HVAC Systems Overview

HVAC — Heating, Ventilation, and Air Conditioning — is the mechanical system responsible for maintaining indoor environmental comfort. It controls four variables: temperature, humidity, air quality, and air movement. In most of North America, buildings require heating in winter, cooling in summer, ventilation year-round, and humidity control in certain climates. The HVAC system is typically the single largest energy consumer in a building, accounting for 40–60% of total energy use, making system selection a critical decision with decades-long cost implications.

Heating and Cooling Loads

Before selecting any HVAC equipment, engineers calculate the building's heating load (how much heat must be added in winter) and cooling load (how much heat must be removed in summer). These loads depend on:

• Climate — outdoor design temperatures (e.g., 0°F winter / 95°F summer for Chicago)
• Building envelope performance — insulation R-values, window U-factors, air leakage rates
• Internal gains — heat from occupants (approximately 250 BTU/hr sensible + 200 BTU/hr latent per person), lighting (approximately 1 W/ft² for LED), and equipment (computers, copiers, servers)
• Solar gains — direct sunlight through windows (can exceed 200 BTU/hr per square foot of glass on west-facing exposures)
• Ventilation requirements — ASHRAE Standard 62.1 requires minimum outdoor air quantities (typically 15–20 CFM per person for offices) which must be heated or cooled

Load calculations follow ACCA Manual J (residential) or ASHRAE methods (commercial). Oversizing equipment is a common and costly mistake — oversized units short-cycle, fail to dehumidify properly, and waste energy.

Refrigeration Cycle

All air conditioning and heat pump systems rely on the vapor-compression refrigeration cycle, which exploits the fact that fluids absorb heat when they evaporate and release heat when they condense:

1. Evaporator — liquid refrigerant absorbs heat from indoor air and evaporates into a low-pressure gas
2. Compressor — the compressor pressurizes the gas, raising its temperature well above outdoor conditions
3. Condenser — the hot, high-pressure gas releases its heat to the outdoor air (or water) and condenses back into a liquid
4. Expansion valve — the high-pressure liquid passes through an expansion device, dropping its pressure and temperature, and the cycle repeats

A heat pump is simply a refrigeration system with a reversing valve that can switch the direction of refrigerant flow — the indoor coil becomes the condenser in heating mode and the evaporator in cooling mode.

Split System (Forced Air)

The most common residential HVAC system in North America is the split system: a gas furnace and evaporator coil inside the house connected to an air-cooled condensing unit outside. A blower in the furnace or air handler pushes air through a duct system to supply registers in each room; return air grilles bring air back to the unit.

• Heating: Gas furnace (80% or 95%+ AFUE efficiency) burns natural gas; heat exchanger transfers heat to airstream
• Cooling: Refrigerant lines connect indoor evaporator coil to outdoor condensing unit; typical efficiency 14–20+ SEER2
• Distribution: Sheet metal trunk-and-branch or flex duct system; supply registers in each room; central or distributed returns
• Control: Single thermostat (or multi-zone with dampers)
• Typical capacity: 2–5 tons cooling (1 ton = 12,000 BTU/hr); 40,000–100,000 BTU/hr heating

Heat Pump (Air-Source)

Air-source heat pumps provide both heating and cooling using the refrigeration cycle. They are increasingly popular in moderate climates and, with cold-climate technology, in northern climates as well.

• Types: Ducted split systems (look identical to conventional split systems); ductless mini-splits (wall-mounted indoor units with individual refrigerant lines to outdoor unit)
• Efficiency: 8–13 HSPF2 heating; 14–22+ SEER2 cooling — significantly more efficient than gas heating in moderate climates
• Cold climate limitation: Heating capacity and efficiency decrease as outdoor temperature drops; modern cold-climate heat pumps (e.g., Mitsubishi Hyper-Heat) maintain rated capacity down to 5°F and operate to -13°F or below
• Backup heat: Electric resistance strip heat or gas furnace (dual-fuel system) provides supplemental heat during extreme cold

Hydronic (Boiler-Based)

Hydronic systems circulate hot water through pipes to radiators, baseboard convectors, or radiant floor tubing. Common in the Northeast and upper Midwest.

• Boiler types: Gas-fired cast iron (80% AFUE), condensing gas (95%+ AFUE), oil-fired, electric
• Distribution: Copper, PEX, or steel piping; circulator pumps; zone valves
• Terminal units: Baseboard convectors, panel radiators, cast iron radiators, radiant floor tubing (PEX embedded in slab or stapled under subfloor)
• Advantages: Silent operation; no ductwork; zoning is simple (one thermostat per zone loop); radiant floor provides superior comfort
• Limitations: No cooling (requires separate system — typically ductless mini-splits); no air filtration; no ventilation (requires separate system such as ERV/HRV)

Packaged Rooftop Units (RTUs)

Rooftop units are self-contained HVAC systems (compressor, condenser, evaporator, fan, filters, heating section all in one weatherproof cabinet) mounted on the roof. They are the most common HVAC system for single-story commercial buildings — retail, restaurants, small offices, warehouses.

• Capacity: 3–150 tons per unit
• Heating options: Gas-fired heat exchanger, electric heat, heat pump
• Distribution: Ductwork drops through roof curb into ceiling plenum
• Advantages: No mechanical room needed; all maintenance on roof; easy replacement
• Energy codes: ASHRAE 90.1 and IECC set minimum efficiency requirements; economizer (free cooling with outdoor air) required in most climate zones for units above 54,000 BTU/hr

Variable Refrigerant Flow (VRF)

VRF systems use a single large outdoor condensing unit connected by refrigerant piping to multiple indoor fan coil units throughout the building. A branch selector box routes refrigerant to individual zones.

• Capacity: 6–60+ tons per outdoor unit; systems can be combined for larger buildings
• Key advantage: Simultaneous heating and cooling — some zones can be in heating mode while others are in cooling mode (heat recovery VRF)
• Indoor unit types: Wall-mounted, ceiling cassette (4-way blow), concealed ducted, floor-standing
• Efficiency: Highly efficient at part load due to variable-speed inverter compressors
• Applications: Hotels, office buildings, retrofit projects (small refrigerant piping is easier to route than ductwork)
• Limitations: Refrigerant charge limits per ASHRAE 15 in occupied spaces; specialized installation and service training required

Chilled Water Systems

Large commercial buildings (100,000+ sq ft), campuses, and high-rises typically use chilled water systems. A central chiller produces cold water (typically 42–44°F), which is pumped through insulated piping to air handling units (AHUs) throughout the building. The AHUs blow air across chilled water coils to cool the supply air.

• Chiller types: Air-cooled (condenser fans reject heat to outdoor air); water-cooled (condenser water loop rejects heat through a cooling tower on the roof)
• Capacity: 100–2,000+ tons per chiller; large buildings use multiple chillers for redundancy and part-load efficiency
• Distribution: Chilled water piping (steel or copper) with variable-speed pumps; primary-variable or primary-secondary pumping arrangements
• Cooling towers: Evaporative heat rejection devices on the roof; water cascades over fill media while fans draw air through, evaporating a small percentage of water and cooling the rest. Require water treatment for scale, corrosion, and biological growth (including Legionella prevention)
• AHUs: Large air-handling units (draw-through or blow-through configuration) with chilled water cooling coils, hot water or steam heating coils, filters, fans, and economizer dampers. Serve one or more floors; air distributed through ductwork.
• VAV systems: Variable Air Volume boxes downstream of AHUs modulate airflow to each zone based on thermostat demand. VAV with reheat is the most common commercial HVAC distribution system for office buildings.