Displacement ventilation best practices
- 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.
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.
Mechanical best practices
A thorough commissioning process is essential for a DV system. All trades should be involved in the commissioning process. It should be identified that the contractor is responsible for the tightness of the plenum, sealing every single penetration through the raised floor tile and through the slab/structure.
Remember, this system is not a low-first-cost solution and offers performance obstacles if not constructed correctly.
All floor penetrations must be carefully sealed around an insulated duct at each penetration in a raised-floor system. If solid-slab (non-raised floor) is chosen, coordinate with the architect whether supply air will be ducted from ceiling space below the floor slab or within the same floor level from above. Supply air ducts to the DV diffusers are recommended to come from above in the same floor space. Allow for chases or thickened wall profiles at each drop.
Do not place diffusers where items such as rack storage (or file cabinets) or other obstacles will block the airflow from reaching the heat sources (building occupants).Consider final furniture layouts (if known) early in your layout, wall space isn’t always available or close enough to the occupants.
Most floor diffuser outlet manufacturers can provide both a polymer-based outlet for lighter loads such as normal walking, or aluminum construction for heavier rolling loads such as carts or rolling furniture systems. Consider pencil-proof and/or high-heel-proof openings. Always select an outlet with a collector cup under the outlet ring. It is not desirable to collect a 32-oz soda (or two) in the plenum space.
Carefully select a displacement-type outlet, rather than a turbulent diffuser that would throw the air too high and cause a mixing effect.
Manually controlled floor-mounted outlets can be cost-effective for large open spaces, allowing the occupants to custom-control their comfort zone. Floor outlets should be specified with minimal limiting controls to ensure minimum ventilation rates are provided.
In perimeter heating, place fan-powered variable-air-volume (VAV)-controlled heating terminals around the perimeter of the space within the raised floor in areas where access can be afforded (hallways or traffic paths) located out from under proposed furniture locations. Locate the perimeter zones and the heating elements far enough so as not to mix with interior cooling zones. A perimeter zone is typically 15 ft from the exterior glass with no occupants within these 15 ft. Ductwork from the terminal unit to the floor outlets is required for this system. Extending duct from the underfloor discharge to within 10 ft to 15 ft of the intake to the terminal units will ensure enough air will reach these terminal boxes. Hard duct can be cumbersome to maneuver under a raised floor with the support stanchions associated with such a system. Flexible fabric ducts can be very forgiving in routing around the obstacles of floor supports.
Alternately, condition the perimeter zone via linear "troughs" using either electric or hydronic heating elements. The natural convection from the troughs coupled with a "night-setback" control strategy (activating heating elements without air-terminal activation) can add efficiency to the overall system.
Wall-mounted displacement diffusers with heating elements should be used in perimeter zones for solid-slab systems. Overhead radiant panels may not be as effective as air-based systems in the floor, but such water-based systems are generally more energy-efficient than air-based delivery systems. A finned-tube radiator integrated into the top of a wall-mounted DV outlet is a common configuration.
Smaller enclosed spaces, such as conference rooms, require independent VAV-box control serving those spaces, with minimum settings to provide required ventilation rates and occupancy sensors to minimize or shut off air delivery when not in use.
The decentralized air-handling blower units in the floor plenum spaces should be compact and accessible. Key components of the units are a supply fan with variable-speed control, primary cooling coil, MERV 8 filters (used with prefilters), control dampers for outside air and return air, and smoke detectors. A key component would be a bypass heating coil that performs the following:
- Keeps the plenum from super-cooling in a low-load condition.
- Allows humidity control and keeps the plenum supply air from condensing.
Humidity control is an important design consideration in DV because ventilation of outdoor air can account for 50% to 80% of the building moisture load itself, and the occupants generate a lot of moisture. It is important not to let excess moisture into a plenum floor space to prevent unwanted biological growth. Over-ventilating the space with unnecessary outside air amounts is not a good practice in DV.
An overhead mixing system is normally designed to induce the outside air into the entire volume of the space. An important aspect of DV is the ventilation air is designed for the breathing-zone space of the occupancy. Because of the buoyant aspect of the displacement delivery near or at the floor, it effectively moves contaminants up and away from the occupants, to the high-return grilles.
Per the Engineering Guide to Displacement Ventilation:
"Typically, mixed-ventilation systems … have an average effectiveness of e=0.9. DV systems … have a ventilation effectiveness of at least e=1.2, and have the potential for greater ventilation effectiveness when used in combination with a dedicated outdoor air system and radiant heating/cooling systems."
Best practice in any DV system lends itself to using a dedicated outdoor air system (DOAS). This provides measured, preconditioned outdoor air to the air blowers throughout the facility using DV. Drying the outdoor air first in the DOAU can aid in control of a desired 30% relative humidity (RH, or less) in the plenum space. By using a total enthalpy wheel in the DOAU (with or without a desiccant wheel for humidity removal) is a very efficient method of energy saving in a DV system.
Steven M. Dailey is a principal and the mechanical engineering discipline leader at Harley Ellis Devereaux.