Radiant Systems
Many different types of radiant systems are used in HVAC applications. These systems provide or remove heat using heat exchange based on electromagnetic radiation due to the difference in temperature between two surfaces. Often, one of these “surfaces” is the skin or clothing of a person, and it is experienced by the warmth of the sun or by the chill of being near a window in the winter. Based on the temperature difference and the thermal mass of the “sending” surface, radiant systems can be put into a number of categories:
- Infrared heaters, such as those used by restaurants for outdoor seating, warehouse loading docks, and many other applications, are high-temperature heaters where the value of not heating the air but only the people and objects exposed to the radiation of the heater is what is important.
- Baseboard radiators use hot water, steam, or electricity to offset the skin losses of a building during the winter, especially near windows.
- Radiant panels that provide heating and cooling inside buildings, often at the ceiling, are low-temperature systems where the thermal mass of the panels is low (often they are metal) so that they can respond to changing conditions in the zone and primarily use water to add or remove heat from the panel.
- Embedded surface systems using water circulated by hydronic tubes within the concrete, often in the floor slab, warm or cool the larger thermal mass and take longer to respond to changing conditions in the zone and typically use water to add or remove heat.
- Similar to embedded surface systems, thermoactive building systems (TABS) exploit the thermal mass even more to provide peak shaving of electrical use by pre-warming or pre-cooling the thermal mass, often during unoccupied hours, to reduce overall building electrical demand.
This page will focus on the last three options described that often provide both heating and cooling.
How It Works
Radiant heating and cooling is based on radiation heat transfer, one of the three main forms of heat transfer, which also includes convection and conduction. For HVAC applications, almost all radiant systems include at least some convection heat transfer component as well and are usually considered “radiant” if at least half the heat transfer takes place due to radiation. The form of thermal radiation uses infrared wavelengths, which pose no health risks and simply transmit heat from one surface to another. The factors that determine the quantity of heat transferred are:
- The temperature of the emitting surface and receiving surface
- For the radiating surface, the emittance surface property
- For the receiving surface, the absorptance surface property and the transmittance surface property
- The angle between the emitting and receiving surfaces, which is often described as a view factor
From a human perception perspective, the sense of being comfortable in an environment is dependent on both the air temperature and the mean radiant temperature (a metric used to quantity the amount of radiant heat exchange between a person and their surroundings, see https://en.wikipedia.org/wiki/Mean_radiant_temperature), as well as other factors such as airspeed and humidity that the person experiences. Due to the comfort being directly impacted by the radiant exchange of heat around the person, radiant systems can create a sense of comfort in spaces where the air temperature alone would not be considered comfortable.
Radiant Panels
Ceiling panels used in a radiant system are typically metal panels where the back of the panel is in good thermal connection with a piping or tubing system carrying hot or cold water. The panels often look very similar to typical suspended ceiling tiles, usually using flexible quick-connect fittings to the piping headers so that they can be easily removed for service. The thermal connection between the piping or tubing and the panel needs to be very good since even a small gap can greatly reduce the capacity of the individual panel.
Often, insulation is behind the piping so that most of the heat transfer is to the space instead of the plenum. Since the thermal mass of the metal panels is low, it is easy to control the conditions in the space since the response time to changes in loads is quick. For buildings, it is not uncommon for the panels to only provide cooling for the core spaces since they generally have loads driven by internal gains while providing either heating or cooling to perimeter spaces, which have loads driven by both internal gains and from the building envelope. While most applications of radiant panels use the ceiling, panels can also be used on walls. During heating, the surface temperature of a ceiling panel should not exceed about 95F for typical ceiling heights so that the occupants do not experience discomfort. This temperature can be higher for higher ceilings. Due to temperature limits on both heating and cooling, the capacity of radiant panels is limited, so they are best suited for well-insulated buildings. The vertical space needed for radiant panels, including smaller ducts necessary to deliver outside air, is often much smaller than an all-air HVAC system, so this can significantly reduce the floor-to-floor height in a building, reducing capital costs. Since the temperatures needed are often significantly closer to the space temperature than a typical air-based HVAC system, the return water from a water-to-air heat exchanger coil is often sufficient to supply the radiant panels. By utilizing the water that has already been through the water-to-air heat exchanger coil, the radiant system itself returns water to the chiller or boiler even closer to the space temperature, which usually allows for more efficient operation of the heating or cooling system.
Embedded Systems
One of the most common types of embedded radiant systems is hydronic piping or tubing being incorporated into concrete slabs by being supported as part of the form when the concrete is being poured. Another type of embedded radiant system involves hydronic piping or tubing being attached to the metal lath prior to a thick coat of plaster (1 to 2 inches) being applied to secure it in position. Often, the other side of the lath is then finished and faces the room, although sometimes the side of the lath with the hydronic tubes is finished and faces the room. Sometimes, a metal plate is added to aid in the distribution of heat. In any of these configurations, the piping or tubing is tightly connected to the thermal mass of either the concrete or plaster. These larger thermal masses mean that response time is significantly longer between a call for additional heating or cooling and when the occupants feel the change in comfort. The thermal mass of a concrete slab is usually significantly greater than that of plaster due to its thickness and properties. Radiant systems embedded in floors also have very local comfort restrictions since people, even during weather that needs cooling, will be uncomfortable if their feet get too cold. One type of embedded radiant system is called a thermoactive building system (TABS), which is more related to how it is controlled but often also purposely includes very high thermal mass, such as thicker concrete. TABS systems are purposefully controlled to anticipate the type of heating or cooling load expected later in the day and pre-cool or preheat the thermal mass of the building so that the building can ride through those portions of the day without any heating or cooling being added to the slab. The control systems for TABS are often anticipatory, but in shoulder seasons, simply keeping the slab near the mid-point of the comfort window will often allow the building to coast through the middle of the day without any heating or cooling.
Outdoor Air and Condensation
Radiant heating and cooling systems are almost always combined with another HVAC system in commercial buildings in order to provide outdoor air. The air system may also be used to offset a portion of the load since radiant systems are often constrained by their dimensions and range of temperature differences. One of the most common configurations for new buildings is to combine a radiant system with a dedicated outdoor air system (DOAS) that provides 100% outdoor air but uses sensible heat recovery from the exhaust air stream to temper the outdoor air, see figure below. This configuration works in some climates, but in areas that experience higher humidity, a cooling coil may be used to reduce further the humidity of air being supplied to the zone.
Due to the DOAS being the only reason to provide ducting to the zone, this will often result in much smaller ducts and registers and smaller floor-to-floor heights. With a radiant cooling system, it is especially important to control humidity in the zone since condensation on ceilings, floors, or walls can result in damaged property or even injury due to wet floors. This is so important that sensors specifically to detect condensation are often deployed in buildings that use radiant cooling that shuts off or changes the operating temperature of the system when such conditions exist. For simulation purposes, it is unlikely that such condensation controls are actually simulated since that would indicate improperly specified controls, but the dewpoint temperature should be regularly monitored to avoid incorrect simulations. Since this is likely to occur in zones with high latent loads, such as high occupancy or equipment that adds humidity to the space, such as in a kitchen, such zones may be better served by non-radiant cooling systems. If they are served by radiant cooling systems, their conditions should be closely monitored. In addition, in locations that have high outdoor humidity, the simulation results for the peak humidity times of the year should be examined closely. Using radiant cooling systems along with standard air conditioning systems such as VAV is possible, but again, times that provide humid air to the zone should be examined closely, and central control of humidity becomes more important.
Information Needed for the Model
Not all simulation programs will model radiant systems well. Simulation programs that use heat balance methods are generally more accurate when modeling radiant systems than those that use simplified methods such as Radiant Time Series or Weight Factor methods. Programs that use more fundamental heat balance methods, including EnergyPlus, IES-VE, TRNSYS, ESP-r, and others, will be more accurate when it comes to the thermal mass and radiant heat exchange impact between surfaces than those that use simplified methods. Since radiant systems often are either directly controlling the system based on mean radiant temperature or are at least focused on describing comfort, it is important to review the comfort outputs such as predicted mean vote (PMV) available in the software, which is often based on ASHRAE Standard 55.
For the radiant systems, depending on the type and the specific simulation program, the following may be specified:
- Location of the hydronic tubing within surface construction
- Source of heat in the radiant system (electric or hydronic)
- Tubing arrangement details (length, diameter, conductivity, spacing, number of circuits)
- Availability of the heating and cooling
- Connections to the plant for a hydronic system
- Heating and cooling capacity of the system
- Maximum and minimum water flow rate for heating and cooling
- Weight of the water
- Sensor location
- Sensed variable (air temperature, mean radiant temperature, operative temperature, surface temperature, outdoor temperature, etc.)
- Fixed or variable flow
- Control algorithm and throttling range
- Condensation control type (turning off or varying the flow or temperature)
- Condensation control dewpoint offset
- Temperature control schedule
- Minimum and maximum temperature for heating and cooling
- Rated inlet water temperature
- Rated water flow rate
- Fraction of the total heat transfer that is radiant rather than convective
- Fraction of the radiant heat transfer going to people
- Fraction of the radiant heat transfer going to surfaces
- Fraction of radiant energy to other surfaces
Common Measures
Once radiant heating and cooling are chosen for the building, many different types of measures can be considered to make the building energy efficient:
- The system for providing fresh air can be a conventional VAV, a DOAS, or natural or mixed-mode ventilation.
- The amount of mass for embedded systems
- Various sources of lower-temperature water, such as recovered from other HVAC or non-HVAC systems
- The exact configuration of hydronic tubes in radiant ceiling panels or embedded in concrete or plaster
- The use of floor covering or bare concrete for floor-based radiant systems
- Variations in temperature
- Using variable speed pumping instead of constant speed
- Using variable temperature instead of fixed temperature
Common Control Options
Radiant Panels
Radiant panels are usually controlled at the zone level. The control system is focused on both providing comfort and preventing condensation on the surface. For core zones, since only cooling is required, a chilled water supply and return pipe are all that is needed. Control is often achieved by modulating the flow or the overall supply temperature. By either monitoring the zone humidity or using condensation sensors, the control system will either stop flow through the panel when near the dewpoint temperature or increase the water temperature so that it is further from the dewpoint. To increase the temperature, warmer water is injected, such as from the return or from the heating supply. This often does require the use of three-way valves, which are more expensive and reduce the capacity of the ceiling panels. Condensation sensors are often attached to the headers of supply piping in each zone since that is the coldest point in the room and are very sensitive to any condensation forming.
Embedded Systems
Due to thermal inertia, the controls for a high thermal mass radiant system, such as a concrete slab, often require controls that may be more directly linked to the overall load of the space, such as being dependent on the averaged outdoor temperature rather than the indoor temperature. Both regulating the flow rate of water through an embedded hydraulic system or the entering temperature is used. Predictive controls are sometimes employed. Often, spaces with radiant systems in concrete slabs do not benefit from night setback controls because the savings of changing the temperature at night are smaller than the energy needed to bring the concrete slab back to the daytime temperature.
Common Applications
Probably one of the most common applications for radiant heating and cooling systems is high-performance office buildings, but they can also be applied to other building types as well. They are poorly suited for applications with high humidity, so buildings with high outdoor air requirements would be poor fit. They have been applied to museums. For heating-only applications, radiant systems can be included in almost all building types but are especially well suited for buildings with large exposed concrete floors.
Model Output Checks
For radiant systems involved in cooling, it is probably most important to ensure that no condensation occurs in the spaces being served. To do this, look at the dewpoint temperature of the space and the surface temperature of the radiant cooling system and make sure they are at least one degree apart under all conditions. This will require looking at the hourly or timestep results for the zones. If the simulation software offers controls to protect against condensation, examine the hours that such a control is utilized and the hours before and after those times to ensure that the algorithm is working properly.
Confirm that the surface temperatures for the radiant panel or embedded system are as expected based on the design. The surface temperature drives the radiant exchange and is a fundamental metric for determining if the simulation is working properly.
Confirm that the controls operate as expected.
Related Energy Code Requirements
ASHRAE Standard 90.1-2019 does include several requirements or exceptions related to radiant heating systems but no really specific requirement related to radiant cooling systems:
- Section 5 requirements related to heated slab-on-grade floors
- 6.4.3.3.2 Setback Control
- 6.4.3.3.3 Optimum Start Controls
- 6.4.4.1.5 Radiant Floor Heating
- 6.5.4.8 Buildings with High-Capacity Space-heating Gas Boiler Systems
- 6.5.8.1 Radiant Heating Systems - Heating Unenclosed Spaces
- 6.5.8.2 Radiant Heating Systems - Heating Enclosed Spaces
Most of these sections also appear in 90.1-2022.
Similar or Related Systems
It is common to pair a radiant heating and cooling system with a dedicated outdoor air system (DOAS).
Passive chilled beam systems are similar to radiant systems but rely on convection to provide cooling. An active chilled beam adds a small fan or central air into the zone to increase convection.
Additional Resources
2019 ASHRAE Handbook - HVAC Applications. Chapter 55 - “Radiant Heating and Cooling.”
2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 6 - “Radiant Heating and Cooling.”
Radiant Heating and Cooling Handbook. Richard D. Watson and Kirby S. Chapman. McGraw Hill. 2002.
Citation: Bizzarri, M.; Conti, P.; Glicksman, L.R.; Schito, E.; Testi, D. Radiant Floor Cooling Systems: A Critical Review of Modeling Methods. Energies 2023, 16, 6160.
Wikipedia article https://en.wikipedia.org/wiki/Radiant_heating_and_cooling
Center for the Built Environment
References
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