometer, even though real heat energy is being moved.
• Total cooling capacity is the sum of sensible heat removal (temperature drop) and latent heat removal (dehumidification) — a system's sensible-to-total ratio describes how that capacity splits between the two.
• Dew point: the temperature at which air becomes saturated and moisture begins to condense.
• Slower airflow across the evaporator coil increases dehumidification (more contact time for latent heat transfer) but reduces sensible cooling capacity.

• Balancing dampers at each branch allow airflow to be adjusted room by room without changing blower speed.
• Zoning systems use motorized dampers and a bypass or variable-speed blower to serve different areas independently.
• Always balance a system with all registers open and filters clean — balancing a dirty system just bakes in the wrong numbers.

• MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles - higher MERV means finer filtration.
• MERV 8-11 is typical for residential systems - a good balance of filtration and airflow.
• MERV 13+ filters capture much finer particles but add significant static pressure - verify the blower and duct system can handle it before upgrading.
• A filter that's "too good" for the system can starve airflow just as badly as a dirty one.

• Modern tightly-sealed homes and commercial buildings often need mechanical ventilation to maintain healthy indoor air quality.
• ERV (Energy Recovery Ventilator): exchanges heat and humidity between incoming and outgoing air streams - good for humid climates.
• HRV (Heat Recovery Ventilator): exchanges heat only, without transferring humidity - better suited to drier climates.
• Ventilation airflow is balanced separately from the conditioning system's airflow, but both interact with total static pressure and must be considered together.