The basics of radiant ceiling cooling
No drips allowed.
For decades, well-designed and -installed hydronic heating systems have earned a reputation for superior comfort. Still, potential customers who understand and desire the benefits of hydronic heating often ask, “But how does this system provide cooling?” Until recently, the response has usually been, “It doesn’t.”
The discussion then shifts to how a completely independent cooling system could be installed. This can be done with a standard central air-conditioning system or with the increasingly popular mini-split systems now on the market.
Unfortunately, the combined cost of a hydronic heating system and a separate cooling system often strains the construction budget to a point where something has to go. That something is often the hydronic heating option. It’s gets trumped by a lower-cost forced-air system that delivers both heating and cooling, albeit at often reduced comfort.
This doesn’t have to be the case. Several methods exist by which hydronic heating can be combined with hydronic cooling in smaller buildings. The latter is accomplished using chilled water supplied from a geothermal water-to-water heat pump or from an air-to-water heat pump. In either case, chilled water temperatures in the range of 45° to 65° F does the work of moving “cooling effect” around the building.
Start with loads
The total design cooling load of a building is calculated as the sum of the sensible cooling load and the latent cooling load. Sensible cooling maintains an acceptable interior air temperature by absorbing heat from gains through the building envelope, as well as heat generated from sunlight, artificial lighting, people and equipment.
Latent cooling deals with moisture removal from interior air. The moisture comes from a variety of sources such as incoming ventilation air, air leakage, respiration and perspiration from occupants, and activities such as washing and cooking.
On average, a typical residential cooling load is about 70% sensible and 30% latent. However, these proportions can change depending on weather, location and building use. To maintain acceptable interior comfort, a properly designed cooling system must provide the proper proportions of both sensible and latent cooling.
The proportions are determined through proper load calculations, such as those defined by ACCA Manual J procedures, and used in software based on those procedures.
The traditional method of chilled-water cooling circulates the water through insulated piping, leading to one or more chilled-water air handlers, such as the unit shown in Figure 1.
Any air handler or fan coil intended to provide both sensible and latent cooling must be equipped with a condensate drip pan and drainage tube. That tube can be seen in the lower right corner of the air handler in Figure 1.
It probably doesn’t surprise any of you that air handlers use air, pushed by a blower, to deliver both sensible and latent cooling. This obviously works, but it takes much more electrical energy to move the total cooling effect around a building, compared to what’s possible using water.
As I’ve discussed in many past columns, water is vastly superior to air when it comes to absorbing heat on the basis of volume. A cubic foot of water can absorb 3,457 times more heat than a cubic foot of air for the same temperature increase. This is the physics underlying a rapidly developing global market known as radiant cooling.
By switching as much of the sensible cooling load as possible to water-based delivery rather than air-based delivery, the electrical energy used by the cooling delivery system can be drastically reduced. One reference indicates potential savings in overall cooling energy use of more than 40% by using hydronic radiant cooling rather than an air-based cooling system.
At present, radiant cooling primarily is used in larger buildings. Still, the basic concepts are applicable to smaller systems such as those used in houses and light commercial buildings. Although radiant cooling can be embodied within a building in several ways, I want to focus on an approach that allows one radiant panel to provides heating and cooling. That panel is located in the ceiling, and one reliable and efficient way to construct it is shown in Figure 2.
Some readers may recognize this construction detail as something I’ve shown in previous columns. It’s a system I developed with my friend, Harvey Youker, for use in our small office building back in 1999. It’s been performing well, as a heating panel, ever since. Since then, I’ve designed this radiant ceiling into several other projects.
Its thermal output in heating is very respectable, with downward heat output of 0.71 Btu/hr/ft2 for each degree that the average water temperature exceeds the room temperature. For example, if the average water temperature in the panel is 110° and the room below it has an operative temperature of 70°, the panel releases about (0.71) x (110-70) = 28.4 Btu/hr/ft2. That’s right up there with many floor panel systems operating at comparable water temperatures. The very low thermal mass of this panel also allows it to respond quickly, especially in comparison to high-mass concrete floor panels.
Give and take
The radiant ceiling panel shown in Figure 2 also can serve as an excellent heat absorber. The rate of heat absorption can be calculated using Formula 1 (see slideshow above).
q = rate of heat absorption (Btu/hr/ft2)
TR = average of room air and room mean radiant temperature (°F)
TC = average lower surface temperature of ceiling (°F)
1.1 = an exponent (not a multiplier).
For example, suppose the room’s operative temperature (e.g., the average of its air temperature and mean radiant temperature) was 75° and the average temperature of the ceiling surface was 70°. According to Formula 1, the ceiling could absorb about (see Formula 1a in slideshow above).
Lowering the ceiling’s average surface temperature to 65° would increase heat absorption to about 18.6 Btu/hr/ft2.
Keep in mind that the temperature of the water in the tubing within the radiant panel has to be lower than the ceiling’s surface temperature. For the panel in Figure 2, the difference between the average ceiling surface temperature and the average water temperature in the circuit can be found using Formula 2.
∆Tsw = difference between average water temperature and average ceiling surface temperature (°F)
q = rate of heat absorption (Btu/hr/ft2)
For example, if the rate of heat absorption is 8.7 Btu/hr/ft2, as calculated in the previous example, the ∆T between the average water and ceiling surface temperature would be (see Formula 2a in slideshow above).
Thus, the average circuit water temperature needs to be about 4° lower than the average ceiling surface temperature.
There are definite limits to how cool the lower surface of the ceiling, or other components in the cooling system, can get before a major problem arises. That problem is condensation and it occurs on any surface that cools down to the dewpoint of the surrounding air.
The dewpoint of air can be determined based on its dry bulb temperature and relative humidity. The graph in Figure 3 is one way to find the dewpoint based on these conditions.
For example, if the dry bulb air temperature is 75º and the relative humidity is 50%, the air in the room has a dewpoint of about 55°. If the temperature of any object in this room is at or below 55°, condensation will immediately form on that object. If the object happens to be the ceiling, you can imagine what follows — it’s not pretty.
Thus, all radiant cooling panels must be operated at a temperature a few degrees above the current dewpoint temperature of the interior air. Various references suggest that chilled-water supply temperature to such panels be 3° above the current dewpoint. This provides a small safety margin that’s compatible with current sensor accuracy.
Designers should remember that dewpoint temperatures vary both with time and location within the building. The dewpoint temperature within an entry vestibule, subject to frequent door openings on a sultry summer day, could be several degrees above the dewpoint temperature within an interior space. If the chilled-water supply temperature to radiant panels in both spaces is controlled by a single mixing device but the dewpoint is only sensed within the interior space, it’s likely that condensation will form on the vestibule ceiling due to localized higher humidity.
To prevent such issues, designers must consider when and where localized sources of moisture may occur, and provide each of those areas with separate dewpoint sensors and chilled-water mixing systems.
Figure 4shows an arrangement where a single three-way motorized mixing valve can control chilled-water supply temperature based on dewpoint temperature during cooling, as well as warm-water supply temperature based on outdoor reset control during heating.
This approach simply changes the control logic used to regulate the mixing valve’s actuator depending on the mode of operation. The low-mass radiant ceiling provides excellent response and comfort in both heating and cooling mode operation.
Radiant panel cooling only addresses a portion of the total cooling load. Specifically, cooled ceilings should be sized and operated to absorb as much of the sensible cooling load as possible. The remaining latent load (e.g., moisture removal) must come from a device equipped to handle the resulting condensate.
In most systems this will be an air handler with a chilled water coil and drip pan. The airflow rate and coil should be selected to provide the design latent cooling load using a supply water temperature not lower than 45°. Higher chilled-water temperatures in the range of 50° to 55° may be possible depending on the latent cooling capacity of the coil and the location.
Drier climates with lower latent loads favor higher chilled-water temperatures. These higher temperatures improve Energy Efficiency Ratios for both geothermal heat pumps and chillers using air-cooled condensers. The air handler that provides latent cooling also could be used to distribute ventilation air. The savings come from significantly reduced air flow rates, which means smaller blowers and small ducting.
Although separate controllers are currently available for dewpoint control and outdoor reset, I would love to see new products that combine both functions into a simple, standalone, plug-and-play controller, complete with sensors. This controller would regulate operation of the mixing valve, as well as manage chilled-water flow through the coil of the air handler, providing latent cooling and ventilation.
I believe the North American market for radiant cooling in residential and light commercial buildings will increase in the future. The rate at which this takes place depends on several factors, including the cost of electricity, the availability of plug-and-play control systems and the growing market for hydronic heat pumps.
It’s certainly a great opportunity for those already involved in hydronic heating. It also provides a solid answer to that question, “But how does this system provide cooling?” Expect to see more on radiant panel cooling in my future columns.