📈 Stratification Height
🌡 Supply Air Temperature
Design Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
|---|
How to Use the Displacement Ventilation Calculator
Choose the closest match for a typical supply air temperature difference range, or select Custom to enter your own project-specific value.
Enter floor area and ceiling height. Displacement ventilation generally needs at least 3 to 4 metres of ceiling height to develop meaningful stratification.
Enter the sensible cooling load and the height of the occupied zone where people actually spend time, typically 1.8 to 2.0 metres for standing occupants.
Click Calculate Displacement Ventilation to see supply airflow, supply air temperature, and where stratification height falls relative to the occupied zone.
Understanding Displacement Ventilation Design
Displacement ventilation takes advantage of buoyancy rather than fighting it. Cool air enters slowly near the floor, warms as it passes people and heat sources, and rises naturally toward the ceiling where it is exhausted. Done well, this means the system only has to condition the occupied zone near the floor to comfort conditions, not the entire room volume, an approach the ASHRAE Handbook, HVAC Systems and Equipment volume covers alongside other air distribution strategies, which can be a significant advantage in a tall atrium or auditorium.
Why Ceiling Height Matters So Much
The buoyant plumes rising from people and equipment need vertical distance to develop and separate the clean lower layer from the mixed upper layer. In a low-ceiling room, there is not enough height for this separation to happen before the plumes hit the ceiling and mix back down, which defeats the purpose of the strategy entirely. Diffuser discharge velocity, worth checking against the air velocity calculator, also has to stay low enough not to disturb this buoyant separation, which is why displacement ventilation is mostly seen in spaces with generous ceiling height, generally at least 3 to 4 metres and often considerably more in atriums and auditoriums.
Stratification Height Versus Occupied Zone
Stratification height is where the cool lower layer meets the warmer upper layer. For the system to deliver its intended comfort and air quality benefit, this height needs to sit clearly above the occupied zone, meaning above head height for standing or seated people, within the vertical temperature gradient limits ASHRAE Standard 55 sets for head-to-ankle thermal comfort. If stratification height falls too low, occupants near the top of the occupied zone end up breathing air from the upper, more contaminated layer instead of the clean lower layer the system is meant to provide.
Low Temperature Difference, Not High Velocity
Displacement diffusers deliver air at low velocity and a smaller temperature difference than typical mixing systems, usually 3 to 6 degrees Celsius compared to 10 to 15 degrees for a ceiling-mixing system. Because the air enters directly at floor level near occupants' feet, a larger temperature difference would create noticeable cold drafts. This low-velocity delivery is also part of why displacement ventilation suits auditoriums and theatres so well, since it avoids the diffuser noise a higher-velocity mixing system can add, a factor worth checking against the NC curve calculator for a space with a strict background noise target. This lower delta T does mean displacement systems often need more supply airflow than an equivalent mixing system to remove the same cooling load, which is a real tradeoff against the comfort and air quality benefits.
Where Displacement Ventilation Does Not Work Well
Spaces without a clear buoyant heat source, or spaces with strong mechanical air movement from fans or equipment, can disrupt the stratification the strategy depends on. Low-ceiling office space, for instance, generally performs better with conventional mixing ventilation or the underfloor air distribution approach, which shares some conceptual similarity to displacement ventilation but is designed around plenum pressure and floor diffusers rather than open floor-level supply.
Integrating with the Rest of the Ventilation System
The airflow calculated here should be checked against the minimum outdoor air requirement from the commercial ventilation calculator, since displacement ventilation airflow driven by cooling load may already exceed or fall short of the code-minimum outdoor air requirement depending on room occupancy. If the space uses a dedicated outdoor air unit to deliver the displacement supply, size that unit with the DOAS calculator.
Frequently Asked Questions
Mixing ventilation supplies air at high velocity from the ceiling to blend thoroughly with room air, conditioning the entire volume uniformly. Displacement ventilation supplies cool air slowly at floor level, letting buoyancy from heat sources like people and equipment carry warm, contaminated air upward to be exhausted near the ceiling, so only the occupied zone near the floor needs to be conditioned to comfort conditions.
Stratification height is the elevation above the floor where the cool, clean supply air layer transitions into the warmer, more contaminated upper layer. For a displacement system to work as intended, this height needs to sit above the occupied zone, typically above head height for seated or standing occupants, so people remain in the clean lower layer rather than the mixed upper layer.
Displacement systems rely on a smaller temperature difference between supply air and room air, typically 3 to 6 degrees Celsius, compared to 10 to 15 degrees for mixing systems. A larger temperature difference at floor level would create noticeable cold drafts around occupants' feet and ankles, since the air enters the room directly at floor level rather than being thrown and mixed from the ceiling.
Displacement ventilation works best in spaces with high ceilings, at least 3 to 4 metres, and a clear heat source like occupants, lighting, or equipment that generates a buoyant plume. Atriums, auditoriums, theatres, and some large classrooms are common applications. Low-ceiling spaces and rooms without a clear buoyant heat source generally do not develop the stratification the system depends on. See the underfloor air distribution calculator for a related low-ceiling alternative.
Related HVAC Tools
Low-ceiling floor-based alternative
Check minimum outdoor air
Dedicated outdoor air source
General air handler sizing
Classroom and gymnasium loads
Recover energy from exhaust air
Cooling and heating load
Pairs well with displacement systems