Radiant-cooling systems have progressed in the last few decades, garnering attention as a comfortable and energy- and material-efficient option.¹ Compared with air systems, radiant-cooling hydronic systems use approximately half the horsepower and materials to move heating and cooling energy within a building. Although radiant-cooling systems have been used in Europe for years, they are starting to be examined — and installed — in the United States.

Low-flow injection-pumping systems can help make radiant-cooling systems even more efficient. Injection-pumping systems deliver heating and cooling energy to a variety of terminal units, including chilled ceiling panels, chilled beams, fan coils, and heat pumps, in the same piping-distribution system, even if each unit requires a different temperature.

THE EUROPEAN EXPERIENCE

Like many hydronic-based-system developments, commercial radiant-cooling HVAC systems originated in Western Europe during the 1980s. Traditionally, European commercial buildings were supplied only with heating systems. With the introduction of personal computers, these buildings began to require sensible cooling.

Europeans then developed radiant chilled ceilings to satisfy the need for indoor cooling. Radiant chilled ceilings consist of metal panels with hydronic tubing attached. Chilled water is circulated through the panels to produce radiant and convective cooling.² Although many European commercial buildings have limited ceiling space, these systems require only a minimum amount of room because small pipes, rather than large ducts, are used to transport cooling energy. Newer chilled panels use plastic pipes embedded in ceilings, walls, or floors.

Approximately 50 to 60 percent of the heat transfer from a radiant chilled panel is radiant, while 40 to 50 percent is convective. Chilled-water temperature must be above dew point — between 55°F and 60°F — to prevent condensation from forming on the bottom of the panels. Therefore, the driving force — or temperature difference — between chilled water and a room is reduced to 15°F to 20°F, while a conventional chilled-water system using 40°F to 45°F chilled water reaches temperature differences of 30°F to 35°F.

As a result, higher chilled-water flow rates are required to achieve reasonable capacities. With chilled-water temperature differences or delta-Ts of 4°F to 5°F, flow rates range from 4.5 to 6 gpm per ton. Conventional chilled-water systems have flow rates of 2 to 3 gpm per ton and chilled-water delta-Ts of 8°F to 12°F. Chilled-water flow rates for chilled panels and ceilings are approximately double those of conventional chilled-water systems (Figure 1).

FIGURE 1. Chilled-water flow rates for chilled panels and ceilings.

Even with higher flow rates, radiant-chilled panels and ceilings have relatively low capacities, ranging from 20 to 40 Btuh per square foot of panel area. While this is adequate for cooling loads of interior spaces, it may not be adequate for exterior spaces.

Exterior spaces with larger glass areas can approach 60 Btuh per square foot of floor area. For full sensible cooling in exterior spaces, the cooling load must be reduced or the ceiling panels supplemented with other cooling sources. Solar load can be reduced by using window-shading devices, such as interior blinds or exterior sun shades, that close when windows are exposed to direct sunlight. Additional cooling sources include chilled walls or floor panels.³

Because the chilled-water temperature supplied to a radiant chilled ceiling is above dew point, radiant chilled panels cannot provide latent cooling capacity. However, providing a 100-percent dedicated-outside-air system (DOAS) that can accomplish latent cooling allows the combination decoupled system to provide sensible and latent capacity.⁴

The biggest advantage to decoupling sensible and latent loads is substantial airflow reduction. A typical air-based cooling system will require 7 to 10 air changes per hour (ACH) of recirculated and outside air. A radiant-cooling system employing a DOAS will require 1 to 2 changes of outside air per hour only. This substantially reduces the horsepower and materials required to move cooling energy.

PASSIVE AND ACTIVE CHILLED BEAMS

Europeans discovered that, by lowering chilled panels below a ceiling, an individual panel's convection-cooling component could be increased. This satisfied the need for increased cooling loads caused by the expanded use of computers. In addition, higher cooling capacities were needed for exterior zones to deliver better overall comfort.

By lowering the panels, chilled-panel capacity can be increased to approximately 120 to 150 Btuh per square foot of the beam or beam coil area. Also known as a passive chilled beam, this configuration resembles a beam when mounted below a ceiling and is passive because natural convection is the convective-cooling component.