Selecting radiant ceiling cooling and heating systems (part 1)
This three-part series provides an overview of the most commonly available and applied commercial building radiant cooling systems. There are also a wide variety of radiant “heating-only” devices that are available, such as low- and high-intensity fuel-fired infrared heaters and electric radiant heating panels and sheets, but the scope of this series is to examine active radiant cooling applications for commercial buildings.
Editor’s note: This three-part series provides an overview of the most commonly available and applied commercial building radiant cooling systems. There are also a wide variety of radiant “heating-only” devices that are available, such as low- and high-intensity fuel-fired infrared heaters and electric radiant heating panels and sheets, but the scope of this series is to examine active radiant cooling applications for commercial buildings.
In our search for energy-efficient building systems, a lot of old technology is being re-invented. Two millennia ago, the Romans used the hypocaust systems consisting of a fire in the crawlspace of a heavy masonry building with the hot flue gases routed up through wall cavities to provide radiant floor and wall heating, using the thermal mass of the masonry structure to spread out and store the heat.
Similarly, cool stream water was channeled through heavy masonry building walls in hot climates to provide radiant space cooling in the Middle East. Today, we are rediscovering these systems and how to apply them for energy-efficient buildings that provide excellent human comfort. The active systems that are readily available are: suspended radiant ceiling panels, applied capillary tubes, fabricated PEX piping, and radiant slab systems (concrete core conditioning).
These radiant systems can separate the space sensible temperature control function from the ventilation function to arrive at an energy efficient system. Once the space sensible temperature is taken care of by the radiant heating/cooling system, the air system only needs to be a fresh air (100% outdoor air) delivery system, with latent (humidity) control, with no specific room temperature control function. This outdoor air can be heated, cooled, humidified, or de-humidified as required to supply a constant volume of constant temperature fresh air into the space.
This fresh air delivery system could also take the form of a natural ventilation system in mild climates where outdoor humidity and temperature is in the comfortable range. With a forced dedicated outdoor air ventilation system, controlled on an occupancy basis, just the “right” amount of fresh air could be supplied to just the occupied areas, resulting in even more energy efficiency.
Human comfort and mean radiant temperature
Many studies have shown that human comfort (body temperature regulation) is made up of three basic components: +/-45% radiation, +/-35% convection (air movement/air temperature), and 10% to 20% perspiration (relative humidity). [Reference 1] Human skin has a very high absorptivity and emissivity rate (0.97), which is greater than any other known material, flat black surfaces included. Given that the skin contains the bulk of our sweat glands, capillary blood vessels, and nerve endings, this makes us highly sensitive to changes in the mean radiant temperature of our surroundings.
Humans also need a source of fresh air to breathe and to dispel local air pollution. Most conventional building air conditioning systems (“all air“ systems) address only two aspects of human comfort: air movement (convection/temperature) and relative humidity (perspiration).
Conventional building HVAC systems use “conditioned air” as the primary space temperature control medium, using a mix of recirculated building air and outdoor air, supplied at warm or cool temperatures to provide climate control inside the building. For individual thermal zone temperature control, auxiliary terminal heating or cooling devices are employed: hot water or electric air reheat coils, variable air volume control boxes, chilled water/hot water fan-coils, wall fin heaters, and suspended radiant heating panels in the ceiling. Humidity control is provided as required for the specific climate zone the building is located in by treating the air with steam humidifiers or de-humidification equipment as needed.
These “all-air systems” are well understood and supported by the building systems design and construction industry. However, they have limitations, and require a great deal of technology and control systems to adequately address energy-efficient operation, let alone satisfying occupant comfort. The system maintenance requirements also are quite high for most conventional all-air HVAC systems. Witness the rise in “sick building” lawsuits and issues in the past decade—if these conventional systems are not maintained properly, serious risks are created.
The key space temperature design concept to be used in a radiant heating and cooling system design is the space “resultant” temperature. This is the temperature that the human body would “feel”—a combination of the ambient radiant temperatures, air temperature, and relative humidity. Terms like “wind chill temperature” and “humidex readings” are other terms for specific types of resultant temperatures.
In a radiant cooling environment, for example, a space with an air temperature of 78 F and a radiant ceiling surface temperature of 64 F will provide a “resultant” space temperature of +/-73 F. While conventional commercial HVAC temperature sensors currently cannot measure this “resultant temperature,” the radiant systems designer must consider controls setpoints and controls algorithms to accommodate this.
The key to finding energy efficiency in any HVAC system is to first concentrate on the building envelope to reduce the outdoor climate fluctuations, and then find the most energy-efficient methods of transporting heating and cooling energy around a building for human comfort.
“Air” has a relatively light thermal mass, while water has a much higher thermal mass. Hydronic, or water-based systems can provide much more energy-efficient building energy transfer systems. A building HVAC system engineer’s job must start by working with the building design team to minimize thermal loads in the occupied space first, in order to minimize the building climate control requirements.
General radiant/ventilation concepts and design issues
Radiant temperature control systems operate based on infrared energy heat exchange, which functions at the speed of light. Generally speaking, an overhead (ceiling) radiant temperature control system provides the best human comfort effect since it mimics the overhead sun and clear night sky effect that we have been subject to for millions of years of evolution, and provides the largest mean radiant surface area relative to our bodies. Radiant systems provide “sensible temperature control,” which means that the air system that is still required for ventilation air supply simply has to be a dedicated outdoor air system that provides de-humidified or humidified tempered ventilation air only.
Radiant systems are based on providing surface areas (radiant emitters) that are controlled to a certain surface temperature to provide radiant heat exchange to our bodies. The more radiant surface area you have to work with, the less extreme the radiant surface temperatures have to be to provide the necessary heat exchange for comfort control.
Large surface areas operating at relatively small surface temperature differences relative to the occupied space can provide excellent comfort control and stable indoor climates. Small radiant surface areas would have to operate at much higher temperature differences relative to the occupied space to provide adequate temperature control.
Radiant heating and cooling systems use warm or cool water (hydronic systems) to provide the necessary heat exchange medium, which is far more efficient than using masses of warm or cool air. Water can hold 3,400 times as much energy per unit volume compared to air. Air moving fans consume 7 to 10 times as much energy compared to water pumps to move the equivalent amount of energy around inside a building. Therefore, a moderate temperature hydronic radiant building HVAC system will provide a very energy-efficient building system.
Note that if the radiant cooling system is being operated above the ambient dewpoint temperature of the local space, it would not normally require pipe insulation, allowing for further mechanical systems savings. For long pipe runs through warm service plenums, however, it would be prudent to insulate the main pipe runs to maintain the supply water setpoints to the radiant cooling terminals.
Be wary of moisture control
A key fundamental limitation to any radiant cooling system is the ambient dewpoint temperature in the controlled space. The radiant cooling surface temperature cannot go below the room dewpoint temperature for any period of time due to the risk of condensation. This means that in humid climates, the indoor ambient relative humidity must be controlled, and in dryer climates, more radiant cooling capacity may be found by using lower radiant cooling water temperatures.
This also means that high solar gains and high cooling loads cannot be handled by a radiant cooling system due to the lower limit of radiant surface temperatures they must operate at. Solar gain control at the windows of the building and internal heat gains must be carefully considered and controlled if a radiant cooling system is to be used for space temperature control.
The ventilation air system can be used for “second stage” space temperature control “trim” to a certain extent, to deal with some of these higher transient cooling or heating loads in the space, if necessary.
Graphics by Chad Doi
1. National Research Council of Canada, Paper CBD-102 : “Thermal Environment and Human Comfort”, by N.B. Hutcheon.