Fundamentals of displacement ventilation

Displacement ventilation offers a clever approach to keeping indoor spaces comfortable. It relies on natural air movement instead of forcing air to mix everywhere. In this article we walk through how it works, why it creates distinct air layers, and what engineers consider during design. You will see that the system uses simple physics to deliver fresh air exactly where people need it most.

Core mechanics

Displacement ventilation works through thermal buoyancy. Cool supply air enters the room at low speed and stays low because it is denser than the warmer air above. We introduce this air at about 18 °C with a velocity between 0.2 and 0.5 m/s. The air spreads gently across the floor and forms what we call the cool pool.

Heat sources such as people, computers, and lights create rising columns of warm air called convective plumes. These plumes act like the engine of the system. They pull the cool floor air upward as they rise. Contaminants and heat ride along with the plumes all the way to the ceiling. Exhaust grilles then remove the warm, polluted air from the upper part of the room. This process keeps the breathing zone cleaner because the rising plumes carry away unwanted particles before they spread sideways.

You can picture the cool air resting on the floor like a shallow layer of water. The plumes act like invisible chimneys that suck this layer upward. Fresh air reaches occupants from below while stale air stays above and leaves through the ceiling.

1Interactive model

The room in motion

Watch the cool pool feed the plumes while the exhaust clears the upper layer. Toggle the contaminants to follow them upward.

Show contaminants

Fluid dynamics: the thermal plume

The real driver here is how plumes grow as they rise. Friction along the edge of each plume pulls in surrounding air. This process is called entrainment. The plume gets wider and carries more air the higher it goes.

For a single heat source, engineers use the Morton-Taylor-Turner equation to calculate the volume of air moving inside the plume at any height:

$\dot{V}_p = 0.005 \cdot \dot{Q}_c^{1/3} \cdot z^{5/3}$

Here is what each term means:

• $\dot{V}_p$ is the volume flow rate inside the plume in cubic metres per second.
• $\dot{Q}_c$ is the convective heat released by the source in watts.
• $z$ is the height above the heat source in metres.

Plume flow grows quickly with height because of the $z^{5/3}$ term.

As many plumes rise together, they eventually entrain all the fresh supply air. The height where this balance happens is called the stratification boundary, written as $z_{strat}$. At this level the total mass flow from all plumes equals the mass flow of the supply air. Above $z_{strat}$ the plumes have no fresh air left to pull in, so they start recirculating the already polluted upper air. This creates a stable layer of warm, contaminated air near the ceiling that never mixes back down into the occupied space.

2Interactive model

How a plume grows

Adjust the heat source and the supply airflow, then move the probe to read the flow inside the plume.

Plume flow at probe31 L/s

Stratification height1.58 m

Breathing zoneBoundary above

Convective heat100 WProbe height1.20 mSupply airflow50 L/s

The vertical temperature profile

The room naturally divides into three thermal layers. Understanding these layers helps you design a comfortable environment.

Near the floor, below about 0.1 m, the supply air warms up quickly as it touches the floor slab. This creates a thin sub-layer with a sharp temperature rise.

In the occupied zone, from roughly 0.1 m up to the stratification boundary, temperature increases steadily. The rise comes from the heat picked up by the plumes as they pass through this region.

Above the stratification boundary you find the upper mixed zone. Air here is hot, fairly uniform in temperature, and contains most of the contaminants. It stays there until the exhaust removes it.

Designers must control the temperature gradient in the occupied zone. Standards such as ISO 7730 and ASHRAE 55 recommend that the vertical temperature difference should not exceed 2.5 to 3.0 °C per metre. A steeper gradient would make ankles feel too cool while the head feels too warm. You can avoid discomfort by choosing the right supply air volume and diffuser layout.

3Interactive model

Three layers, one profile

Change the supply temperature, the heat load, and the airflow to see how the profile and the gradient respond.

Exhaust26.3 °C

Near floor22.1 °C

Stratification height1.83 m

Gradient2.4 °C/m

Supply temperature18.0 °CTotal heat load1600 WSupply airflow160 L/s

The occupied-zone gradient is within the 3 °C per metre comfort limit.

Analytical models for early-stage design

When we design a displacement ventilation system we need reliable ways to predict how the air will behave long before construction starts. Engineers choose different calculation methods depending on the height of the room. Each model helps us estimate the stratification height and the way temperature changes from the floor up to the ceiling.

The Skistad model for low-height spaces

For typical offices and classrooms that are about 3 m tall, the Skistad model works well. It rests on a simple rule of thumb called the 50 percent rule. To keep the idea easy to follow, we begin with the temperature difference rather than jumping straight to the full equation.

As the cool fresh supply air spreads across the floor, the floor slab beneath it is warmed from above. The warm ceiling and the upper wall surfaces send long-wave radiation downward onto the slab. The slab then passes some of this heat into the thin layer of air that rests directly on it. In an ordinary room the radiative path that brings heat onto the slab and the convective path that carries heat into the air share the load in roughly equal measure.

Because the two paths are balanced, the rise in temperature from the supply air to the air sitting just above the floor is only half of the total temperature rise that occurs between supply and exhaust:

$T_f - T_s = 0.5(T_e - T_s)$

Here:

• $T_f - T_s$ is the temperature rise from the supply air to the thin layer of air at floor level.
• $T_e - T_s$ is the overall temperature rise from supply all the way to exhaust.

If we rearrange this difference equation by adding $T_s$ to both sides, we arrive at the familiar form of the 50 percent rule:

$T_f \approx T_s + 0.5(T_e - T_s)$

You will meet the same balance again in the Mundt model. When the convective and radiative heat-transfer coefficients are equal, the Mundt equation reduces exactly to this halfway result. The total heat load divided by the mass-flow rate and the specific heat of air is simply the overall temperature rise from supply to exhaust. The 50 percent rule is that ordinary balance written down as a fixed assumption.

This quick method gives good results in standard rooms of ordinary height. However, it does not account for the stronger radiation that comes from high ceilings. That is why the model becomes inaccurate once the room is much taller.

Plume entrainment and the Mundt model for high-height spaces

In atria, theatres, or industrial halls taller than 5 m, radiation from the hot ceiling and upper walls heats the floor slab more intensely. This extra heat changes the temperature profile near the floor and makes the vertical gradient steeper. For these spaces we turn to the Mundt model, which combines the calculation of plume entrainment with a detailed radiant heat balance.

The model first finds the stratification height by balancing the volume of air entrained by the rising plumes against the volume of fresh supply air we deliver. Once that height is known, it calculates the near-floor air temperature:

$T_f = T_s + \frac{\dot{Q}_{tot}}{\dot{m} \cdot C_p} \cdot \left( \frac{1}{1 + \frac{h_c}{h_r}} \right)$

Each term means:

• $T_f$ is the air temperature just above the floor.
• $\dot{Q}_{tot}$ is the total heat load in the room.
• $\dot{m}$ is the supply mass-flow rate.
• $C_p$ is the specific heat of air.
• $h_c$ is the convective heat-transfer coefficient.
• $h_r$ is the radiative heat-transfer coefficient.

The fraction that contains $h_c$ and $h_r$ shows how the convective and radiative paths share the heat load. When radiation is strong the floor becomes warmer than the supply air. This warming steepens the temperature gradient in the occupied zone. As a result you may need to supply more air than a simple model would predict if you want to keep people comfortable.

By using the Mundt model we avoid under-sizing the system in large, high spaces. The extra calculation effort pays off because it gives a more accurate picture of the temperatures people will actually experience near the floor.

4Interactive model

Two rooms, two models

Compare a 3 metre office with a tall hall side by side. Raise the hall and adjust the coefficients to see how the stronger radiation warms the floor beyond the 50 percent rule.

Skistad · 3 m room22.0 °C

Mundt · tall room22.6 °C

Hall floor vs office floor+0.6 °C

Radiation dominant · hall floor is warmer

Tall room height9 mSupply temperature18.0 °CExhaust temperature26.0 °CRatio hc / hr0.75

Hardware and heating integration

The diffusers that deliver the supply air play a key role in making displacement ventilation work well. They must have large perforated face areas so the air leaves at very low speed. This keeps the discharge gentle, avoids drafts, and lets the cool air form a proper pool across the floor. We choose the diffuser type based on the room layout and the amount of air we need to supply.

5Interactive model

Three ways to deliver the air

Select a diffuser type to see how it spreads the supply air.

Wall-mounted (flat or semi-circular)Mounted on the wall, sending cool air out along the floor with a clear adjacent zone where it first spreads. Most common in offices and classrooms because they take no floor space.Free-standing (cylindrical)Stand alone in the room and release air in a full 360-degree pattern. They handle large volumes at low exit velocity, so they suit industrial halls and other high-bay spaces.Floor-mounted (flush grilles)Sit flush with the floor under fixed seating in auditoriums and lecture halls. Air joins the rising plume immediately, which makes the system highly efficient.

Heating limitations and system integration

Pure displacement ventilation cannot heat a space. If we supply air warmer than about 20 °C at low level, the warm air rises straight to the ceiling and skips the occupied zone. This destroys the stratification we need for good ventilation.

In heating climates we therefore combine displacement ventilation with separate perimeter heating such as trench heaters or radiant panels. The perimeter equipment takes care of all the heat losses through the building envelope. The floor or wall diffusers stay dedicated to cooling and indoor air quality. They continue to supply air at a steady 18 °C so the thermal layers remain stable throughout the year.

6Interactive model

Why the supply must stay cool

Warm the supply air past 20 degrees Celsius and watch the layers collapse. Switch on the perimeter heating to see how the two systems share the work.

Supply air18.0 °C

LayersStable stratification

Supply temperature18.0 °C

Perimeter heating