Displacement ventilation best practices
Displacement ventilation (DV) is a low-velocity, non-turbulent cooling system for commercial buildings that is not always fully understood. Learn when and where these DV systems can be best used.
- Examine the important, yet not always apparent, best practices of displacement ventilation systems.
- Classify the architectural and mechanical best practices of designing such systems.
While underfloor air distribution (UFAD) and displacement ventilation (DV) have had a place in industrial and commercial service for many years now, how many designs have been completed and operated with that "if I only knew about that" feeling while commissioning the system? DV is a delivery system where non-turbulent air is delivered at or near floor level at higher temperatures than overhead mixing-type systems. It achieves its cooling effects for occupants by the cooler air at floor level contacting the heat sources (people or equipment) in the room, allowing a "thermal plume" rising at the heat source—and it is the driving force in transferring heat from the source via the plume. The heat is carried up to a higher stratified level at the ceiling of the space being cooled, then returned to the air handler from this higher level. All this is accomplished at a very low velocity, typically at 40 ft/min, providing a comfortable, non-turbulent environment.
Where DV is often provided from a raised floor system, it should not be confused with a UFAD system common in office buildings and data centers, with this type of system being generally turbulent (highly mixing) in nature. A UFAD system, while sharing aspects of DV, has different design objectives not shared with DV systems.
DV systems offer benefits of modularity and flexibility in the workplace, reduced energy costs, reduced particulate contaminates in the occupied spaces, and individual user control of the workplace ventilation. A DV system is a good choice if:
- The contaminants are warmer and/or lighter than the room air
- Supply air is cooler than the room air
- The room height is 9 ft or more
- Low noise levels are desired.
Consideration of a traditional overhead mixing-supply system may be more advantageous if:
- Ceiling heights are 8 ft or lower
- Disturbances to room airflow are strong
- Contaminants are colder and/or denser than the ambient air
- Cooling loads are high and radiant cooling is not an option.
Considerations in designing a DV system include early upfront coordination with all design trades (architectural, mechanical, electrical, and structural) in addition to consideration of the construction of the facility itself. Only a contractor highly experienced in DV projects (particularly in a raised-floor delivery) should be selected. The contractor selected should present its own list of "best practices" in the bid submission for the contract. The quality of construction is paramount to the success of such a system. Note that DV is typically not the low-first-cost solution for a facility.
Architectural best practices
If a raised floor system, the floor system should be chosen carefully with minimal leakage and quality seals fixed to the supports, without the ability for the seals to become dislodged if the floor panels are removed and replaced. Whereas DV can be accomplished without a raised floor, the cost of air-distribution material (minimal compared to above-floor delivery) in a raised floor is attractive, and the system is aesthetically concealed. The costs of a quality flooring system may run $8/sq ft to $10/sq ft.
Underfloor DV systems are ideal for the open-floor-plan layout. Open-floor plans lend themselves to flexibility and renovation in the future. The engineer should plan for multiple air-delivery systems (e.g., 5,000 cfm to 8,000 cfm) throughout the floor plate creating multiple control zones. This not only adds a tighter degree of temperature control throughout the open space, but helps maintain compliance with ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings for fan-power limitation. With similar sized and model air handlers, serviceability will be standardized through interchangeable parts (belts, fans, filters, etc.).
Redundancy has now been built into the floor's systems by allowing the other units to ramp up temporarily to compensate for a down-blower unit. The use of vertical fan units in lieu of horizontal units can reduce floor space. Enclosed-space floor plans require independent terminal zone control and dedicated ventilation measurement and control, more so than the open-floor space scenario, adding cost. If the floor space is comprised of mostly smaller offices or enclosed spaces, careful consideration should be given to a fully ducted centralized supply unit or an overhead mixing system instead.
An open-floor layout aesthetically lends itself to an open-ceiling appearance. Whereas the Price Industries' Engineering Guide to Displacement Ventilation indicates that a 9-ft ceiling height is minimum, remember how a DV system works: Colder air is introduced at floor level, it warms as the air rises across the heat source in a vertical fashion, and it accumulates at the top of the space at a much higher temperature where it is then returned.
A 9-ft lay-in ceiling height, with a 3- to 4-ft-high plenum space above, may accomplish this adequately, but what about the open-ceiling scenario? A 13- to 14-ft-high floor-to-deck height is recommended for the open-ceiling configuration. Return air should either be ducted to the air-supply blowers spaced throughout the floor, or point returns can be used as high as possible from each blower system as long as the distance from the return inlet to the exterior wall is no more than 40 ft to 50 ft. This point-return layout should be analyzed to ensure noise is not transmitted from the fan room into the space, with consideration of using silencers to mitigate possible noise.
In either ceiling scenario, provide insulation on a floor deck above a DV-serviced floor. An uninsulated floor deck will allow heat in the stratified hot air to migrate to the floor above. This can cause nuisance higher temperatures in the floor above in a traditional overhead mixing system, and can wreak havoc in attempting to control an underfloor DV system if above such a DV system below.
Insulation types such as foam, board, or batt are economical and effective when used for a concealed, dropped-ceiling plenum, but an open-ceiling system may require a more attractive solution. Experience has lent to using an epoxy spray-on mixture that, while more costly than traditional insulating materials (approximately $3.80/sq ft at 30-mil to 35-mil thickness), provides an R-0.08 insulating value, is paintable, and does not disrupt the appearance or texture of the finished deck. It certainly is not as effective as traditional insulating materials, but provides some insulating value with a clean finish.
Open-communicating stairways between floors should be avoided. The supply air, delivered at floor level in a DV system, is colder than the median room temperature and crawls across the floor and right down the stairwell. A glassed-in enclosure with a door at the landing is an alternative to help contain the supply air on the floor it is serving.
Column enclosures should start at the floor slab and extend through the panel with the floor panel sealed to the column enclosure. The enclosure should have an airtight seal at the top and bottom. Construction tightness with a raised-floor system is very important; without positive seals at the top and bottom of a column enclosure, the whole assembly could act as an air duct allowing the pressurized supply air to escape the underfloor plenum.
Wet areas designed within a raised-floor area should be identified early in the design as monuments, and the floor slab height raised to match the proposed raised floor height. Any moisture spilled into a plenum from a kitchen or toilet area could breed biological growth unwanted in any airstream delivery system. Raise the height of the slab in these areas and supply with low-mounted wall-type DV diffusers.
If solid slab, avoid floor-sill exterior glass. Raise fenestration above the floor at least a foot and seal the foot or knee wall extremely well. In a raised-floor design, use a second "plenum wall" construction immediately adjacent to the exterior wall that can effectively stop infiltration (or exfiltration) from the plenum.
Considerations of operable sash areas in a DV-supplied building are complex. If an operable sash is a desirable aspect to your design, limit the areas to small enclosed spaces, rather than open-floor-plan areas; carefully seal the floor, walls, and ceiling above and below the raised floor (if present); and provide high-quality, very tightly sealed, shut-off dampers (automatically actuated based on sash switches) within metal transfer ducts to effectively isolate these areas from the rest of the DV system.
A thermograph study of the enclosure of the building is highly recommended prior to the fit-out of the mechanical systems during the enclosure construction phase. The thermograph, performed on the building's enclosure using temporary air delivery, will identify trouble spots on the building enclosure that could hinder a raised-floor plenum performance. This is especially critical if the discussion is made to retrofit an existing building with a DV system.