I’m a long-time reader of JLC and a first-time contributor. I'm also the founder and principal of Positive Energy (www.positiveenergy.pro), a building-science consulting and engineering firm headquartered in Austin, Texas. Our company motto is, “Design around people, a good building follows.”1One of our functions in the local and national market is mechanical system design, which we do partly because we recognize that this is an area where the industry and our society generally is due to evolve.
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Our perspective is that now is the time and we are the professionals and advocates called on to promote human well-being and truly sustainable building practices that recognize both operational and embodied energy and avoid exergy destruction. “Exergy,” as distinct from “energy,” is a useful practical concept in thermodynamics: It means the available useful energy in a system. Using some numbers to illustrate exergy, consider how exergy-inefficient it is to use a gas flame at 3,500°F to heat indoor air to 70°F or domestic water to 120°F. By contrast, using the otherwise unusable low-temperature heat in the ground to heat a building via a ground-source heat pump is an example of exergy efficiency.
For these reasons and for the thermal comfort of our team at Positive Energy and our clients, we’ve been taking a close look at radiant heating and cooling for the past few years; we have a working system in our office and several designs on the books.
Radiant heating is a well-established idea in the comfort industry2. People know that it’s a solution that provides great comfort and high efficiency. In fact, relative to forced-air heating and cooling, radiant systems can rely on low heating temperatures and high cooling temperatures. Though this may seem backward, what it’s pointing to is exergy efficiency.
Radiant cooling is a new idea for most people, but in principle it’s the same as radiant heating: It’s an efficient way to deliver great thermal comfort. But just as radiant heat is fundamentally different from forced-air heat, radiant cooling is a radically different concept from traditional air conditioning.
Air conditioning is aptly named. It’s not the same as occupant conditioning. Air conditioning systems cool and dry your air, but they don’t directly cool you. When you live or work in an air-conditioned space, you don’t experience the air temperature directly. Your predominant thermal experience in any space is of the temperature of the surrounding surfaces in that space. In a room with radiant heat, your skin directly receives the warm infrared rays that radiate from the radiant floor, wall, or ceiling surfaces. In a space with radiant cooling, you have the same experience in reverse. Your body becomes the radiator and cooler surrounding surfaces absorb your heat.
This works because the main way that human bodies shed and absorb heat is through radiation: Long-wave infrared radiation goes in a straight line at the speed of light from the radiator (your skin) to the absorber (the cooler surfaces that surround you). Your skin, at about 90°F, is like a radiator that radiates heat to those floor, wall, and ceiling surfaces, and that’s what cools you off. When we operate an air conditioning system to cool the people who live or work in a space, we’re accomplishing the same thing, but indirectly: We’re cooling large volumes of air and moving them through the space in order to cool the surfaces in that space, which then in turn cool off the building occupants by absorbing the radiant heat that those occupants emit plus the heat that leaks in from the outdoors.
So why not cool the surfaces directly? That’s the idea behind radiant cooling, one that imbues the enclosure with more of an active, engaged role with the occupants. Instead of moving high volumes of air through the living space to cool the interior surfaces of a structure, we move low volumes of glycol and water coolant through the surfaces themselves to do the same thing, more efficiently. Exposed to those cool surfaces, people’s bodies cool themselves by radiating heat. If you’ve ever experienced the coolness inside an old stone building, or even a new parking garage, you have experienced radiant exchange with cooler surrounding surfaces. In our offices in Austin, we have installed what we believe is the first radiant cooling system in the state of Texas. We built the system using components generously supplied by Messana Radiant Cooling (www.radiantcooling.com). Let’s take a walk through the system.
OVERVIEW
The radiant system uses cool ceiling panels as the heat-absorbing surface. These are pre-manufactured panels that consist of aluminum heat conductors sandwiched between a layer of gypsum board and a 1.5-inch layer of EPS insulation that isolates the cool fins and gypsum board from the framing of the building. Polyethylene tubing filled with glycol and water fluid runs through the aluminum fins in a serpentine pattern to transport heat into or out of the panels. Because of the EPS insulation, the radiant exchange from the hydronic tubing occurs more strongly with the room-facing panels than the framing-facing structural members. This allows for a rapidly variable panel surface temperature, which is important in a humid, cooling-dominated area like central Texas.
In operation, the gypsum board ceiling is maintained at about 68°F. Humans (and their dogs) in the space radiate heat to the gypsum board, which absorbs the heat and conducts it into the aluminum fins, which conduct it to the tubing. The fluid circulating through the serpentine tubing at about 60°F absorbs the heat and transports it to a manifold, which directs the warmed fluid into the return line leading to a buffer tank in the mechanical room. We have four rooms in our office, so our manifold has four loops. But you could manage as many zones as you wish by extending the manifold.
The buffer tank in the basement separates the source side from the system side. The buffer tank plays an important role in maintaining equilibrium for the system, because the fluid in the system maintains a reserve of heat and cooling that is always available to answer short-term demands. Our system has a 30-gallon buffer tank, maintained at about 60°F during the summer cooling months. If there were to be a massive call for cooling upstairs, and we needed to send an increased volume of thermally absorptive fluid up there, the reserve in the tank would be available to answer that call and our cooling source (an air-to-water heat pump located outside) would be able to carry on at its usual measured, consistent pace.
So we separate the source from the system using the buffer tank. The buffer tank can be considered a thermal battery, and could be larger than 30 gallons if needed for the application. For example, we could design an off-grid system with sufficient thermal storage that the high COP heat pumps only run when solar photovoltaic power is available during the day. In that case, we could use an array of 80-gallon or 100-gallon buffer tanks to store cool water that could answer the demand for cooling without the need for an energy-hungry compressor to operate. For larger projects or projects with critical loads, these thermal storage reservoirs can be extremely well insulated and hold thousands of gallons of thermal fluid, or even a combination of thermal fluid and phase change materials (PCM), capable of absorbing or delivering tens of thousands of Btu of energy.
The buffer tank provides a thermal mass that lets the system smoothly ride out fluctuations in the demand for cooling or heating. But the bottom line is that, in cooling mode, the fluid is taking heat from the indoors and sending it outside the building. Outside the building is our source: a 3-ton SpacePak Solstice3 air-to-water heat pump. Here, the heat that was originally absorbed as direct radiation from the bodies of people in the space into the ceiling panels is finally shed from the system out into the outdoor air. This same energy could have been shed directly into the ground around our building using a ground-coupled heat exchanger and a pump to circulate the hydronic fluid. In our climate, we can avoid the installation cost, soil and ecosystem disruption, and embodied resource use with ground loops by rejecting the waste heat into the air, or first into a hot-water storage tank, while maintaining high COPs.
Of course, radiation between people and their surroundings is not the only way that people get warm or cool. You’re also affected by direct contact with the air in a space. In a room with radiant cooling, such as our offices, this process also takes place. Warm air in the rooms expands, becomes buoyant, and rises to the ceiling; when it contacts the ceiling, it cools off and falls toward the floor. So people in the space are continually bathed in a gentle cascade of cooled air falling from the ceiling and trickling over us. But there’s no need for fans or ductwork to move that cool air around.
It is important to bear in mind that a radiant cooling or heating system is focused on occupant thermal comfort. The fact that we also live our indoor lives immersed in an indoor pool of air means that reliable supplemental systems for ventilation, filtration, and dehumidification for healthy IAQ are a must.
WHAT ABOUT CONDENSATION?
Outdoor air here in Austin, Texas, can be very humid in the summer--we regularly hit 78°F dew point and even the low 80s--thus cool radiant surfaces are well below outdoor dew points and run the risk of significant condensation and sorption into materials. The last thing we want is for moisture in our building to condense on our ceiling drywall and support mold, mildew, bacteria, and other indoor microbiomes associated with damp buildings and poor occupant health. Fortunately, the system is designed to prevent that.
In the first place, we control humidity in the space using a dedicated, ducted dehumidifier (an Ultra Aire 98H). That unit is continually paying attention to humidity and removing any excess moisture, and it circulates dry air all the time. The indoor relative humidity is held right around 50%, so the 68°F temperature of our ceiling radiant absorbers is well above the dew point. But the amount of ductwork required for fresh air supply and dedicated dehumidification in a radiantly cooled space is much less than the amount of ductwork that would be needed to provide traditional air conditioning. It is important to note that dedicated dehumidification is important in humid, or green grass, climates generally, and not just when radiant cooling systems are used.
Even with the dedicated dehumidifier, if someone were to open up the doors and windows and let a blast of humidity into the building, there could be a risk of condensation on the ceiling drywall. However, this is a low-mass system that can be controlled rapidly to maintain the surface temperatures above dew point. The controls for the setup include humidity sensors that know if there’s a sudden spike in the indoor relative humidity. If so, the flow of cooling water is instantly halted and the cool drywall surface is allowed to become adiabatic with the space conditions so that it remains above the dew point in the space. At that point, it’s time for the occupants to close the doors or windows so that the dedicated dehumidifier can remove the excess humidity until the cooling system can start up again.
At our office, we also have a dedicated, enthalpy tempered, ventilation system using IFTTT logic to come on as needed. Another option would have been to use the dedicated ventilation port, damper, and controller logic available with the UltraAire98H.
FINAL THOUGHTS
Building science is systems theory applied to homes and buildings. The power of building science comes from its ability to recognize and understand cause and effect relationships involving heat, mass, and moisture flows between systems, assemblies, and components. Yet, somehow our prevailing industry view, even at building-science conferences, is that the enclosure and its mechanical systems are somehow separate. This has manifested as a clear decoupling in the practice of architecture and design. Now is the time for the "hegemony of enclosurism"4 to give way to the more accurate perception that a building can do more than just contain indoor air and exclude outdoor conditions. When we use a warmed or cooled building mass to supply heating or cooling, the building itself can directly engage with its occupants as the primary heating and cooling system.
Contrast that reality with what is normal today. In the summer, we use energy to cool our buildings for comfort while separately and often simultaneously expending energy to heat water for bathing or swimming. In the winter, we produce and deliver heat to the conditioned space while we also reject heat from AV closets and wine rooms to the exterior. More broadly, systems that want to reject heat include the air conditioner, dehumidifier, wine-room chiller, and AV closet/computer room; systems that want to absorb heat include the domestic-hot-water tank and the humidifier as well as the pool, spa, and snow-melt systems. With the more widespread adoption of hydronic designs and installations, not only do we expand the palette of architectural colors by reducing reliance on large air ducts, but we also allow for systems that can aggregate and buffer these heat flows for the overall benefit of energy and exergy efficiency.
Taking a thermodynamic systems perspective, it is clear that there are tremendous thermodynamic synergies that we have yet to achieve at a meaningful scale and radiant cooling and heating systems are a symbol of the need to rethink our unexamined biases and approaches. The needed retooling of the construction industry is currently held back not by lack of technology but rather by lack of industry and societal understanding and will. Those of you reading this are a self-selected group who have the power (and perhaps responsibility) to continue to deepen your understanding so that you can engage, educate, and advocate in a way that moves our industry out of its current state of complacency. Why are the very places where we live and love our families held back in a laggard state of technology?
Footnotes
1. Credit for that motto and the interest in radiant cooling both go to Robert Bean, my mentor and good friend.
2. For an excellent historical overview and current context of radiant systems I refer you to Keil Moe’s book “Thermally Active Surfaces in Architecture”.
3. Generously donated by Mestek.
4. The phrase is mine; I’m joking — but only sort of. Over the last decade or so, an imbalance has developed in our industry between those who focus on enclosures and see them as the primary solution, and those who recognize that the poignancy of elegant thermodynamics includes the active, energy-using systems. Yes, thermal comfort and IAQ are invisible, but that’s not a reason to ignore them. We start with a good enclosure by focusing on the passive elements of the building for which there is only one good chance to get them right (this includes air distribution systems!), and then we thoughtfully approach heating and cooling for comfort, and dehumidification, ventilation, and filtration for health and well-being.
