HVAC Components & Distribution

Understanding HVAC systems at the component level is essential for construction professionals. During construction, HVAC equipment must be specified, procured, delivered, set in place, connected, and commissioned — and every component has specific space requirements, structural support needs, utility connections, and access clearances. During the building's life, components require maintenance and eventual replacement. This lesson examines the major components of HVAC systems and the ductwork distribution networks that deliver conditioned air to occupied spaces.

Furnaces

A residential gas furnace contains a gas burner, a heat exchanger, a blower, an air filter, and controls — all housed in a sheet metal cabinet. The burner ignites natural gas (or propane); combustion gases flow through the heat exchanger tubes, transferring heat to the building air stream flowing over the exterior of the tubes. Combustion gases exhaust through a flue.

• Standard efficiency (80% AFUE): Non-condensing; single heat exchanger; metal flue vented through roof
• High efficiency (90–98% AFUE): Condensing furnace with a secondary heat exchanger that extracts additional heat by condensing water vapor from exhaust gases. PVC flue vented through sidewall. Produces acidic condensate (pH 3–4) that must be drained and may require neutralization
• Sizing: Measured in BTU/hr input and output; typical residential sizes are 40,000–120,000 BTU/hr output
• Blower: Direct-drive ECM (electronically commutated motor) blower for variable-speed airflow; older units use PSC (permanent split capacitor) motors at fixed speeds

Air Handlers

An air handler (AHU) is a factory-built assembly containing a blower, heating and/or cooling coils, filters, mixing box, and dampers. Residential air handlers pair with heat pumps (the AHU contains the evaporator coil and may include electric strip heat). Commercial AHUs are larger — from closet-sized units serving a single floor to truck-sized custom units in mechanical rooms.

Commercial AHU components (in order of airflow):

1. Outdoor air intake with bird screen and motorized damper
2. Mixing box — blends outdoor air with return air
3. Pre-filter section — MERV 8 pleated filters for large particle capture
4. Final filter section — MERV 13 or higher for fine particle capture (required by many codes and ASHRAE 62.1)
5. Heating coil — hot water, steam, or electric
6. Cooling coil — chilled water or DX (direct expansion refrigerant)
7. Fan section — centrifugal or plenum fan with VFD (variable frequency drive) for speed control
8. Sound attenuator — lined duct section or silencer to reduce fan noise
9. Supply duct connection

Condensing Units and Heat Pumps

The outdoor unit in a split system contains the compressor, condenser coil, condenser fan, and controls. For heat pumps, it also contains the reversing valve.

• Placement: On a ground-level concrete pad (residential) or on the roof, steel dunnage, or grade (commercial). Minimum clearance on all sides for airflow and service access (typically 12"–24" on sides, 48"+ on service side)
• Refrigerant lines: Insulated suction line (large, cold) and uninsulated liquid line (small, warm) connect the outdoor unit to the indoor coil. Lines must be properly sized, supported, and insulated per manufacturer specs.
• Electrical: Dedicated circuit with disconnect switch within sight of the unit

Ductwork is the distribution network that carries conditioned air from the AHU or furnace to the occupied spaces (supply) and returns air back to the unit (return).

Duct Materials and Types

• Galvanized sheet metal — The standard for commercial and high-quality residential ductwork. Fabricated from 26-gauge to 16-gauge galvanized steel depending on size and pressure class. Joints sealed with mastic and/or tape (UL 181A/181B listed). Supports are trapeze hangers (angle iron or all-thread with channel) per SMACNA standards. Round duct is more efficient aerodynamically; rectangular duct fits better in tight spaces.
• Flexible duct — Insulated flexible duct (inner liner, insulation, outer jacket) is common for residential supply runs and commercial connections from rigid trunk ducts to diffusers. Advantages: easy installation, inherent insulation. Disadvantages: higher friction loss (especially when kinked, compressed, or excessively long), shorter life, limited to 6" pressure class. Maximum recommended length is 25 feet per run; excess length must be cut, not bundled.
• Duct board — Rigid fiberglass board (faced with foil) fabricated into rectangular duct sections. Integral insulation and sound attenuation. Common in residential and light commercial. Not suitable for high-velocity or high-pressure systems.
• Spiral (round) duct — Factory-fabricated round duct with a continuous spiral seam. Inherently stronger, tighter, and lower-friction than rectangular duct. Increasingly used in exposed-duct architectural designs.

Duct Sizing and Design

Duct sizing follows two primary methods:

• Equal friction method: Sizes each duct section to maintain a constant friction rate (typically 0.08–0.10" w.g. per 100 ft for residential, 0.08–0.20" for low-velocity commercial). Simple and commonly used.
• Static regain method: Sizes each duct section so that the static pressure regain from velocity reduction at each branch takeoff offsets the friction loss in the following section. Results in more uniform pressure at all outlets; used for large commercial systems.

Critical duct design principles:

• Minimize turns and abrupt transitions (use turning vanes in rectangular elbows; use long-radius elbows for round duct)
• Maintain adequate velocity (600–900 FPM for residential supply; 1,000–2,500 FPM for commercial low-velocity; up to 4,000 FPM for high-velocity)
• Seal all joints (duct leakage wastes 15–30% of HVAC energy in many buildings)
• Insulate supply ducts in unconditioned spaces (R-6 minimum in most codes; R-8 recommended)
• Provide access doors at dampers, coils, and fire dampers for maintenance

Supply diffusers distribute conditioned air into occupied spaces:

• Ceiling diffusers: Square (4-way throw), round, linear slot (continuous strips in ceiling), perforated face — the most common type in commercial buildings
• Sidewall registers: Rectangular grilles with adjustable vanes — common in residential
• Floor registers: Metal or wood grilles in the floor — common in residential with basement ductwork

Return grilles collect room air for recirculation:

• Typically located high on walls or in ceilings (commercial) or low on walls or in hallways (residential)
• Sized for low velocity (300–500 FPM face velocity) to minimize noise

Dampers control airflow:

• Volume dampers (manual or motorized) — adjust airflow in branch ducts for balancing
• Fire dampers — spring-loaded dampers with fusible links that close automatically when duct penetrates a fire-rated wall or floor assembly. Required by code at all rated penetrations. Must be accessible for inspection and testing.
• Smoke dampers — motorized dampers controlled by the fire alarm system to prevent smoke spread through ductwork
• Combination fire/smoke dampers — serve both functions

Thermostats are the primary user interface:

• Residential: Programmable or smart thermostats (Wi-Fi connected with occupancy sensing, learning algorithms, remote access)
• Commercial: Zone thermostats connected to a Building Automation System (BAS) or Building Management System (BMS)

Building Automation Systems (BAS):

• Centralized computer system that monitors and controls HVAC (and often lighting, security, and fire alarm)
• DDC (Direct Digital Control) controllers at each AHU, VAV box, and major piece of equipment
• Sensors throughout: temperature, humidity, CO2, pressure, flow
• Automated sequences: occupied/unoccupied scheduling, optimal start, economizer control, demand-controlled ventilation, alarm management
• Communication protocols: BACnet (ASHRAE 135), LonWorks, Modbus