An induction unit is a type of terminal device used in HVAC systems that delivers heating or cooling to a space by leveraging the Venturi effect to induce room air circulation across a conditioning coil. High-pressure primary air from a central air handling unit is discharged through a series of nozzles at high velocity, creating a low-pressure zone that draws in secondary air from the occupied space. This secondary air passes over a heating or cooling coil before mixing with the primary air and entering the room as a blended, conditioned airstream.
Technical Details and Specifications
Induction units operate with primary air supplied at pressures typically ranging from 2 to 6 inches of water gauge (500 to 1,500 Pa). The primary air volume usually accounts for only 20% to 30% of the total supply air delivered to the space, with the remaining 70% to 80% consisting of induced room air. This ratio, known as the induction ratio, is a key performance characteristic and commonly falls between 2:1 and 4:1 depending on the unit design and nozzle configuration.
Unit capacities can range from several thousand to over 10,000 BTU/h, depending on the coil size and water temperatures used. Coil options include chilled water for cooling, hot water for heating, and in some cases electric resistance elements. Two-pipe and four-pipe configurations are available, with four-pipe systems offering simultaneous access to both heating and cooling for greater flexibility in changeover seasons. Because the units contain no fans or moving parts at the terminal, they require minimal maintenance and produce very low noise levels compared to fan coil units.
Common Applications
Induction units are well suited to buildings that require individual zone control along exterior perimeter zones, where solar loads and transmission losses vary significantly from room to room. Typical installations include:
- Commercial office buildings with large glass facades
- Hospitals and healthcare facilities requiring precise temperature control in patient rooms
- Hotels and high-rise residential buildings where quiet operation is a priority
- Institutional buildings such as universities and government facilities
Units are commonly mounted beneath windows in sill-height enclosures or concealed within ceiling cavities, depending on the architectural design and air distribution strategy.
Relevant Standards and Design References
The design, testing, and application of induction units are addressed in ASHRAE Handbook chapters covering air distribution and terminal units. ASHRAE Standard 130 provides methods for testing air terminal units, including performance metrics relevant to induction-type devices. Selection and layout guidance is also available through SMACNA duct design standards and manufacturer-specific engineering data. Engineers should reference local energy codes and ASHRAE Standard 90.1 when evaluating system-level energy performance, as the central plant and ductwork pressurization requirements directly influence overall system efficiency.
Practical Significance
Induction units reduce the volume of air that must be transported from the central plant to each zone, lowering ductwork size and fan energy consumption compared to all-air systems. The absence of local fans eliminates motor maintenance and reduces electrical demand at the terminal. These characteristics make induction units an energy-conscious choice for perimeter zone conditioning, particularly in buildings where quiet operation, long equipment life, and individual room control are priorities. While largely supplanted by active chilled beams in some modern designs, induction units remain a proven and reliable technology with decades of successful field performance.