Getting started
Duct design workflow: from airflow to a pressure check Connect an established airflow requirement to clear duct geometry, constructible routes, compatible losses, and an equipment pressure boundary in a traceable sequence. Choose the duct calculation that matches the job Start with the decision you need to make, then choose the airflow, sizing, pressure, component, or system check that supports it. How to read a duct calculator result A useful result includes the number, its unit, its method, its assumptions, and the next check the number still needs.
Airflow measurement
ACFM vs. SCFM: actual airflow, reference conditions, and density Convert airflow between two explicitly stated moist-air conditions while conserving dry-air mass flow; SCFM has no single reference state unless the convention is specified. Blower-table airflow: an estimate, not a measurement Use the exact equipment blower table at a matching measured pressure boundary, and keep the table estimate distinct from field airflow measurement. CFM, FPM, duct area, and airflow capacity Use clear inside area and representative average velocity to solve airflow, or use airflow to find average velocity for one duct section. Plan and interpret a duct traverse Plan measurement points from clear duct dimensions, collect representative readings, and reduce them using the matching instrument method. What an airflow pickup K-factor means Use a product-specific airflow coefficient with the correct pickup ports, flow basis, density treatment, and documented signal range.
Duct sizing and geometry
Aspect ratio, clearance, and selecting a constructible duct size Carry the required area through aspect-ratio and installation-envelope constraints, then select only among the candidates and criteria actually supplied. Plan duct transitions and offsets from clear dimensions Separate centered rectangular transitions, offset end planes, and two-elbow offsets, then carry the selected geometry into a fabrication and pressure review. Equal-area vs. equal-friction duct conversion Choose whether a shape conversion preserves open area or matches the stated straight-duct friction correlation; the resulting dimensions answer different questions. Flexible duct sizing, sag, compression, and installed performance Treat flexible duct as an installed condition: product data, stretch, compression, sag, bends, support, and test boundaries all affect the pressure result. Round and rectangular duct sizing use different decisions The airflow relationship is simple; the sizing method depends on velocity, friction, shape, and the conditions around the duct. Target velocity vs. equal-friction duct sizing Choose a target-velocity screen or a friction-based straight-duct size by identifying the governing input, air properties, geometry method, and next pressure check.
Pressure loss and system checks
Available static pressure, TEL, and design friction rate Build a compatible pressure allowance for ducts and fittings, then divide it by a matching total effective length. Add system losses and identify the critical path Build compatible complete paths through a duct system, preserve shared segments and endpoint pressure requirements, then identify the route that governs a single-fan duty. Fitting K-factor vs. equivalent length Choose one fitting-loss representation that matches the geometry, flow path, velocity reference, and source data. Static, velocity, and total pressure: where TESP fits Keep fan-plane pressure, equipment TESP, velocity pressure, and total pressure tied to their correct definitions and measurement locations.
Leakage and balancing
Measured airflow balancing: compare, adjust, and remeasure Use a complete simultaneous terminal schedule and explicitly selected criteria to separate design-band checks, group totals, and proportional balancing references. Duct leakage testing: CFM25, leakage class, and operating leakage Compare a leakage test with a supplied class or allocation criterion, fit a same-specimen pressure curve when appropriate, and keep test leakage separate from operating airflow.
Components and terminals
Use matched filter and damper performance curves Match product, size, condition, curve basis, and pressure boundary before using a filter or damper pressure result in a system budget. Select grilles, registers, and diffusers using free area, pressure, and throw Screen the opening geometry first, then use matching product data for pressure, noise, and throw before selecting a terminal. VAV terminal selection across operating states Check a terminal, controller, coil, pressure, and catalog-noise envelope across required states instead of selecting from airflow or inlet size alone.
Ventilation and air balance
ACH vs. outdoor-air ventilation rate Distinguish total air changes from outdoor-air ventilation, then choose the calculation that matches the room or dwelling task. Bathroom exhaust airflow and installed duct resistance Calculate an initial intermittent bathroom exhaust target, then check whether the fan can deliver it through the complete installed route. Closed-door return-air pathways and transfer airflow Compare complete parallel return paths using documented pressure-flow curves, a net room-to-hall airflow, and an explicit project pressure limit. CO₂ ventilation estimates: steady state vs. decay Choose a steady-generation balance or a source-free decay model, and interpret the estimated flow only within its measurement assumptions. Commercial hood exhaust and louver selection from matched data Use named product and installation data to screen hood exhaust or louver candidates while preserving unmet pressure, weather, geometry, and code boundaries. Heat-recovery temperature ratio vs. effectiveness Keep a measured supply temperature ratio separate from sensible exchanger effectiveness when the two air streams have different heat-capacity rates. Makeup-air flow balance vs. building pressure Account for simultaneous exhaust and incoming streams, using mass balance when their actual air densities differ. Mixed-air temperature and outdoor-air fraction Choose a temperature-only estimate or a two-state moist-air balance, and keep actual volume fractions separate from dry-air mass fractions. Single-zone vs. multizone outdoor-air calculations Use a single-zone relationship for one zone and a multizone method when central primary flow, concurrent populations, zone efficiencies, and critical-zone effects interact.
Fans and energy
Blade-pass frequency: a frequency check, not a fault diagnosis Calculate blade-event frequencies from blade count and fan shaft RPM, then compare their locations with a compatible measured spectrum without assigning a sound or fault level. Fan balance and vibration criteria: what the readings can establish Keep residual rotor balance, factory vibration, and installed bearing-housing readings separate because their quantities and limits are not interchangeable. Compare fan energy cost across a full operating schedule Compare electrical energy and energy-only charges using average inputs or a complete annual schedule, while keeping shaft power separate from electrical consumption. Fan laws and density: when operating-point comparisons apply Scale a documented fan operating point only when geometry, density, pressure basis, and similarity assumptions match the calculation. Fan power: match pressure basis with efficiency Convert a fan duty point from air power to shaft and electrical power only after matching total or static pressure with the corresponding efficiencies. Fan speed, belt ratios, pulley geometry, and tip speed Calculate the geometric speed relationship and impeller tip speed first, then check the actual belt, motor, fan, vibration, and performance limits. Fan and system curves: operating point and installation effects Find supported intersections between supplied fan and system curves, then keep installation effects separate from ordinary resistance. FEI vs. FEG: what the rating represents Keep Fan Energy Index and Fan Efficiency Grade separate: they use different inputs, boundaries, methods, and evidence.
Thermal and materials
Dew point, duct condensation, and insulation limits Compare measured duct surface temperature with local dew point, or screen an explicit steady thermal-resistance model. Duct heat loss, fan heat gain, and outlet temperature Estimate steady sensible duct heat transfer or fan heat gain while keeping their input boundaries and air-property assumptions separate. Duct material takeoff: area, weight, wrap, and sealant Estimate straight-duct sheet, insulation wrap, or sealant quantities using actual geometry and product-specific inputs. Duct thermal expansion and free movement Estimate the free linear movement of a straight duct run from length, wall-temperature range, and an established material coefficient before reviewing guides, anchors, and flexible connections.
Acoustics
Trace a duct sound path: branch division, breakout, and end reflection Build a frequency-by-frequency acoustic path from source spectrum to branch, wall, or termination effects without claiming a complete room-noise result. NC vs. dBA: sound pressure, sound power, and frequency bands Use complete octave-band data and keep NC curves, A-weighted summaries, sound power, and sound pressure on their correct measurement bases. Select a silencer using insertion loss, generated noise, and pressure drop Rank silencer candidates only when actual airflow, pressure, space, source spectrum, receiver budget, insertion loss, and regenerated noise are supported by matching data.