Perhaps the U.S.’s largest to-date Dedicated Outdoor Air System (DOAS) coupled with radiant ceiling panel (RCP) cooling and heating mechanical system is currently under construction at the Washington College of Law at American University (Washington). The design’s compelling lifecycle and first-cost story was documented by Cindy Cogil, P.E., of SmithGroupJJR in an article in this magazine last September. This month, additional members of the mechanical engineering team report on some important design topics, lessons learned, and progress-to-date.
While this article focuses on a specific case study, general system design considerations will be reviewed regarding the DOAS system, demand control ventilation, radiant ceiling panel (RCP) system design, RCP heating and cooling water temperatures, and heating and cooling system ΔT’s.
American University’s Washington College of Law is a 300,000-sq-ft academic project in the Tenleytown neighborhood of our nation’s capital. Its anchor is Capital Hall, a historic campus building which is being fully renovated. A central open atrium connects Capital Hall to two new wings of the complex, Yuma Hall and Nebraska Hall. These two large buildings, totaling nearly 250,000 sq ft, were designed with the DOAS/Radiant system described above.
The buildings are all served by a common central cooling and central heating plant. A 720-ton central chilled water plant that includes two magnetic bearing centrifugal chillers and two cross-flow induced draft cooling towers with a variable primary pumping arrangement provides chilled water to the buildings. A 6,000-MBH central heating plant that includes three pulse-modulating, gas-fired condensing boilers with a variable primary pumping arrangement provides heating hot water to the buildings. The existing building and atrium are ventilated and conditioned by a mixture of rooftop AHUs and fan coil units. The two new buildings are ventilated by centralized DOAS systems, and space loads are met using passive RCPs.
While hydronic space conditioning has been common in Europe and Asia for decades, the U.S. has largely focused on air systems. But as energy in America becomes less reliable and more expensive, the thermal efficiency of water-based systems beckons. However, a typical comment from owners and facilities managers sums up a very real concern: “I don’t want rain in my building!”
Fan coil units — a much more common conditioning scheme in the U.S. — achieve high capacity by utilizing low cooling water temperatures in conjunction with fans for convective cooling. An important and deliberate secondary function of such a system is the ability to condense water at the cabinet’s coil, collect it and then drain it, effectively dehumidifying the space locally. In the case of radiant ceiling panels, there is no provision for collection and drainage of liquid water. If the space air’s dewpoint temperature reaches the chilled water temperature in a radiant ceiling panel, condensation will begin to form on the panel surface. To prevent the formation of liquid water, a radiant cooling design must carefully consider space dew point temperatures and RCP chilled water temperatures.
The radiant ceiling panels provide targeted space conditioning, but code-required ventilation is still accomplished at the central AHU via the DOAS. This decoupled approach reduces the size of the air distribution system as much as safety allows, while optimizing the thermal power of water as a transfer medium. While the primary purpose of the DOAS is to provide code-mandated ventilation, a useful secondary role of this ventilation air is to behave as a gaseous sponge. Careful calculation of the latent loads in a space produce an air handler design that precisely conditions the supply air to the humidity and temperature necessary to meet latent loads. Demand controlled ventilation (DCV) adjusts the quantity of air as the space usage increases and decreases.
Washington experiences hot, humid summers, relatively mild shoulder seasons that can still experience waves of high humidity, and mild winters that are often (but not always) fairly dry. The AU project is a law school with a full-service commercial kitchen and cafeteria with lots of latent input, and a full range of course offerings during peak humidity. A dead band temperature — 3°F, in this project — acts as a buffer against localized dewpoint temperature rises that might temporarily occur in the system, either through outside air intrusions or as a result of internal activities.
Extremely detailed latent load calculations were performed early on in the design, when operable windows were planned for the extensive office wings of the complex. Window switches were included for each operable sash, to close the CHWS valve whenever an occupant chooses natural ventilation. Controls were designed to delay bringing the chilled water back on line in that space until the DOAS has ample opportunity to deliver enough conditioned air to absorb the introduced moisture. Ultimately, the operable windows were removed from the architectural design during VE, and window switches were no longer necessary.
The process of choosing air and water characteristics was necessarily iterative. Warmer CHWS temperatures through the RCPs decrease condensation concerns but also greatly reduce capacity. Colder CHWS temperatures provide more cooling power, but increase condensation problems. Colder ventilation SA temps aid in cooling the space (the typical goal in our climate), but negatively impact thermal comfort.
It is important to note that radiant systems accommodate different thermal comfort temperatures than air-based systems. According to ASHRAE 55, a space conditioned by a radiant system that is set to 78°F has the feel of an air-based system conditioning a space to a set point of 75°F. To maximize the energy savings for the air-based system, therefore, AU’s space design conditions were selected to be 78°F, 45% RH for cooling, and 72°F for heating.
At the chosen cooling setpoint conditions, the corresponding dew point temperature is 55°F. Using the 3°F dead band noted previously, the chilled water supply temperature must be no less than 58°F to avoid condensation.
Of course, this relatively high temperature cooling water has negative implications for the capacity of the RCPs, and their cooling capability is already at the low end, due to the lack of forced convection. In general, a chilled beam system has much greater capacity than a straight radiant system. But the design team’s extensive study indicated that significant airflow savings could not be achieved using chilled beams (see the previous article, by C. Cogil, P.E.). In addition to the plenum space required for larger ductwork, the chilled beam devices themselves require more ceiling and plenum depth. Ceiling height is always a driving factor in Washington (due to a height limitation for historical reasons), and the AU project had a vast cadre of stakeholders, including neighborhood committees and historic societies, all of whom desired as low a structure as possible. In addition, the client preferred the sleek appearance of RCPs over chilled beams.
The DOAS system supplies ventilation air at 62°F 54°F WB to diffusers in each space. This is slightly lower than ambient temperature, and slightly drier than the targeted space humidity, in order to pick up latent loads. The air was chosen to be slightly cooler than space neutral in order to provide some cooling to help keep panel area requirements down, but not so low that interior cooling only zones would need to have heating capabilities to maintain thermal comfort.
The next step was to calculate the quantity of SA at these conditions which would be needed to offset the latent load in each space (Equation 1), and then compare this total to the code-mandated ventilation air (based on occupancy type, as outlined in the “Procedures” of ASHRAE 62.1, 2010.)
Vdpt = 0.68 x Qlatent
ventilation airflow required to offset the space latent load under design conditions, CFM
latent cooling load assigned to the space due to all internal sources, CFM
humidity ratio of the space, grains of moisture per pound of dry air
humidity ratio of the ventilation supply air, grains of moisture per pound of dry air
0.68 = Conversion factor
The greater of these two values was chosen as the SA primary flow. In several banks of offices which were less than 150 sq ft, the very low SA totals (both as calculated for ventilation and for latent loads) were increased to 30 cfm and passive airflow regulators were specified.
An important energy-saving measure to consider for any building that includes dense occupancy spaces of variable occupancies such as classrooms and conference rooms is reducing the outside air using a DCV strategy. There are several ways to setup DCV for a system. Occupancy sensors used for lighting can be specified to tie-in to the mechanical controls system or CO2 sensors can be placed inside the room and modulate the amount of ventilation air in response to CO2 levels inside the room. Theoretically, the more occupants in the space, the greater the CO2 levels and latent loads. In order to account for highly unpredictable part loads and the reliability of a single CO2 sensor in a room, a controls sequence was written that utilizes both occupancy sensors and CO2 sensors to regulate ventilation airflows to the high occupancy spaces.
Zehnder Rittling was chosen for the Basis of Design, in part due to their compelling testing and documentation, which seems to be a result of the more robust European standards for these system components. The team used catalog values to derive trendlines for the various panel sizes and mounting methods in order to analyze circuit combinations and flow volumes to determine optimal design parameters. Subsequent to this initial exercise, the manufacturer has developed a useful Excel-based template which serves a similar function, and produces capacities per panel and per square foot of area, as well as leaving water temperatures, flow characteristics, and velocities.
The AU mechanical team determined to limit any single circuit to 36 sq ft or less, to maintain fairly uniform (comfortable) panel surface temperatures across each space. In order to estimate materials takeoffs and piping and valving requirements, the team chose flow rates of 0.42 gpm in cooling and 0.27 gpm in heating, which reasonably balanced capacity with efficiency (minimum mass flow). This allowed global sizing and scheduling — with nearly 250,000 sq ft of ceiling area, it was vital to use a broad brush for the first pass.
In spaces with both heating and cooling loads, some circuits are designated as Cooling Only (CO), while others are dual-duty Heating and Cooling (H/C). Pressure Independent Characterized Control Valves (PICCVs) at the head of each circuit maintain design pressures despite the fluctuations that result from the Washington College of Law’s diverse programming. Motorized diverter valves for each circuit allow four-pipe system operation while maintaining optimum panel areas for either heating or cooling.
SUCCESS WITH A CEILING
All radiant systems rely on line-of-sight for their impact, so the layouts must reflect this. While radiant floors can be quite comfortable, they often don’t accommodate furniture and interior partition shifts. PEX piping is buried in concrete or below permanent floor assemblies, and it can’t be adjusted as a building transforms to accommodate changing user needs. Radiators mounted on walls or at toe kicks are easily blocked by furniture and often can’t meet the large surface area requirements to fully condition a space. Radiant ceiling panels maintain direct sightlines with occupants despite the furniture layout, and can even be removed and re-installed to accommodate program changes.
Still, ceilings are popular real estate, so coordination with lights, fire protection devices, controls, sensors, and architectural features is paramount. Although most panel manufacturers can custom-cut penetrations in RCPs, the long lead time and high expense make it prohibitive. The AU project benefitted from strong coordination between engineering disciplines and the design architects, as well as a robust BIM model to limit this solution.
The design team developed a spreadsheet to track the loads impact of all ventilation airflows, crediting the space in cooling mode and documenting reheat requirements in heating mode. We took care to reference available ceiling area in each space and document what percentage would be required to fully meet the cooling load. In some cases, there simply was not enough ceiling space in the thin perimeter zone to meet 100% of the space load. Because distance is not a limiting factor for thermal radiation, it was possible to do quite a lot of sharing between spaces.
Comfort is achieved both by having the RCPs radiate to and from all building components (floor, furniture, even window glass) and by presenting a warmed or cooled surface to the occupants. For instance, the second and third floors of Nebraska Hall house a law library, with seating at perimeter curtain walls facing east, south, and west. Whereas an all-air system would have been designed with large quantities of air washing the glass, potentially creating uncomfortable drafts in these quiet study areas, the RCP system is silent and still. The radiant panels which meet the perimeter load extend out into the open interior seating areas quite a distance and are still ‘visible’ to patrons near the exposed curtainwalls. This makes perfect sense when you consider that occupants in the interior of a large open space can still thermally “see” the cold window glass.
Even with this blurring of traditional zone boundaries, the team was hard-pressed to fully meet cooling loads in this particular situation. Although interior shades are scheduled for all of these areas, they are not included on the BMS with electronic sensors and actuators. Not only was it not possible to take credit for shading in the energy model, but all peak loads would have to be fully met by the mechanical systems. At one point, fan-powered boxes were considered to replace the RCPs in these zones. This would have undermined the design intent, however, and would have introduced fan noise, access panels, and unnecessary draft.
The architectural team came to the rescue by adjusting their ceiling design. The standard capacity of radiant ceiling panels is indexed to a free-hanging or “cloud” condition, with insulation applied to the top-side of the panels. Capacity is de-rated when the RCPs are mounted in a closed ceiling, which is the typical condition throughout the project, whether in gypsum or ACT grids. We omitted the insulation to achieve some cost savings and a small improvement in heating capacity. On the other hand, if the panels (without insulation) are free-hanging to allow convective currents to develop, the cooling capacity can be increased quite a bit. Note that in the case of cooling, this increased capacity directly benefits the room, but in heating mode, the heat generated on the topside of the panel simply collects below the ceiling and doesn’t translate to genuine capacity in the space.
Open areas of libraries required coordinated ventilation and RCP design approaches (see Figure 1):
- RCPs in perimeter zones were installed in “sail” configurations to boost capacity, while interior area panels are flush with the ceiling grid.
- Perimeter loads were partially met by RCPs in neighboring interior spaces close enough to be in occupants’ “thermal sight-line.”
- Nozzle diffusers mounted at a soffit sidewall “wash” the RCPs to boost capacity.
- Dashed lines indicate perforated panels for ambient noise attenuation, as dictated by the acoustical consultant.
- Diagonal hatch indicates dual-duty Heating/Cooling circuits
- Heavy dashed outlines indicate zones controlled by PICCVs (subsidiary circuits are limited to 36 sf or less).
The AU project broke ground in the summer of 2013, with an expected occupancy for the fall semester, 2015. A mockup of a typical RCP ceiling layout is expected to be ready for inspection at about the time of this printing. A third and final article is planned for this ad hoc series, which will cover several interesting lessons in BIM and documentation issues, as well as important additions to construction notes and specifications that should be helpful for mechanical designers considering a similar system.
- https://www.ashrae.org/standards-research--technology/advanced-energy-design-guides/50 percent-aedg-free-download