In the concluding paragraph of the Oct. 7, 2021, Engineered Systems’ article titled, “Health Care Design: Beyond Code Minimum – Creating Healthier, More Efficient Environments,” it was proposed: “Physical laws can also be exploited to contribute to workload reduction to meet health care HVAC needs…” Understanding the physical laws of heat transfer — i.e., evaporation (vapor compression cycle or evaporative cooling) and conduction (coil heat transfer) — to effectively transfer discrete amounts of energy to or from a building is a given. HVAC designs apply the laws of physics to calculate building cooling and heating loads and, there upon, the amount of energy required to execute the work to be done to maintain stable, comfortable, and healthy building environments. It is not uncommon for engineers to then add a 15%-20% contingency to ensure system layouts have enough energy available to overcome loads that were not anticipated and/or other operational variances. The statement’s intent was to convey the idea that a mechanical brute-force approach to heat transfer is not the only available methodology. A design team can adopt another mindset that passively applies, at least partially, to all methods of heat transfer in a manner that will reduce the amount of system work to be done.
Dedicated outside air systems (DOAS) offer design teams such an approach when applied to passive hydronic HVAC systems. Unlike conventional designs that apply only two heat transfer modes, evaporation, and conduction, engineering teams can propose more efficient systems by exploiting all four natural heat transfer methods: conduction, convection, radiation, and evaporation. This article will review and evaluate passive radiant heating and cooling systems as an effective design approach that exploits the properties of all heat transfer modalities for enhanced system efficiency with the added benefit, when properly designed, of creating healthier built environments.
A Comparative Technology Review
- Before reevaluating why passive systems offer such proposed advantages, let us begin by reviewing conventional all-air systems. All-air systems use air as a building’s heat transfer medium. However, the specific heat of air is low compared to water. Consequently, large volumes of air are required to transfer a building’s energy gain to the outdoors when in cooling mode or, in the case of heating, add energy to maintain space temperatures.
- To overcome increasing thermal energy levels when in a cooling mode, a discrete volume of cool (55°F) air is normally supplied at 20° below the room design set point temperature of approximately 75°. The cool air is generally mixed into the zone and maintains a room temperature band of approximately +/- 2°. Depending on zone loads and air-mixing effectiveness, room air at elevated temperatures (75°-80°) is drawn from a space through return air grilles to an air-handler or fan-coil unit. Return air is then often mixed with humid outside air (ventilation air) in an air-handling unit’s (AHU’s) mixing section before passing through a cooling coil to cool and dehumidify the supply air. Supply air is cooled to the saturation point for dehumidification and mildly warmed (in draw-through configurations) by fan and motor heat by approximately 1°-3°. Consequently, many designs target a 55° DB/54° WB discharge air condition off the coil. Cooling coils are sized to meet a building’s total load (i.e., sensible and latent loads), and the supply air condition is designed to both maintain building thermal comfort and control building humidity levels. As a general rule of thumb, building supply airflow rates would historically range between 0.9-1.2 cfm per square foot, depending on building use and geographical region. As a result, AHU requirements with all-air systems could be quickly approximated. Initial estimates for a 100,000-square-foot building may have begun with AHU airflow rates of approximately 100,000 cfm for peak summer cooling loads. Heating loads, depending on climate zones, generally benefit from lower total airflow rates due to much larger approach temperatures than cooling airflow rates. In recent years, all-air systems' total supply-air (cfm per square foot) volumes have been dropping to meet reduced sensible loads. Upgraded construction standards, enhanced materials, lower lighting loads, and reduced plug loads are contributing to lower sensible heat ratios. However, it is not uncommon for the fan energy of all-air HVAC systems to represent upwards of 41% of a building’s overall energy costs to circulate this low-grade heat transfer medium.
Decoupled 100% Outside Air Systems
- Decoupled hydronic 100% outside air systems (OAS) are not designed to condition a building’s total load using solely the AHU’s cooling coil. Unlike conventional all-air systems, these systems exploit the advantage of a more dense heat transfer medium — typically water — to manage the sensible loads in the space, while the ventilation and dehumidification requirements are processed via the introduction of the primary air. In other words, a zone’s total load (sensible and latent load) is decoupled. Space sensible loads are removed (cooling) by a chilled water loop piped directly to each zone. Water, unlike air, has approximately 3,400 times the heat carrying capacity of air due to its specific heat and density advantage. As a result, it is far more efficient to extract sensible heat from a space using water and processing the ventilation loads separately than it is to displace the warm air-volume from a room by supplying approximately 55° air from AHUs. Shifting a zone’s sensible energy to a chilled water loop results in a reduction of fan-system horsepower and overall energy consumption. Air volumes can be reduced significantly, at least in the Southwestern climate zone, to between 0.25-0.8 cfm per square foot, depending on the use and latent loads of the zones served. It is not uncommon for decoupled ventilation systems (e.g. passive radiant and beam systems) to yield a 30% to 50%-plus energy savings compared to conventional medium-pressure, constant air volume (CAV) systems.
Active beams are conductive-only heat-transfer terminal devices. This equipment requires fan pressure to create induction at the beam to recirculate and process the room air through the beam-mounted cooling coil. The cooling coil is held above the room’s dew point temperature. However, passive radiant heating and cooling equipment does not rely on pressurization to drive room air currents. This setup yields enhanced system efficiencies and comfort by exploiting all four heat-transfer modalities, resulting in reduced system work. Heat transfer by radiation and convection is also applied when designing with radiant panels and sails. One might say the transfer is “exploited.” Radiant devices generally are sized in square or rectangular shapes installed at the ceiling level or on walls. By controlling a room’s exposed surface temperature profile with either chilled or hot water panels and/or sails, radiant energy is emitted from warmer room surfaces (cooling mode) to the thermally controlled cooler radiant device or emitted to a room’s cooler surfaces (heating mode) from controlled warmer devices. Due to the physics and properties of electromagnetic radiation, high-energy states move to low-energy states. Heat transfer is instantaneous and occurs at the speed of light. Consequently, energy emitted from an occupant’s warmer outer extremities will be absorbed by the cooler surfaces of radiant panels or sails located within the zone.
In cooling, a secondary conductive heat transfer effect also occurs between the surface of the radiant device and the warm air near the boundary layer of the panel and/or sail. Air in close proximity to the radiant chilled surfaces is cooled via conduction. As a result, the air increases in density and falls from the radiant panel and/or sail into the occupied zone, creating a convective pumping action that allows for the recirculation of the room air. To enhance conductive heat transfer and convective flow, a zone’s thermal profile is designed to be stratified between the floor, thermostat, and ceiling. A thermally stratified environment (see Figure 1), depending on a room’s sensible heat load and ceiling height, is designed to an approximate temperature profile of 70° at the floor, 75° at the thermostat, and 78°-80° at the upper levels of a room.
Displacement ventilation is the air delivery system of choice to suit a stratified room air-distribution strategy. Space ventilation is delivered using 100% outside air supplied locally to each zone through low in-wall displacement diffusers or through underfloor air plenums. Airflow rates are calculated to meet minimum ventilation rates as prescribed by ASHRAE 62.1. Airflow rates can be greater, depending on zone load, occupancy density, or an owner’s intent for enhanced IAQ. Unlike medium-pressure CAV/VAV or active chilled beam systems, displacement ventilation designs can be configured as low-static systems that may result in reduced fan horsepower. Zone heat sources move air via thermal convective plumes in lieu of fan energy. Each heat source, if it has a surface temperature greater than a room’s set point temperature, warms ambient air local to each source (e.g., an occupant’s body, a computer, etc.). Heat energy is drawn to the upper levels of a room by convective forces, i.e., less dense air currents. A human occupant with a body temperature of approximately 98° located in a room set at 75° will generate a thermal plume of an average velocity 30 feet per minute (fpm) (see Note 1). Displacement air systems supplying approximately 65° DB air also have an added advantage in the Southwestern desert, where cool shoulder and winter seasons are dry. Economizer hours can be doubled, adding an additional layer of performance efficiency in Arizona’s Maricopa County. Note that with decoupled systems, air-side free cooling has a limited ability to contribute to the overall zone cooling capacity in certain geographic locations. It is for this reason that water-side cooling becomes an important consideration for extended chiller-free operating hours using evaporative or hybrid cooling towers or dry-coolers. Water consumption and treatment must be considered relative to capital and operating costs if water-side free cooling includes evaporative cooling towers as a part of the mechanical solution.
Passive Radiant Device and System Design Considerations
When designing radiant cooling systems, it’s important to evaluate a building’s latent cooling requirement to assess the appropriate use of decoupled strategies and to calculate the radiant device capacities. To evaluate the suitability of a building or space for a decoupled ventilation solution, and terminal sensible cooling/heating using radiant panels/sails, it's relatively simple to determine the air-side load fraction (ALF). The ALF determines the overall contribution to the sensible cooling of the space delivered by the primary air to the control zone to suit the greatest of the following three parameters:
1) Minimum ventilation per ASHRAE Standard 62.1;
2) Minimum mass-flow of primary air/outside air to satisfy the latent loads of the space. This is driven by the dew point difference between the primary air off-coil condition and the design condition of the zone and can be calculated using the following simplified equation for projects at sea-level altitude; and
CFM (QL) = QL / (0.68*∆Gr)
CFM (QL): Cfm of primary/outside air to manage the latent load in the zone. QL: Zone latent load (Buth) ∆Gr: Grain difference between the room design dew point and the primary air off-coil moisture content.
3) Determine if any additional airflow may be required to supplement and/or support the terminal radiant sensible cooling solution.
As a general rule, if the ALF ≤ 0.8 (see Note 2), the zone could be a suitable candidate for decoupling the sensible loads in the room from the air-side loads (see Table 1). Higher ALF values are acceptable but typically represent a lower potential fan-energy savings than those spaces exhibiting lower ALF values.
Once the ventilation and latent control is calculated and managed in the space via the delivery of the outside air/primary air, the balance of the cooling/heating would then be provided by the radiant ceiling/sail devices, also known as the terminal devices, and any supplemental primary air that may be required to bolster the control in the space. It should be noted that DOAS discharge air should be designed to a low enough dew point to control building humidity, approximately 6°-8° dew point depression off the cooling coil. Some spaces exhibit cooling loads beyond that which radiant terminal devices can yield with their permissible ceiling coverage area. As a result, supplemental primary/outside air may be required to offset these shortfalls. A balance between capital cost and auxiliary primary/outside air will typically limit ceiling coverage to approximately 80%. The manufacturer’s software or an architecturally iterative solution will yield a balance of these terminal products and the consumed supply air.
Variations of Performance of Panels and Sails and Why
Radiant terminals can be supplied in either panel or louvered sail designs, each having a different performance capacity. The typical radiant terminal unit's sensible cooling capacity is driven by the approach temperature. The approach temperature for either of these devices is described as the difference between the room design dry-bulb set point temperature and the average uniform surface temperature of the radiant panel/sail. The anticipated capacity of these terminal products ranges between 21-44 Btuh per square foot at an approach temperature of 12°. Radiant panels are approximated to yield 50% of their sensible cooling effect via radiant exchange within the space, and their remaining capacity is the result of downward convective currents from the chilled surfaces of the panels.
Radiant sail technologies raise the overall sensible cooling effect in the space but do so by lowering the radiant cooling effect in the space and raising the downward convective current’s contribution to the total (See Figures 2 and 3).
The highest-performing sails yield approximately 30% radiant and 70% convective cooling in the zone. It’s important to note that as a result of their convective contribution, radiant sails are typically mounted below the ceiling approximately 8 inches in order to provide a thermal convective pathway for the air within a control zone to circulate (see Note 3 and Figure 4).
- Warm air rises to the ceiling level and migrates behind the sail to be cooled at a chilled surface.
- Chilled surfaces at the sail act as a heat sink via radiant heat exchange to the occupants and cool the warm air above the terminal unit.
- The sail is serviced with chilled water and held no less than 3.6° above the room dew point temperature to prevent condensation and falls into the space as the convective cooling contribution to the sail’s overall sensible cooling total.
- Convective currents are established as cooled air falls from between the sail blades then warmed and recirculated to the ceiling after being heated by the sensible loads of the space.
Chilled Water Loop Design Considerations, Piping, and Control
Large panel field arrays are generally piped in reverse-return circuits within the zone to minimize the use of water-balancing valves across the field. Two-pipe changeover systems are common and oftentimes serviced with six-way valves. In colder climates, four-pipe solutions are generally favored but are restricted to panel solutions, as radiant sails are not generally fabricated for heating applications in four-pipe configurations. When sails are used for low-temperature heating applications, the convective portion of the sail’s output is excluded from the net heating effect provided by the sail, as these convective currents rise to the ceiling surface and do not meaningfully contribute to the sensible heating of the space. Additionally, integral reverse-return piping solutions are available from some manufacturers to provide an additional pallet of elegant piping solutions to those consultants looking to solve water distribution challenges of longer continuous runs of perimeter radiant heating with lower hot water temperatures.
Secondary chilled water servicing these terminal products must be 3.6°F warmer than the room design dew point, in order to prevent condensation from forming on the surface of these devices. Additionally, in order to ensure the flow rate within the tubing servicing the panels/sails remains fully turbulent and experiences proper heat transfer, the flow rate must hold a Reynolds number of Re ≥ 4,400. In result, modulating chilled water flow rates are typically restricted to sail products, as this can add a layer of controllability relative to the overall cooling effect and enhance the downward convective airflow control. Radiant sails, as referenced in the sentence above, and panels operating in cooling mode are generally controlled using two-position valves. When considering high-temperature perimeter heating applications, a combination of modulating water-flow control and a hot water boiler reset strategy, indexed to the outside ambient temperature, help to mitigate the risk of radiant asymmetry for occupants near the perimeter.
Chilled Water Reset When Humidity Is Compromised in a Building
Compromised zone humidity levels result in a chilled water lockout (valves drive to closed) condition to prevent condensation from forming on the chilled ceiling/sail. However, two-pipe changeover applications allow for the conversion of the loop to heating to purge the loop volume from the terminal products and further limit the risk to any zone-level condensation. A chilled water reset strategy can also be employed for off-peak shoulder seasons to further extend chilled-water energy savings.
On July 28, 2021, the U.S. Department of Energy (DOE) issued a final determination that ANSI/ASHRAE/IEW Standard 90.1-2019 will become the referenced energy efficiency standard and offered instructions to each state to “…review the provisions of their commercial building code regarding energy efficiency and, as necessary, update their codes to meet or exceed Standard 90.1-2019.”
The American Institute of Architects' (AIA’s) 2030 commitment to achieve net-zero carbon emissions will very likely require reexamination of HVAC design strategies to meet the commitment’s objective. According to Architecture 2030, HVAC systems generate approximately 40% of a building’s carbon footprint. Other national and local initiatives are driving building design teams to reevaluate how buildings are made and assess embodied and operational carbon impacts. At the same time, there is a national effort to improve IAQ in buildings. On Oct. 10, the White House held its first IAQ Summit and opened a Clean Air In Buildings Webpage, offering building owners an IAQ challenge. Historically, better IAQ came at a cost: greater energy consumption. One-hundred percent outside air passive radiant systems are applicable design strategies to span the divide between meeting or exceeding ASHRAE Standard 90.1-2019 performance guidelines while maintaining healthier environments through enhanced IAQ. The opportunity for superior IAQ becomes apparent as thermally stratified environments result in a single pass of clean, conditioned, 100% outside air across occupant breathing zones. By exploiting all four modes of heat transfer, 100% OSA/passive radiant cooling and heating systems provide building owners the best of both worlds by reducing operating costs while securing a healthier more comfortable built environment that promotes an occupant’s personal sense of well-being.
- Thermal plume profile: An occupant’s rising thermal plume velocity profile varies across the occupant’s body from approximately 2-5 fpm at the feet and lower legs up to 45-50 fpm at thoracic region of greater mass and energy density.
- Air-side load fraction definition is the percentage of the sensible load that is serviced by the primary air that is needed to meet the ASHRAE 62.1 requirements, and/or the latent loads in a space. Any value less than unity “1.0” represents fan-energy savings potential. The lower the air-side load fraction, the greater the opportunity there is for primary-air fan-energy savings. During the design phase, this calculation should be performed on a zone-level basis to prequalify spaces as suitable candidates for decoupled ventilation, or it can be used to provide evidence that an alternative HVAC strategy may be required for spaces where the ALF exceeds 1.0.
- Unlike radiant panels, sails do not provide any acoustic attenuation within the space. Supplemental acoustic treatment may be required within the occupied zone, depending on the acoustic design criteria. See acoustic treatment fastened to the ceiling above the sail at position No. 2 in Figure 4.
- Reverse-Return Piping explanation: In radiant heating/cooling panel applications, where large distributed piping arrays may exist, this copper tubing circuit option allows the first tube to act as a supply “main” and the last tube on the panel to act as the “return.” This has the advantage of lowering the distribution piping, which must be hung in the space to service the panels, and adds the benefit of having this distribution piping pre-insulated by the factory. This strategy can help to minimize field piping labor and materials to insulate the mains in some cases.