A terminal unit in an HVAC (Heating, Ventilation, and Air Conditioning) system is a device that regulates the flow of conditioned primary air from a central air handler into an occupied space. Essentially, it's the last stop for the air before it enters your room, office, or building zone.
Purpose of Terminal Units
Terminal units serve several crucial functions:
- Air Volume Regulation: They precisely control the amount of heated or cooled air delivered to a specific area to maintain the desired temperature.
- Zone Control: They allow for individualized temperature control in different zones or rooms within a building. This is far more efficient than relying on a single thermostat for the entire structure.
- Fresh Air Introduction: Many terminal units also incorporate the introduction of fresh, outside air, ensuring adequate ventilation and air quality, as per occupancy requirements.
- Noise Reduction: They can also help to reduce noise from the central air handling unit, creating a more comfortable environment.
Factors in Determining Air Volume
When determining the required air volume for each space, HVAC engineers consider:
- Thermal Conditions: The heating or cooling load requirements of the space based on factors like room size, insulation, windows, and occupancy.
- Fresh Air Requirements: The amount of fresh air needed to maintain acceptable air quality based on the number of occupants and the activities taking place.
Types of Terminal Units
Several types of terminal units are available, each with its own characteristics and applications. Some common types include:
- Variable Air Volume (VAV) Boxes: These are the most common type. They adjust the volume of air supplied based on the zone's heating or cooling needs.
- Fan-Powered Boxes: These units include a fan to supplement the airflow from the central air handler, improving air distribution and mixing. They can be series or parallel fan powered boxes.
- Reheat Coils: Located at the terminal unit, these coils reheat cooled air to provide individual zone temperature control when cooling isn't needed. They are often used in conjunction with VAV boxes.
- Induction Units: These units use a jet of primary air to induce the circulation of room air over a heating or cooling coil.
- Chilled Beams: These use chilled water to cool the air in a space, offering a more energy-efficient alternative to traditional air-based systems.
Example Scenario
Imagine a large office building with multiple zones. A central air handler provides conditioned air to all zones. Each zone has a VAV box that adjusts the airflow based on the temperature settings for that particular zone. If the conference room is occupied and requires more cooling, the VAV box will open further, allowing more cool air to enter. Conversely, if an individual office is unoccupied and requires less heating, the VAV box will reduce the airflow, saving energy.
