Displacement ventilation in commercial spaces

Displacement ventilation supplies cool air at low velocity near the floor and removes warm air near the ceiling. Instead of mixing the whole room to a uniform temperature, it lets fresh air pool across the floor, rise around people and heat sources, and carry heat and contaminants upward and out. The result is a room that is stratified on purpose: cooler and cleaner at breathing level, warmer near the ceiling.

Used in the right spaces, displacement ventilation can improve air quality in the breathing zone and reduce energy use. Used in the wrong spaces, or designed like a mixing system, it causes cold drafts at the ankles and fails to heat. This guide explains when and how to use it.

How it works

Every warm object in a room, including people, computers and lights, creates a rising column of warm air called a thermal plume. In a displacement system:

1. Supply air enters through large, low-velocity outlets near the floor, a few degrees cooler than the room.
2. The cool air spreads across the floor as a thin layer.
3. Thermal plumes draw this air upward around heat sources, including around occupants' bodies, bringing fresh air to the breathing zone.
4. Warm, contaminated air rises above the occupied zone and forms a warmer upper layer.
5. Return or exhaust air is taken from at or near the ceiling.

The height at which the upward plume flow equals the supply airflow is often called the stratification height. Below it, air is relatively clean; above it, air is warmer and more contaminated. A good design keeps this height above the breathing zone of occupants.

Where displacement ventilation works well

Displacement is less suited to spaces with low ceilings (below about 2.7 m or 9 ft), high cooling loads per unit floor area, strong downdrafts from cold windows, or where the dominant need is heating.

Key design parameters

Supply air temperature

Supply air is typically delivered only a few degrees below the room setpoint, commonly around 17 to 20 C (63 to 68 F). Supply air that is too cold creates uncomfortable drafts at ankle level, and comfort standards limit the allowable vertical temperature difference between head and ankle.

Supply velocity and outlet sizing

Displacement outlets are large and deliver air at low face velocity, often in the range of about 0.2 to 0.5 m/s (40 to 100 fpm) depending on the outlet and space. Manufacturers publish an adjacent zone or near zone, the area around each outlet where velocity exceeds a comfort limit (often 0.2 m/s or 40 fpm). People should not be seated within this zone.

Vertical temperature gradient

ASHRAE Standard 55 limits vertical air temperature difference between head and ankle height for seated occupants; designers typically keep the gradient within the occupied zone to a few kelvins. See thermal comfort and ASHRAE Standard 55 for the comfort criteria.

Cooling capacity limits

Because supply air can only be a few degrees cooler than the room, and velocity must stay low, displacement systems have a practical upper limit on cooling capacity per unit floor area. Spaces with high sensible loads may need supplementary cooling, such as chilled ceilings or radiant panels, or may not be good candidates.

The ventilation benefit

ASHRAE Standard 62.1 recognizes the benefit of good air distribution through the zone air distribution effectiveness factor (Ez). For a well-performing mixing system Ez is 1.0. Standard 62.1 assigns displacement ventilation a higher value of 1.2 when the design meets the standard's conditions, including low supply velocity and a stratified upper zone. A higher Ez reduces the outdoor airflow required at the zone. Check the edition adopted in your jurisdiction, as the table and conditions have been revised over time. The guide on the ASHRAE Standard 62.1 Ventilation Rate Procedure explains how Ez fits in.

Heating: the main limitation

Displacement ventilation is fundamentally a cooling and ventilation strategy. Warm air supplied at floor level rises immediately, short-circuiting to the ceiling return and mixing the room. Standard 62.1 assigns a lower Ez when warm air is supplied at floor level with ceiling return.

In Canadian climates, heating must be considered from the start:

• Perimeter heating: hydronic radiation, radiant panels or convectors along exterior walls offset envelope losses and block cold downdrafts from windows.
• Radiant floors: in-floor heating pairs well with displacement ventilation, since the system supplies ventilation air while the floor provides heat.
• Neutral air in winter: displacement outlets can supply near-neutral air during heating, with heat coming from another source.
• High-performance envelope: better glazing and walls reduce perimeter heating needs and cold drafts.

Energy considerations

Displacement ventilation often uses warmer supply air than overhead mixing, which:

• Increases hours of economizer free cooling, particularly valuable in Canadian climates.
• Allows higher chilled water temperatures, improving chiller efficiency.
• May reduce outdoor air requirements via a higher Ez.

Offsetting these, it needs more supply airflow for a given sensible load because of the small temperature difference, which can raise fan energy if not managed. An energy model comparing displacement and mixing designs is the reliable way to quantify the trade-off.

Design procedure outline

1. Confirm the space is suitable: ceiling height, load density, occupancy pattern and heating strategy.
2. Calculate cooling loads, separating loads in the occupied zone from those above it (such as lights).
3. Determine supply airflow based on the allowable temperature difference and occupied zone load, using ASHRAE or REHVA displacement design methods.
4. Check ventilation requirements with the appropriate Ez value.
5. Select outlets and locate them where adjacent zones will not affect occupants.
6. Place returns high, and thermostats in the occupied zone away from outlets.
7. Design heating separately.
8. Check vertical temperature gradient and draft risk against Standard 55.

Frequently asked questions

Can displacement ventilation be used in an existing building?

Sometimes. It requires floor or low-wall outlet locations, a way to route supply ducts to them and a high return. Classrooms and assembly spaces with high ceilings are the most practical retrofit candidates.

Is displacement ventilation good for infection control?

It can improve breathing-zone air quality by removing exhaled air upward. However, people's own thermal plumes and movement can disturb the flow, and it is not a substitute for the specific ventilation requirements of CSA Z317.2 in healthcare spaces. Discuss options with a healthcare HVAC specialist for clinical areas.

Key takeaways

• Displacement ventilation supplies cool air at low velocity near the floor and relies on thermal plumes and stratification.
• It suits tall, densely occupied spaces such as classrooms, theatres and assembly areas.
• Supply air is only a few degrees below room temperature, which limits cooling capacity.
• Standard 62.1 credits it with a higher zone air distribution effectiveness, reducing required outdoor air.
• Heating must come from a separate source, which is critical in Canadian climates.