Radiant Conditioning Of The Indoor Environment
To realize the benefits and avoid the pitfalls, hydronic radiant systems must be designed, modeled, and operated correctly. Perhaps more than any other conditioning approach, hydronic radiant systems depend on holistic, integrated building design.
FUNDAMENTALS OF HYDRONIC RADIANT SYSTEMS
Radiant conditioning relies on the transmission of heat energy from warmer objects to cooler ones via infrared radiation. This radiant exchange depends on the difference in temperature of two objects and the “view factor,” or how much each is exposed to the other. Because human comfort depends on both the air temperature and the radiant environment, a measure that combines them, known as operative temperature, is a better measure of comfort than air temperature alone. Radiant heating and cooling systems are one way to achieve comfort by controlling the radiant portion of the operative temperature.
Radiant heating and cooling systems in the U.S. typically consist of piping in the floor, wall, or ceiling using water to transport thermal energy in or out of the occupied space and radiant surfaces to exchange heat with people and objects. These hydronic systems are more energy efficient than conventional all-air HVAC systems because water requires far less volume to transport the same quantity of thermal energy, saving space and fan energy.
Radiant floors typically range between 66ºF and 84º and ceilings between 60º and 120º, allowing boilers and chillers to operate more efficiently and save energy. As a result of these limitations, the capacity of radiant floor heating is about 20 to 35 Btuh/sq ft and radiant ceiling cooling is about 5 to 15 Btuh/sq ft, allowing these systems work particularly well in buildings with moderate heating and cooling loads.
Systems that use radiant exchange, unlike air systems that rely on convection, convey only sensible heat and do not affect the latent heat (humidity) of the air. So while radiant systems can address sensible heating and cooling loads, they can only reduce the size of a traditional air-based HVAC, not replace them entirely because there is still a requirement to supply a minimum amount of outdoor air for ventilation and humidity control.
If the hydronic system handles the entire sensible load, the ventilation requirement is met by a dedicated outdoor air system (DOAS), which may supply only one-fifth the volume of an all-air system. As a further benefit, depending on system configuration, ASHRAE 62.1 can allow DOAS up to a 30% reduction in ventilation air below all-air, recirculating VAV systems. Because they require less ductwork, hydronic systems can reduce or eliminate plenum space, either increasing ceiling heights in existing facilities or reducing floor-to-floor heights in new buildings.
Radiant systems require much milder water temperatures than conventional HVAC systems. Warmer chilled water temperatures can mean significantly improved chiller efficiencies, while the cooler heating water temperatures will allow for condensing boilers which operate at much higher efficiencies than non-condensing boilers.
DESIGNING HYDRONIC RADIANT SYSTEMS
The radiant floor. Radiant floors are better at delivering heat than radiant ceilings, as air warmed by the floor rises through the space to warm the occupants by convection. Hydronic radiant floor heating works well in larger spaces with uniform and moderate heating or cooling loads, such as lobbies, atriums, school classrooms, and offices.
Designers divide the conditioned space into permanent thermal piping zones from the early design phase, requiring careful coordination with the partitions and fixed furniture to avoid damage to the piping during build out. Care should be taken since the system will be fixed within the floor, making it difficult to rearrange zones to meet significantly different thermal loads down the road. The main piping of a hydronic radiant floor system supplies manifolds, or distribution points, throughout the building with hot or cold water, based on demand. Smaller piping circuits run from these manifolds out to area zones. The piping in each zone should be as close to the radiant surface as possible.
Hydronic floor piping may run within the concrete topping slab, (i.e., a “wet” system) or in channels inside proprietary modular floorboards (i.e., “dry” system). Both cases require careful coordination with other building elements such as structure. All hydronic systems should be pressure tested before and after installation to check for leaks before they are charged with water. Wet systems in particular require early coordination for the weight, height, and rebar design and schedule coordination to allow the concrete to fully cure.
For one new, 60,000-sqft wing of a LEED® Gold rated school, hydronic radiant floor heating was the optimal solution because the area’s mild coastal climate did not require a cooling system and the building was designed in contemporary style without ceilings in most areas. The greatest challenge here was the layout of piping and manifolds so that each manifold was close to the zones it served and yet able to economically receive the distribution from the main piping. For this reason, manifolds were kept as close as possible to the area being served to keep zone piping short, using a single length of pipe to help prevent leaks. Practically, a maximum of four to six zones (eight to 12 pipes to/from each zone) should connect to each manifold. More piping can lead to overheating near the manifold and difficulty in physically arranging all the pipes so close together in the slab. Because it is embedded in the massive concrete slab, a “wet” hydronic floor heating system has a substantial time lag, which must be considerend in the controls sequences of operation.
The radiant ceiling. Radiant ceilings are more effective at cooling a space than they are at heating, because cooled air naturally drops into occupied zones by convection. Radiant ceilings become less effective above about 10 ft, a critical consideration in system selection. In a radiant ceiling system, piping most typically runs throughout the building to ceiling-mounted radiant surfaces, designed to transfer thermal energy into each zone. Rather than embedding piping into the slab, hydronic ceilings typically consist of metal radiant panels.
This relatively lightweight panel makes this approach more flexible and faster acting than its flooring counterpart, but because ceiling panels must be large, they require careful coordination of room size and layout of partitions, lighting, sprinklers, and other fixtures. The radiant ceiling components also require coordination with the DOAS ductwork and diffusers providing ventilation air to the space — whether those diffusers are overhead in the ceiling, displacement ventilation diffusers, or under floor air diffusers incorporated in a raised floor system.
Condensation risk presents the greatest challenge to radiant cooling, whether on the floor or ceiling. Space conditions must be maintained such that the dewpoint temperature never exceeds the surface temperature of the radiant cooling device. Successful designs do this both by controlling the humidity of the air and temperature of the surface. Radiant cooling should not be used in places where condensation cannot be controlled, like a cafeteria or gymnasium due to high latent loads or with natural ventilation. Dewpoint sensors and moisture sensors should be included as part of the instrumentation.
MODELING HYDRONIC RADIANT SYSTEMS
While energy modeling has emerged today as a major tool to estimate a system’s potential viability prior to implementation, it is not always an easy tool with which to assess radiant systems. The majority of energy modeling software packages assume a well-mixed zone, where all air in the room is the same temperature and no radiant heat exchange takes place. A properly designed radiant system aims to achieve a particular operative temperature, which may not be reflected in the software.
For instance, an air temperature of 65° would typically be uncomfortable, but with a radiant heating system providing a mean radiant temperature of 77°, the operative temperature experienced by the occupants is 71°, which is comfortable. The energy savings associated with this wider band of operating temperatures, whether for heating or cooling, can be difficult to cap-ture in most energy modeling software packages. However, energy modeling is still employed for analysis with radiant systems, typically as part of a two-step process where calculations performed outside of the software are combined with the system analysis.
Since energy modeling programs typically don’t simulate radiant systems in the way they perform, a similar, hydronic-based system, such as a fancoil may be modeled instead. The engineer working in the software will remove the energy associated with the fan and adjust the water temperature into the typical operating range of a radiant system, adjusting performance curves as necessary. Finally, the analysis will be compared against the known operating characteristics of existing radiant systems to determine if the simulation is performing adequately.
Energy simulation should be balanced with thermal comfort modeling of the occupied space — whether via computational fluid dynamics (CFD) or some other means. This balanced approach provides a more comprehensive prediction of the effectiveness of the radiant system on occupant thermal comfort and energy efficiency.
CONTROLLING HYDRONIC RADIANT SYSTEMS FOR THERMAL COMFORT
The delay or lag of thermally massive surfaces presents an interesting comfort control challenge. On one hand, the mass virtually eliminates dramatic temperature swings, but on the other hand, the system tends to react slowly to input from a thermostat or sensors. A thermally massive radiant floor can take as long as eight hours to return to neutral temperature after the heating or cooling supply has been shut off. Understanding and planning for this thermal lag using predictive control sequences is crucial to controlling the temperature in the space.
For example, it may be appropriate to pre-cool a radiant floor overnight, when utility rates are usually lower, relying on the thermal inertia of the cool slab for the first few hours of occupancy in the morning. Alternatively, in climates with cool nights and warm days, like in southern California, the system might heat the building for a few hours before occupants arrive, then shut down early and coast through the cool morning on thermal inertia, avoiding overheating as the day gets warmer. While radiant floors may experience significant thermal lag, that is typically not a problem for metal radiant ceilings because the panels have low thermal capacitance and return to space temperature very shortly after the flow of water has ceased, a critical factor in cooling mode.
As mentioned previously, condensation presents a great challenge to radiant cooling regardless of installation. While all radiant floor systems must have temperature sensors for both the room air and the slab, any radiant cooling system, floor or ceiling, must incorporate dewpoint sensors that shut the system off when temperature of the radiant surface is too close to the temperature at which moisture condenses out of the air. Because radiant systems can be slow acting, the controls should maintain cold water temperatures at least 2º above room dewpoint temperature. Furthermore, the air supply must be controlled to prevent humid air from reaching the cool surface, which usually precludes radiant cooling in naturally ventilated spaces or entry lobbies.
At one Midwest airport, a 200-ft dia skylight over a 60-ft-high indoor plaza imposed a significant solar load, risking summer overheating and demanding substantial energy to heat in winter using an all-air system. Instead, a hydronic system in the floor provides heat during the winter and cooling in summer. During the summer months, the system efficiently removes the solar heat load directly at the floor where it occurs, but requires sophisticated dewpoint control to prevent condensation on the floor. While they experience similar sensible loads, radiant cooling was not used in the check-in area and baggage hall because of close proximity to the many exterior doors. Such uncontrolled openings can admit humid outside air, potentially causing condensation right at the entry or exit; presenting a serious slipping hazard for travelers and a potential maintenance issue for the operator.
For all building HVAC systems, maintenance is a critical element to ensuring continued system functionality, although radiant systems generally require less than most. Maintaining a hydronic radiant system consists primarily of checking the operation of the valves and maintaining the right chemical water treatment regime. Radiant ceiling panels may also require occasional cleaning. Thanks to new materials and joining methods, leaks in hydronic piping are preventable. Careful design to minimize joints and select durable materials, coupled with rigorous testing during construction, ensures a durable solution.
Properly designed and operated radiant systems have the potential to provide better indoor environmental quality as well as first and operating cost savings when compared to conventional HVAC systems. As radiant technology continues to improve and the cost of energy con-tinues to rise, the application of hydronic radiant systems for heating and cooling will grow in their adoption as a supplement to, and even substitute for, conventional, all-air HVAC design. ES