Between Velocity and Volume: Mastering the Flow of Air
Air isn’t just air. It’s pressure. It’s friction. It’s the silent courier of comfort, the whisper behind every vent and register. To move air right is to understand how it breathes.
Two Ways Fresh Air Enters a Structure
- Natural Infiltration – through cracks, gaps, and unsealed doors or windows.
- Mechanical Ventilation – through designed systems such as ERVs, HRVs, and intake vents.
Forced-Air vs Natural-Draft
- Forced-Air: Uses blowers to move air across coils and through ductwork.
- Natural-Draft: Relies on temperature-induced buoyancy to move air—low pressure, low control.
Why Some Rooms Need More Airflow
Room size, insulation levels, number of windows, and exposure to sunlight all dictate heat gain and loss. A room facing west in the summer? Needs more airflow than a shaded one in the same home.
Four Desirable Traits of a Forced-Air System
- Even temperature distribution
- Quiet operation
- Responsive to thermostat inputs
- Efficient filtration and airflow control
Units to Measure Pressure
- Inches of Water Column (“wc)
- Pascals (Pa)
Three Pressures in a Duct System
- Static Pressure – Force against the duct wall
- Velocity Pressure – Pressure from moving air
- Total Pressure – Static + Velocity
Instrument for Measuring Duct Pressure
- Manometer – Reads pressure differences using fluid columns or electronic sensors.
What Is an Anemometer?
A tool for measuring air velocity. Types include vane, hot-wire, and cup styles.
Pitot Tube Construction and Use
A slender metal tube inserted into the airstream. One port measures total pressure, another measures static. Subtract static from total = velocity pressure. Essential for accurate airflow diagnosis.
Velocity vs Volume
- Velocity: Speed of air (ft/min)
- Volume: Amount of air (cfm = cubic feet per minute) Higher velocity = more volume, if duct area remains constant.
Propeller Fan vs Squirrel Cage Blower
- Propeller Fan: Moves large volumes at low pressure. Good for exhaust.
- Squirrel Cage: High pressure, low noise. Ideal for ducted systems.
Direct-Drive vs Belt-Drive
- Direct-Drive: Motor shaft connects directly to blower. Fewer parts, less maintenance.
- Belt-Drive: Allows for speed adjustment. More mechanical complexity.
Importance of Duct Sizing
Too small = noise, pressure drop, poor airflow. Too big = wasted materials and slow air. Proper sizing ensures comfort, efficiency, and quiet operation.
Four Common Duct Configurations
- Extended Plenum – Simple, good for small systems
- Radial – Ducts branch out like spokes
- Reducing Trunk – Maintains velocity, balances pressure
- Loop – Circular routing for equal pressure distribution
Duct Section Connections
Joined by S-cleats, drive cleats, flanges, or slip joints. Sealed with mastic or foil tape to prevent air leaks.
Four Common Duct Materials
- Galvanized steel
- Aluminum
- Fiberglass duct board
- Flexible duct
Inertia in Air Movement
Air wants to keep moving in a straight line. Bends, dampers, and filters slow it down. Sharp transitions increase resistance and noise. Smooth airflow = efficient airflow.
Balancing Dampers
Located in branch ducts. Adjust flow to individual rooms. Crucial for system tuning and room-to-room comfort.
Zoning
Divides a building into independently controlled areas. Uses dampers and multiple thermostats. Maximizes comfort and reduces energy usage.
Insulated Ducts
Prevents heat gain/loss. Reduces condensation. Critical in attics, crawlspaces, or unconditioned areas.
Air Blending
Registers and diffusers mix supply air with room air. Ensures even temperature distribution and prevents drafts.
Two Return Air Types
- Central Return – One large grille
- Multiple Returns – Grilles in each room
Friction Losses in Ducts
Every turn, bend, and rough surface adds resistance. This reduces airflow and can cause rooms to starve. Designing for low friction = quiet and efficient.
Determining Average Air Velocity
Use an anemometer or pitot tube. Take multiple readings across the duct cross-section. Average them. Multiply by area to get volume.
Purpose of an Air Friction Chart
Helps size ducts by relating airflow volume, duct size, and pressure loss. Essential tool for layout design.
Troubleshooting Air Distribution Systems
- Low airflow? Check filters, returns, and fan speed.
- Noisy ducts? Look for sharp bends or undersized branches.
- Hot/cold spots? Adjust dampers, inspect zoning, or re-balance.
Air moves like water—but unlike water, it’s invisible and temperamental. Learning to tame it, balance it, and distribute it right? That’s where craftsmanship starts. Next up: fuel, fire, and the combustion curve we ride to keep winter at bay.