While radiant heating is a coveted upgrade in efficiency and comfort for new home design, residential radiant cooling is nearly unheard of.
Typically designed in conjunction with radiant heating, radiant cooling systems circulate cooled fluid through the same network of PEX pipe used for heating, creating a cooled floor, wall or ceiling that can evenly absorb sensible heat energy. This includes radiant energy from solar gain, people, equipment, lights and computers, in addition to convective heat transfer from the air.
Before dismissing radiant cooling due to concerns such as condensation, consider how using the radiant piping network for year-round space conditioning can help achieve energy performance targets while increasing comfort.
At least since the 1930s we’ve known that optimal heating and cooling has more to do with natural human comfort than dry-bulb set-points. Because of the way our bodies perceive operative space temperature (the combination of air temperature and radiant temperature), we feel comfortable with lower air temperatures when surfaces are heated (radiant heating) and with higher air temperatures when surfaces are cooled (radiant cooling).
Therefore, spaces with radiant cooling use space set-point temperatures as high as 77° F, while improving cooling comfort compared to traditional air conditioning. This is because people can radiate excess heat to the cooled surfaces in a room, rather than waiting for moving air to convect the heat away.
In other words, radiant cooling reduces the mean radiant temperature of a space, increasing radiant heat loss of the people, a good thing when it’s warm outside. This helps to reduce air-conditioning loads, cooling air volume and the potential for drafts and air noise. In most cases, a downsized forced-air system is still required to meet fresh air and dehumidification requirements. These downsized air-handling systems cost less than full-sized systems, take up less space and use less electricity to operate smaller fans.
Typical operational energy and cost savings of hybrid radiant/forced-air vs. traditional forced-air systems are 30% to 40%. The initial investment cost is slightly higher than a forced-air-only system.
Successful home design
Radiant cooling is most suited to high-performance home designs, such as those aiming for LEED certification. It is not uncommon for these homes to use ground-source geothermal heat pumps as the source to heat and cool fluid, which is particularly efficient due to the low-temperature requirements of radiant.
The ideal house for radiant cooling is a tightly sealed building designed to minimize air leakage. It has fixed windows or operable windows with contact relays; hard surface flooring (not carpet or wood); concrete floors, walls or ceilings where PEX pipe can be embedded; a source of cooled water (such as a chiller or geothermal heat pump); a downsized HVAC system to provide cooled and dehumidified fresh air; and a smart control system to integrate the hydronic and air-handling systems. Multiple zones of control will be integrated to respond to local loads throughout the house.
If this sounds like a light commercial building, it’s not far off. In fact, many of the houses that are feasible for radiant cooling are large enough to be considered light commercial buildings.
There are exceptions, such as the very arid southwestern United States, where radiant heating and cooling can provide year-round comfort without the integrated HVAC component and sophisticated controls. But for most of North America, radiant cooling is not for the typical house with many operable windows and a single thermostat.
Construction of radiant surfaces includes PEX pipe embedded in concrete floors, walls or ceilings. To absorb heat energy into floors, there should be minimal barrier to heat transfer into the floor. For this reason, radiant cooling is not recommended with carpet or hardwood flooring. Bare concrete, tile or stone floors are ideal. Likewise, exposed concrete is the ideal ceiling construction for absorbing heat energy.
While some buildings use pipe in ceilings for optimal radiant cooling and pipe in floors for optimal radiant heating, most projects use the same embedded PEX piping network in floors for both radiant cooling and heating. However, since it is easier to emit heat from a floor than to absorb heat back into a floor, a few adjustments need to be made during the design process to optimize the piping network for both modes.
Radiant cooling systems in poured floors or ceilings typically use 5/8 in. to 3/4 in. PEX pipe at 6 in. spacing and higher flow rates than radiant heating systems, with a typical fluid delta-T of just 5° to 8° F. While this will require more pipe and larger circulators than heating-only systems, this also results in a more efficient radiant heating system, with faster response time.
Under optimal design conditions, radiant-cooled floor capacities of up to 16 Btu/hr.-ft.² can be achieved, with more typical capacities in the 8 Btu/hr.-ft.² to 12 Btu/hr.-ft.² range. The cooled floor and walls are very effective at absorbing solar gain through nearby windows.
Controls mitigate condensation
Condensation occurs when a surface is colder than the dew point temperature of the air. Although this is desirable for a fan-coil when trying to remove latent energy (moisture) from the air, it is not so desirable on a radiant manifold — and even worse on a radiant slab. Simply pumping cold water through radiant surfaces could be a moist disaster, if the system is not designed correctly. We don’t even want to mention the other “m” word, rhymes with “cold.” Therefore, it’s essential to use technology that avoids condensation.
The coolest point in the fluid’s path through the radiant circuit is, by default, the entering water temperature at the supply header of a radiant cooling manifold. Therefore, effective control systems monitor and control the system fluid from this point. Ensuring the entering water temperature is at least 3° above dew point effectively avoids condensation.
A typical radiant-cooled space will use 75° air temperature with no greater than 50% relative humidity, yielding a low 55° dew point temperature. Floor temperatures below 66° are to be avoided for foot comfort anyway, so there is a good safety margin. Typical supply water temperature to maximize a system’s capacity is 58° to 60°.
In a hybrid radiant/forced-air system, a smart control system is used to monitor relative humidity, increase air cooling and dehumidification if the dew point gets too high, and possibly modulate the radiant fluid temperature so that pipe and surfaces stay above the dew point, while keeping occupants comfortable.
The ability to use the PEX radiant heating pipe network also for cooling addresses the common consumer question about radiant heating, “So how do I cool the space?” Under the right conditions, radiant cooling not only provides superior comfort, it can be the most cost-effective answer.
Perhaps the most important advice before starting your first radiantly cooled home is to seek out a manufacturer with the know-how to design the system and the right building controls to deliver all the benefits that your customers expect.
This article was originally titled “Not just for heating” in the 2015 Radiant & Hydronics Report.
Lance MacNevin, P.E., joined REHAU’s building technology division in 1993. He currently manages REHAU Academy and is the company’s senior codes and standards specialist. An expert in PEX-based technologies, he has primarily been involved in radiant heating and cooling, snow- and ice-melting, and geothermal applications. He can be reached at firstname.lastname@example.org.