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What is AHU and RTU?

Published in HVAC Components 5 mins read

AHU (Air Handling Unit) and RTU (Rooftop Unit) are fundamental components in commercial HVAC systems, each designed to condition and circulate air within a building. While both are crucial for maintaining comfortable indoor environments, they differ significantly in design, location, and application.

When it comes to commercial HVAC systems, two common components you'll often hear about are Rooftop Units (RTUs) and Air Handling Units (AHUs). While both play crucial roles in maintaining comfortable indoor environments, they serve different purposes and are suited for different applications.

Understanding Air Handling Units (AHUs)

An Air Handling Unit (AHU) is a large metal box containing a fan, heating and/or cooling coils, filters, sound attenuators, and dampers. Its primary function is to condition and circulate air as part of a sophisticated HVAC system. AHUs typically draw in outdoor air, mix it with return air from the building, filter it, heat or cool it, and then distribute it through ductwork to various zones.

Key Characteristics of AHUs:

  • Location: Primarily located indoors within mechanical rooms, basements, or dedicated equipment spaces.
  • Components: Include:
    • Fans: To move air through the system and ductwork.
    • Coils: For heating (hot water/steam) and cooling (chilled water/refrigerant). These often connect to external chillers or boilers.
    • Filters: To remove particulates, dust, and contaminants from the air.
    • Dampers: To control airflow, including fresh air intake and exhaust.
    • Sound Attenuators: To reduce noise generated by the fan.
  • Functionality: Designed for precise temperature, humidity, and air quality control. They often serve multiple zones and are customized for specific building needs.
  • Applications: Commonly found in large commercial buildings, hospitals, data centers, hotels, theaters, and any facility requiring high air quality and sophisticated environmental control.

Advantages of AHUs:

  • Customization: Highly configurable to meet specific building requirements.
  • Air Quality: Superior air filtration and humidity control.
  • Energy Efficiency: Can be integrated with advanced central plant systems (e.g., chillers, boilers) for optimal energy performance.
  • Noise Control: Located indoors, reducing external noise pollution and often incorporating sound-deadening features.

Disadvantages of AHUs:

  • Requires significant indoor space.
  • Higher upfront cost and more complex installation.
  • Often needs separate equipment (chillers, boilers) for heating and cooling.

Understanding Rooftop Units (RTUs)

A Rooftop Unit (RTU) is a self-contained, packaged HVAC system designed to be installed on the roof of a building. Unlike AHUs, RTUs typically combine all the necessary components for heating, cooling, and ventilation into a single, pre-assembled unit, making them a "plug-and-play" solution.

Key Characteristics of RTUs:

  • Location: Exclusively located outdoors on the roof of a building.
  • Components: Integrated system, including:
    • Compressor & Condenser: For the cooling cycle.
    • Evaporator Coil: For cooling the air.
    • Heat Exchanger: For heating (often natural gas or electric coils).
    • Supply & Return Fans: To circulate air.
    • Filters: To clean the air.
    • Duct Connections: To connect to the building's internal ductwork.
  • Functionality: Provides all-in-one heating, cooling, and ventilation. They are generally simpler to install and maintain compared to AHU systems.
  • Applications: Widely used in retail stores, warehouses, schools, light commercial offices, restaurants, and other single-story or low-rise commercial buildings.

Advantages of RTUs:

  • Space-Saving: Frees up valuable indoor space, as all components are on the roof.
  • Simplified Installation: Pre-assembled and tested, leading to quicker and less complex installation.
  • Lower Upfront Cost: Generally more affordable for smaller to medium-sized commercial applications.
  • Easier Maintenance: Components are accessible on the roof for service.

Disadvantages of RTUs:

  • Aesthetics: Visible on the rooftop, which can be an aesthetic concern for some buildings.
  • Weather Exposure: Exposed to external elements, potentially leading to faster wear and tear.
  • Noise: Can transmit noise into the building if not properly isolated, or contribute to outdoor noise.
  • Less Customization: Limited options for advanced air quality control compared to AHUs.

AHU vs. RTU: A Comparative Overview

Here's a quick comparison highlighting the primary differences between AHUs and RTUs:

Feature Air Handling Unit (AHU) Rooftop Unit (RTU)
Location Indoors (mechanical rooms, basements) Outdoors (typically on the roof)
Components Modular, separate coils for heating/cooling All-in-one, integrated heating/cooling cycle
Complexity More complex, customized, integrates with central plant Self-contained, simpler, packaged unit
Control High precision for temperature, humidity, air quality Basic temperature control, limited air quality
Installation More involved, longer duration Quicker, easier (pre-assembled)
Space Req. Requires dedicated indoor space Frees up indoor space
Applications Large commercial, hospitals, data centers Retail, warehouses, schools, light commercial
Cost Higher upfront, but potentially lower operating Lower upfront, potentially higher operating

Practical Insights and Solutions

Choosing between an AHU and an RTU depends largely on the building's specific needs, size, budget, and desired level of environmental control.

  • For large, complex buildings like hospitals or skyscrapers, an AHU system coupled with a central plant (chillers, boilers) is often preferred due to its superior control, air quality, and potential for energy efficiency in large-scale operations.
  • For smaller commercial spaces such as standalone retail stores, fast-food restaurants, or light industrial buildings, RTUs offer a cost-effective, space-saving, and straightforward solution.
  • Some projects might even combine both: RTUs for general zones and AHUs for specialized areas requiring stringent environmental conditions (e.g., surgical suites, server rooms).
  • Consider maintenance accessibility. While RTUs are on the roof, meaning technicians need safe access, AHUs are indoors, which can be easier for year-round maintenance, especially in harsh weather.
  • Energy efficiency standards are constantly evolving. Modern RTUs and AHUs often include features like variable frequency drives (VFDs) for fans, advanced controls, and heat recovery options to minimize energy consumption. For more details on efficient HVAC, you might research HVAC energy efficiency guidelines (hypothetical link).

Understanding the distinct roles and advantages of AHUs and RTUs is essential for designing and maintaining effective and efficient commercial HVAC systems.