Radiant Ceiling Cooling

buildings with radiant ceiling cooling systems, also known

as “chilled beam” systems, incorporate pipes in the ceilings

through which chilled water flows.

Figure:1

The pipes lie close

to the ceiling surfaces or in panels, and they cool the room via

natural convection and radiation heat transfer (Figure 1).

Although the technology has existed for more than 50 years,

it has had problems in the past.Condensation of moisture on the cooled surfaces sometimes damaged ceiling materials (e.g., plaster) and created conditions favorable to biological

growth.

As noted by Mumma,1 current

systems usually require

dedicated outdoor air systems

(DOAS) and tight building envelopes

to manage humidity.

Most commercial buildings

avoid condensation on the

chilled panels by using a separate system to maintain the dew

point of the indoor air below the panel temperature. Ventilation

makeup air is the predominant source of peak humidity

load in most buildings. Consequently, humidity loads can be

handled separately from the chilled ceiling by dehumidifying

the makeup air before it enters the space (with enough extra

humidity removal to address internal moisture sources).

Mumma2 reports that with a good base dewpoint control, the

chilled panels can manage temporary increases in local moisture

loads without condensation formation.

A radiant ceiling cooling system delivers sensible cooling

directly to spaces, which de-couples maximum air delivery

from the cooling load. Radiation and natural-convection heat

transfer each account for about half of the approximately 50

Btu/ft2 (150 W/m2) cooling capacity of passive radiant ceiling

panels.3,4 At these heat transfer rates, radiant ceiling panels can

meet peak sensible loads with about one-third of the ceiling

area covered by cooled panels (for a cooling load of 16

Btu/h · ft2 [50 W/m2]). Active chilled beam units that use recirculated

room airflow induced by the ventilation makeup air

supply can supply up to 79 Btu/h · ft2 (250 W/m2). Each unit

can be controlled separately, which simplifies zoning.5

Energy Savings Potential

Radiant ceiling cooling reduces HVAC energy consumption

in several ways. In space cooling mode, energy savings

accrue from delivering higher chilled water temperatures (Tcw)

to the radiant ceiling panels to meet sensible loads, e.g., from

Tcw=50°F6 to 61°F7 (10°C to 16°C) compared to 40°F to 45°F

(4°C to 7°C) for conventional systems. This, in turn, allows the

chiller evaporator temperature to rise and improves cycle efficiency.

Radiant ceilings also reduce the heat dissipated by

ventilation fans within the

conditioned space (discussed

later) and the outdoor air (OA)

volumes that require cooling.

Radiation heat transfer directly

cools the occupants,

which may allow slightly

higher building air temperatures,

decreasing building

cooling loads. Radiant ceilings

used with a DOAS, however,

generally preclude

economizer operation, as

most of these systems do not

include additional ventilation capacity. Overall, radiant ceilings

reduce cooling energy by 15% to 20%.8

The combination of radiant ceiling with a DOAS also reduces

air moving power by moving only the air required for

ventilation (typically 25% to 30% of the airflow rate required

for peak cooling loads in an all-air system). If the ducts of this

DOAS design are matched to this reduced, but constant, flow

requirement, blower power does not decrease at periods of low

load, as in the case with VAV. However, a DOAS can meet

ANSI/ASHRAE Standard 62 ventilation requirements with less

ventilation airflow due to its inherent precision in delivering

required ventilation flows in the aggregate and to individual

zones in the building. An analysis comparing the energy consumption

of a conventional VAV system with a radiant ceiling

with DOAS found that, for a small office building in a Mid-

Atlantic state, the radiant ceiling with DOAS could realize

annual blower-power savings on the order of 25%, with greater

savings in warmer climates.8

In space heating mode, the DOAS saves energy by reducing

the ventilation airflow due to its inherent precision in delivering

required ventilation flows. Simulations show that OA typically

accounts for 50% to 60% of the space heating load. The DOAS

enables approximately a 20% reduction in OA volume, which

decreases space-heating energy consumption by roughly 10%.8

Taken together, these results generally agree with the building

simulations by Stetiu,4 who estimated HVAC savings in cold,

moist areas to be 17% to 42%, and an average savings of 30% in

warm, dry areas. Mumma1 reported similar energy savings (a

23% decrease in HVAC energy expenses) for an office building

in Philadelphia. On a national basis, radiant ceilings, used in

combination with a DOAS, could reduce commercial building

HVAC energy consumption by about 0.6 quads relative to VAV

systems. Relative to a DOAS with a sensible-only VAV system,

radiant ceilings realize more modest savings of about 0.2 quads.

Market Factors

In new construction, the installed costs of radiant ceiling

with a DOAS with enthalpy recovery appear to be similar to

conventional VAV systems. However, this depends on using

other system components: if the system requires separate radiant

heating systems, the radiant ceiling costs substantially

more than an all-air system. For new buildings, Mumma1 posits

that a radiant ceiling with a DOAS (with sensible and enthalpy

transfer devices) costs less to construct than a VAV-based

system. One chilled-beam manufacturer quoted a system price

of 2% more than a VAV system, with large cost reductions for

ducts and fan equipment.9 This parallels the findings of

Springer.6 It is not completely clear, however, if cooling panels

would cost less than a sensible-only VAV plus DOAS (as

advocated by Coad10). The reduced space required by radiant

ceilings (for mechanical equipment and ductwork) translates

into an effective cost reduction by increasing the amount of

usable space.11

Other issues besides first cost appear to impede greater use of

radiant ceilings. Many HVAC system designers and contractors

are unfamiliar with the radiant ceiling approach and often believe

it costs more than other systems. The installation of a radiant

ceiling also has architectural implications, necessitating

early communication on a project between architects and HVAC

system designers. Past problems involving condensation (and

resulting moisture) due to higher infiltration levels in older

buildings and untreated OA also inhibit present use of radiant

ceiling cooling.

References

1. Mumma, S.A. 2001. “Ceiling panel cooling systems.” ASHRAE

Journal 11:28-32. http://www.doas.psu.edu/papers.html.

2. Mumma, S.A. 2001. “Dedicated outdoor air in parallel with chilled

ceiling system.” Engineered Systems 11:56-66. http://www.doas.psu.edu/

papers.html.

3. Frenger Cooling. 2001. Multi Service Chilled Beams Product

Information. http://www.buildingdesign.co.uk/mech/frenger2/frenger2.htm

4. Stetiu, C. 1997. “Radiant cooling in U.S. office buildings: towards

eliminating the perception of climate-imposed barriers.” Doctoral Dissertation,

Energy and Resources Group, University of California at

Berkeley, May. http://epb1.lbl.gov/EPB/thermal/dissertation.html .

5. Dedanco. 2001. Active Chilled Beam Documentation/FAQ.

http://www.dadanco.com.au .

6. Feustel, H. 2001. IES International Energy Studies, Inc. “Hydronic

Radiant Cooling.”

6. Springer, D. 2001. Personal Communication, Davis Energy Group.

8. TIAX. 2002. “Energy consumption characteristics of commercial

building HVAC systems — Volume III: energy savings potential.” Final

Report to US Department of Energy, Office of Building Technologies,

July. http://www.tiax.biz/aboutus/pdfs/HVAC3-FinalReport.pdf .

9. Petrovic, V.M. 2001. Personal Communication, Dedanco.

10. Coad, W.J. 1999. “Conditioning ventilation air for improved performance

and air quality.” Heating Piping and Air Conditioning 9:49-56.

11. Energy Design Resources. 2001. Design Brief: “Radiant Cooling”

Final Draft, prepared by Financial Times Energy, Inc.

John Dieckmann is a principal at the HVAC and Refrigeration

Technology sector of TIAX, Cambridge, Mass. Kurt W. Roth,

Ph.D., is a senior technologist with TIAX. James Brodrick, Ph.D.,

is a project manager, Building Technologies Program, U.S.

Department of Energy, Washington, D.C.

By John Dieckmann, Member ASHRAE, Kurt W. Roth, Ph.D., Associate Member ASHRAE, and

James Brodrick, Ph.D., Member ASHRAE