💨 Total Delivered Airflow
❄ Total Cooling Capacity
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
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How to Use the Induction Unit Calculator
Choose the closest match for a typical induction unit type, or select Custom to enter a manufacturer-specific ratio.
Enter the primary air airflow delivered from the central system to the unit's nozzles, and how many units serve the zone.
Enter room design temperature and the secondary coil water temperatures to calculate sensible cooling capacity.
Click Calculate Induction Unit Sizing to see induced room airflow, total delivered airflow, and total cooling capacity.
Understanding Induction Unit Sizing
Induction units condition perimeter zones using a purely mechanical process: primary air supplied at higher pressure from a central system passes through small nozzles inside the unit, creating a venturi effect that draws, or induces, room air across a secondary coil. There is no fan inside the unit itself, which was a significant advantage in mid-century commercial construction where fan reliability and noise at every perimeter location was a real concern, a system type the ASHRAE Handbook, HVAC Systems and Equipment volume still documents for engineers working on existing installations.
Induction Ratio Drives Total Capacity
The induction ratio describes how much room air the unit draws across its coil for each unit of primary air passing through the nozzles. A 3.5 to 1 ratio means 3.5 litres per second of room air get induced for every litre per second of primary air supplied. Since the secondary coil conditions this larger induced airflow, a higher induction ratio delivers substantially more total capacity from the same primary air supply rate, though it also depends on nozzle discharge velocity, worth checking with the air velocity calculator, and the specific unit geometry to achieve reliably.
Primary Air Handles Ventilation and Dehumidification Centrally
Because induction units have no fan, they rely entirely on primary air pressure to function, and this primary air typically comes pre-conditioned from a central system that also handles building ventilation and dehumidification. This keeps the terminal unit's job limited to sensible heating and cooling through the secondary coil, since primary air already carries whatever latent conditioning the space needs.
Why Induction Units Have Fallen Out of Favor
Modern perimeter zone systems, particularly fan coil units and VAV terminals, generally offer more flexible zone-by-zone control and typically require lower primary air pressure than induction systems need to achieve reliable nozzle performance, a shift the ASHRAE Handbook, HVAC Applications volume reflects in how little space it now devotes to induction systems compared to older editions. See the fan coil calculator for the more common modern alternative. Induction units mostly appear today in renovation projects for older buildings that already have the induction system infrastructure in place.
Sizing the Secondary Coil
Once total delivered airflow is known from the induction ratio and primary air supply rate, the secondary coil capacity follows the same sensible cooling relationship as any other coil: airflow times a constant times the temperature difference between induced room air and coil leaving air temperature, a sensible-only assumption the sensible heat ratio calculator can help confirm holds for a given zone's latent load. This calculator estimates coil leaving air temperature from the coil water temperatures and a typical approach temperature, giving a preliminary capacity estimate before final coil selection from manufacturer performance data.
Frequently Asked Questions
Induction ratio is the ratio of induced room air to primary air moving through the unit's nozzles, typically expressed as a value like 3 to 1 or 4 to 1. A higher induction ratio means the unit draws proportionally more room air across its secondary coil for each unit of primary air, which increases total delivered airflow and capacity from the same primary air supply rate.
An induction unit has no fan of its own. Primary air supplied at higher pressure passes through small nozzles, creating a venturi effect that draws room air across a secondary coil purely through induction, with no moving parts beyond dampers and valves. A fan coil uses an electric fan to actively draw air across its coil, giving it independent control over airflow regardless of primary air supply. See the fan coil calculator for that alternative.
Induction units were popular in mid-twentieth-century commercial construction because they have no moving parts at the terminal unit itself, meaning less maintenance and no fan noise or motor failure risk at each perimeter location. Primary air also handles the entire building's outdoor air and dehumidification centrally, simplifying the terminal unit's job to sensible heating and cooling only.
Induction units are far less common in new construction today, largely displaced by fan coil units and VAV systems that offer more flexible zone-by-zone control and generally lower primary air pressure requirements. They still appear in renovations of older buildings that already have the induction system infrastructure in place, where replacing the entire perimeter system would be more disruptive than maintaining the existing induction approach.
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