HVAC DELIVERY SYSTEMS

• Overview of HVAC Systems

  • Complete HVAC systems consist of various subsystems designed to maintain stable conditions in a space.

  • Components:

  • Cooling Components:

    • Vapor compression refrigeration equipment

    • Occasionally absorption refrigeration equipment

  • Heating Components:

    • Furnaces

    • Boilers

    • Electric resistance devices

    • Heat pumps

  • Heat Transfer:

    • Fluids moved by:

    • Air-handling equipment (ducts, grilles, diffusers)

    • Pumps and piping systems (water)

    • Compressors (refrigerants)

  • Delivery methods vary in complexity and energy consumption. Selecting the appropriate delivery system is critical for performance.

5.1 CONTROL OF HEATING AND COOLING

• Central Systems:

  • Use air-handling units with coils to modify air temperature.

  • Air is delivered by:

  • Fans (direct or through ducts)

  • Multiple spaces can require different heating/cooling loads.

  • Control Methods:

  • Vary air temperature while holding flow constant

  • Vary flow of air while holding temperature constant

  • Vary both temperature and flow or turn the system on/off

• Thermostats:

  • Control heating and cooling loads based on space conditions.

  • Example systems include residential combination HVAC units and rooftop units for small buildings.

• Interior vs. Perimeter Spaces:

  • Interior spaces require continuous air-conditioning year-round due to year-round heat gains.

  • Perimeter spaces require heating in winter due to heat loss through walls and windows.

5.2 ZONING

• Definition of a Zone:

  • An area where temperature is controlled by a single thermostat.

  • Example: A house with a single thermostat is a single-zone system.

  • Larger homes may have multiple zones with separate thermostats.

  • Complex buildings need multiple zones for effective load accommodation.

• Air-Terminal Devices:

  • Serve individual spaces or groups.

  • Each terminal corresponds to a separate control zone.

5.3 CONTROLS AND AUTOMATION

5.3.1 Definition

• Control systems provide the intelligence for mechanical and electrical systems.

• Equipment is selected primarily for design capacity, while controls manage operations across anticipated conditions with minimal human intervention.

5.3.2 Basic Control Systems and Devices

• All HVAC systems require controls (manual or automatic) to manage properties, such as:

  • Temperature: Controlled using sensors with operating temperature limits.

  • Pressure: Monitored using pressure sensors.

  • Flow Rate: Managed by flow rate sensors.

  • Humidity: Controlled through humidity sensors.

  • Speed: Managed via on/off or variable speed controls.

  • Time: Controlled through programmable clocks.

• Types of Control Systems:

  • Electric controls (120 volts or low voltage 12-24 volts)

  • Pneumatic controls (5-30 psi)

  • Direct digital controls (DDC) using microprocessors for intelligent decision-making.

5.3.3 Building Automation and Energy Management

• Components arranged either hierarchically or in a local area network.

• Various names include BAS (Building Automation System) or EMCS (Energy Management and Control System).

• Provides capabilities for information acquisition and adjustment at operator terminals.

• Energy management achieved by adjusting operating parameters, monitoring electrical demand, and optimizing equipment operation.

  • Example: Chilled water discharge temperature adjustment based on humidity levels.

5.3.4 Human Safety

• Interface with fire alarm systems for safety in emergency situations.

• Example: Activating stair pressurization fans and smoke exhaust fans during fire events.

5.3.5 Equipment Protection

• Monitoring specific components for operational parameters to schedule maintenance and prevent damage.

5.4 COMMONLY USED SYSTEMS FOR ZONE CONTROL

5.4.1 Single-Zone Constant Air Volume

• Air is supplied at a constant rate; suitable for small buildings with similar load characteristics.

5.4.2 Single-Zone Reheat

• Uses cooling coils for dehumidification; overcooled air reheated to maintain temperature; high energy consumption.

5.4.3 Multiple-Zone Constant-Volume Terminal-Reheat

• Serves multiple zones from a single air-handling unit; uses terminal boxes to control temperature by re-heating.

5.4.4 Constant-Volume Dual-Duct

• Sends warm and chilled air through separate ducts to control multiple zones; uses mixing boxes for air delivery adjustments.

5.4.5 Multizone System

• Individual ducts run from air-handling units to control different zones; waste energy due to mixing.

5.4.6 Single-Zone Variable Air Volume

• Adjusts airflow based on space thermostat; only suitable for smaller or single spaces.

5.4.7 Multiple-Zone Variable Air Volume

• Utilizes VAV terminals to control airflow and achieve energy efficiency throughout larger spaces.

5.4.8 Variable Refrigerant Flow (VRF)

• System uses variable refrigerant flow to manage heating and cooling through multiple indoor units; can provide simultaneous heating and cooling.

5.4.9 Radiant Panels

• Circulate hot water for heating or chilled water for cooling; effective energy conservation methods.

5.4.10 Package Terminal Air Conditioners (PTAC)

• Window or through-wall units with self-contained compressors and heat exchangers; generally noisy and high-maintenance.

5.4.11 Water-Source Heat Pumps

• Provide individual zone control; can operate in heating or cooling modes with centralized water loops to improve efficiency.

5.4.12 Dedicated Outside Air Systems (DOAS)

• Separate units for ventilation that precondition outside air, leading to improved energy efficiency.