In Part 1 we covered how air moves and how to measure it. Part 2 is the hardware — the blowers, fans, duct materials, registers, and dampers that actually carry and shape the airstream. If Part 1 gave you the gauges, Part 2 gives you the parts list. Written to match the NCCER HVAC curriculum scope so it works equally well for credential study and for the tech standing in front of an open air handler.
- Part 1: Air movement & measurement.
- Part 2 (this article): Equipment & materials — blowers, fans, fan laws, duct, diffusers, registers, grilles, dampers.
- Part 3 (next): System design & energy conservation — layouts, room CFM, insulation, vapor barriers, duct sealing.
1. Blowers vs Fans — The Vocabulary First
In HVAC, blowers and fans both move air, but the terms aren’t interchangeable. The practical distinction the trade uses:
- Blower — a high-pressure air mover. Built to push air through ductwork against significant static pressure. Almost always a centrifugal design with an enclosed scroll housing.
- Fan — a low-pressure air mover. Built to move large volumes of air across short distances with little duct resistance. Usually axial (propeller) or open-bladed.
Residential air handlers and furnaces use blowers. Outdoor condensing units, exhaust hoods, attic ventilators, and through-the-wall propellers use fans.
2. Centrifugal Blowers
A centrifugal blower has a cylindrical wheel inside a scroll-shaped housing. Air enters the center (the “eye”) of the wheel, is flung outward by centrifugal force, and is collected by the scroll, which converts velocity into static pressure as it widens toward the outlet.
This is the workhorse of residential and light-commercial HVAC. The dominant style is the forward-curved (FC) blower, often called a “squirrel-cage” wheel because the many short, forward-leaning blades look like a hamster wheel. FC blowers move a lot of air at a moderate static pressure efficiently and quietly — ideal for residential systems.
Other centrifugal styles you’ll see in commercial work:
- Backward-inclined (BI): Fewer, longer blades leaning backward. Higher efficiency, higher static capability, slightly noisier. Common in rooftop units.
- Airfoil: BI blades shaped like wings. Highest efficiency, used in large commercial AHUs.
- Radial: Straight blades. Tolerates dirty/abrasive air. Common in industrial dust collection.
3. Belt-Drive vs Direct-Drive Blowers
Direct-Drive Blowers
The blower wheel is mounted directly on the motor shaft. No belt, no pulleys. Almost all modern residential air handlers and furnaces are direct-drive. Speed is set by:
- PSC (permanent split capacitor) motors — multi-tap windings; you change speed by moving a wire to a different tap (typically low / med-low / med / med-high / high).
- ECM (electronically commutated motors) — variable-speed; programmed via the control board for fixed CFM regardless of static pressure (within range). Reads ESP and ramps to maintain target CFM.
Advantages: fewer moving parts, no belt adjustment, quieter, more efficient (especially ECM). The trade-off is that capacity changes require a motor or board change, not just a pulley swap.
Belt-Drive Blowers
The motor and blower wheel are on separate shafts, connected by a V-belt running between a motor pulley (sheave) and a blower pulley. Common in commercial rooftop units and older systems.
Speed is changed by adjusting the variable-pitch motor sheave — opening the sheave halves makes the effective diameter smaller, which slows the blower; closing them speeds it up. The relationship is:
Blower RPM = Motor RPM × (Motor Sheave Pitch Diameter ÷ Blower Sheave Pitch Diameter)
Field maintenance on belt-drive blowers: check belt tension (about 1/2″ deflection at midspan with thumb pressure), check pulley alignment with a straight-edge, listen for belt squeal on startup, replace belts in matched sets on multi-belt drives.
4. Fans — Propeller and Duct Fans
Propeller Fans
An axial fan with 3–6 blades on a hub, mounted in a mounting ring or panel. Air moves through the fan in line with the shaft. Used where the airflow needs to be high but the resistance is essentially zero:
- Outdoor condenser unit fans (top-discharge or side-discharge condensers).
- Attic exhaust fans.
- Whole-house ventilation fans.
- Through-wall exhaust fans.
Propeller fans cannot overcome significant static pressure — they’re for free or near-free air movement only.
Duct Fans (In-Line Axial Fans)
Axial fans housed in a tubular shroud, designed to insert into a duct run. Used to boost airflow to a long branch or to exhaust localized areas. Modest static pressure capability (typically <0.5″ wc). Examples: bathroom exhaust duct boosters, grow-room exhaust, server-closet ventilation.
5. The Fan Laws — Why Speed Changes Everything
The fan laws describe what happens to airflow, pressure, and power when you change blower speed, wheel size, or air density. For a given fan moving air through a given duct system, the three core laws are:
- Law 1 (Flow): CFM varies directly with RPM. CFM₂ / CFM₁ = RPM₂ / RPM₁
- Law 2 (Pressure): Static pressure varies with the square of RPM. SP₂ / SP₁ = (RPM₂ / RPM₁)²
- Law 3 (Power): Brake horsepower varies with the cube of RPM. BHP₂ / BHP₁ = (RPM₂ / RPM₁)³
Worked example:
You bump a belt-drive blower from 800 RPM to 1000 RPM (a 25% increase) to get more CFM:
- CFM goes up 25% (800 × 1.25 = 1000 CFM).
- SP goes up 56% (1.25² = 1.5625 — original 0.5″ wc becomes 0.78″ wc).
- BHP goes up 95% (1.25³ = 1.953 — original 1/2 HP would draw nearly a full 1 HP).
That last line is why you can’t just “crank up the blower” on a marginally-sized motor — you’ll trip the overload or burn the windings. The fan laws also explain why doubling the duct length or halving the duct size disproportionately punishes your system: static pressure scales fast.
Fan Curves
A fan curve is the manufacturer’s plot of static pressure (y-axis) vs CFM (x-axis) for a given blower at a given RPM. As CFM goes up, the static pressure the fan can develop goes down — the curve slopes from upper-left to lower-right.
The system curve is plotted on the same chart and represents how much SP the duct system imposes at each CFM (a parabola through the origin, since SP rises with CFM squared). Where the fan curve and the system curve intersect is the operating point — the actual CFM and SP the system will run at.
Practical take: if you measure higher-than-rated ESP and lower-than-design CFM, your system curve is steeper than design assumed (restrictions). Fix the restriction and the operating point slides right (more CFM) and down (less SP) along the fan curve.
6. Duct Materials & Fittings
Galvanized Steel Duct
The traditional rigid sheet-metal duct. Available as rectangular (built up from formed pieces, sealed at seams) or round (snap-lock or spiral). Properties:
- Pros: Lowest friction loss of common duct materials, durable, fire-resistant, repairable, easy to seal with mastic and screws.
- Cons: Heavy, labor-intensive to fabricate, must be externally insulated to prevent condensation in cooling applications, conducts noise.
- Where used: Trunk lines, commercial systems, anywhere durability and low pressure drop matter.
Fiberglass Duct Board
1″ or 1.5″ thick rigid fiberglass panels with a foil/scrim facing on the outside. The board is grooved and folded to form a rectangular duct, then sealed with foil tape and outward-clinching staples.
- Pros: Insulation is built in (no separate wrap needed), absorbs duct noise, lighter than sheet metal, lower installed cost in residential.
- Cons: Higher friction loss than smooth sheet metal, fibers can shed if the inner facing degrades, harder to clean.
- Where used: Common in residential supply trunks, especially in attics and crawl spaces.
Flexible Duct (“Flex”)
A spring-wire helix wrapped in a plastic inner liner, surrounded by insulation, surrounded by an outer vapor jacket. Sold in 25-foot sections.
- Pros: Fast to install, conforms around obstructions, integrated insulation and vapor barrier.
- Cons: Much higher friction loss than rigid duct when bent or not pulled taut — even small sags or kinks dramatically restrict flow.
- Where used: Branch runs from trunk to register boot. Should be pulled tight, supported at least every 4 ft, and bent on a generous radius. Never use flex for trunk duct.
Fittings & Transitions
Fittings change duct direction (elbows), split or join airstreams (tees, wyes, takeoffs), or change duct size (transitions, reducers). Key field rules:
- Use long-radius elbows wherever possible. A radius elbow with throat-to-centerline ratio of 1.0 has about 1/3 the pressure loss of a short-radius elbow.
- Use turning vanes in square elbows. They cut elbow pressure loss by 60–80%.
- Use conical takeoffs, not straight or saddle takeoffs — they significantly reduce branch entry loss.
- Slope transitions gradually — ideally no more than 20° on contractions and 15° on expansions.
7. Diffusers, Registers, and Grilles — Know The Difference
The terms are used loosely in the field, but precisely they mean different things:
- Grille — a fixed louvered face with no damper. Used for returns and transfer air.
- Register — a grille with an integrated damper. Used on supply runs so flow can be balanced or shut at the room.
- Diffuser — a supply outlet specifically designed to shape the discharge airstream (typically ceiling-mounted, with concentric cones or curved blades to throw air radially or in a defined pattern).
Selection considerations:
- Throw — how far the discharge airstream travels before its velocity drops to a target (typically 50 FPM). Must reach the far side of the room without dumping cold air on occupants.
- Spread — the lateral width of the airstream pattern.
- Noise Criteria (NC) — the noise level the diffuser generates at the design CFM. Residential bedrooms typically need ≤NC-25; offices ≤NC-30.
- Face velocity — controls noise. Most residential supplies aim for 500–700 FPM at the face; returns 350–500 FPM.
8. Dampers
Balancing (Volume) Dampers
Manual blade dampers installed in branch ducts (best practice: 2–3 duct diameters downstream of the takeoff). Adjusted once during commissioning to balance airflow between rooms. Most modern systems also have a damper integrated into each register, but branch dampers give you more authority and don’t cause register-face noise.
Motorized Zone Dampers
Powered blade dampers controlled by a zone panel and thermostats. Allow a single piece of equipment to serve multiple thermal zones independently. Use a bypass damper or modulating ECM blower to prevent over-pressurizing the supply when only one zone calls.
Fire Dampers
Spring-loaded steel-curtain dampers installed where ducts penetrate a rated fire-resistive wall, floor, or ceiling. Held open by a fusible link rated typically 165°F (or 212°F where the duct is in a high-temperature area). When the link melts, the spring closes the curtain to block fire spread through the duct.
- Must be installed per UL listing and IMC / NFPA 90A requirements.
- Must be accessible for inspection (access door in the duct).
- Periodic operational testing required in commercial buildings (typically every 4 years per NFPA 80).
Smoke Dampers
Motorized dampers that close on a signal from the building’s smoke detection system to prevent smoke migration through the duct. Often combined with fire dampers as a combination fire/smoke damper in a single assembly. Required in smoke-control systems and at smoke barrier penetrations.
Backdraft / Gravity Dampers
Hinged blade dampers that allow flow in only one direction. Common at exhaust fan discharges to prevent outside air from blowing back into the building when the fan is off.
9. Common Field Issues — Equipment Edition
- Dirty blower wheel → CFM drops 30–50% with no other change. Pull and clean if blades show buildup.
- Belt slipping → blower RPM low; squeal on startup. Tension or replace.
- ECM in “rescue” mode → runs at a fixed reduced speed when it can’t meet CFM target due to high static. Read the control board diagnostic LEDs.
- Flex duct kink → the most common residential airflow killer. Inspect every turn and every support.
- Dropped/disconnected return → equipment pulls unconditioned attic or crawl air; capacity and humidity control both suffer.
- Damaged turning vanes → loud popping or moaning noises during operation; significant SP increase.
- Fire damper accidentally tripped during construction → entire branch dead-headed; check access doors first when one zone has zero flow.
Coming In Part 3
System design and energy conservation: perimeter loop, radial, extended plenum, reducing trunk, and furred-in duct layouts; sizing room CFM for heating and cooling; insulation, vapor barriers, and duct sealing — the practices that turn a working duct system into an efficient one.