Strategies in Architecture: Radiant Cooling Systems

UMass BCT 597SD (2015) - “Solar Energy Systems and Building Design” -

Radiant Cooling Systems: Comfort Distribution Strategies in Architecture

By William A. Womeldorf, Department of Building

Construction Technology at Umass Amherst

Introduction

Mechanical distribution systems for heating and cooling are always evolving and reiterating to achieve a greater efficiency

and a superior form of comfort. The history of cooling in buildings is a relatively new attainment when compared to that of

heating. This paper will discuss in more detail on the current methods used and will tackle the question of radiant cooling

panels in buildings. These systems pose many caveats, especially in residential uses, but do the advantages of this

innovative system out way the disadvantages? More detail will be discussed on the main types of radiant cooling and

which situations where these should and should not be used as a cooling distribution system.

The Fundamentals of Cooling

To effectively understand cooling in a building, one must

comprehend the fundamentals of heat. Foremost, there

are two main forms of heat flow: Sensible heat and latent

heat. Sensible heat is form of energy that can be

measured with a thermometer. It is associated with a

change in temperature of a substance, material or space. [2]

Latent heat results from a release or storage of heat

associated with a change in the phase of the substance

(i.e. Gas to a liquid). Total heat flow is the sum of

sensible and latent flows. The moisture in humid air

holds energy as latent heat so that is why it may feel

hotter than the expressed temperature. Human comfort in

buildings requires careful attention to both of these heat

flows. The study of the psychometric chart explains the

relationship between temperature and humidity and

where maximum comfort is achieved. This is paramount

when designing a comfort distribution system in a

building.

The transfer of sensible heat has three cardinal methods:

Conduction, Convection and Radiation.

Conduction is the transfer of heat between substances

(solids mostly) when they are in direct relationship with

each other. [1]

Some materials conduct heat better, while

others like insulation slow the conduction of heat.

Conduction in a building typically occurs in a building’s

envelope. Heated conditioned air in the winter is

conducting through a wall assembly to the outside, while

in the summer conduction is moving heat into the

building from the outside by the same process.

Convection is the natural movement of heat through a gas

Figure 1 – A psychometric chart can be used to show humidity and

temperature and where ideal comfort can be found. Note that the

comfort zone is a range as humans experience comfort differently. [3]


or liquid by changes in density. As a gas or liquid is

heated, it warms, expands and rises because it is less

dense. [2]

Convection takes place in architecture as a

result of wind or pressure-driven air movement.

Radiation, the last of the three main methods of heat

transfer, occurs when electromagnetic waves travel

through space. This mostly occurs in a building by the

sun in which a space can be heated by direct radiation,

diffuse radiation or by reflected radiation. When these

electromagnetic waves hit an object, they transfer their

heat to that object by the form of radiation. [2]

These three

transfer methods, when understood, give the designer of

the building greater insights when designing a cooling

system in a building.

Air, water or both are the most common mediums used

as methods of transferring heat in a building. It is

important to understand the thermal properties of these

substances to best grasp which comfort delivery systems

are right for the building’s usage. Specific heat is the

amount of heat per unit of mass to raise the temperature

by one degree Celsius. [5]

Water has much higher specific

heat

content than air so less water is needed to store or

transfer heat. This is because water is much denser than

air allowing it to hold and channel more energy per unit

volume. The relationship is a big difference in which it

requires far greater amounts of air to transfer the same

amount of heat as water. That being said, there are some

obvious benefits of using air, like for ventilation and the

quick rapidity of comfort to a space.

“Is supplying chilled water to a room causing

the room to cool by delivering a process akin

to heating but with a “cooling” property?”

An understanding of the concepts of heat flow in a space

and heat sinks are needed to understand how certain

cooling distribution systems are better than others in

certain situations. Heat flows from a higher temperature

but not necessarily from an area of more heat to less heat.

Temperature, not heat content, determines heat flow in a

building. [4]

When studying how a room is heated it is

easy to take note on why water or air with higher heat

content is moved into the conditioned space to raise the

room’s temperature. However, it is much less intuitive to

understand the cooling process in a space. Is supplying

chilled water to a room causing the room to cool by

delivering a process akin to heating but with a “cooling”

property? The answer is no, because instead the chilled

water is absorbing the heat in the space by acting as a

heat sink. A heat sink is a term used to describe a

material or space used to store the heat exited from a

conditioned space. Sometimes the heat sink is in the form

of a cooling tower. Generally, when cooling is occurring

in a building, what is actually happening is the warm air

is being removed by an air conditioning mechanical

system and is being exited from the building in the form

of a heat sink process. In areas where it gets much cooler

Figure 2 – The three main ways heat transfers in a building is by conduction, convection and radiation (respectfully shown, left to right). [3]

Figure 3 – One cubic foot of water can store or

transfer the same amount of heat as over 3000

ft3of air. [4]


at night, the outside air can be used instead as a heat sink

to relieve some of the hot indoor air. A cooling delivery

system utilizes heat sinks to condition the air and provide

comfort for the building’s users.

Mean radiant temperature (or MRT) describes the

radiant environment for a point in space. [4]

It is an

important metric when comparing a radiant dominated

distribution system to other more direct systems. To

determine how much radiant heat or cooling a human in a

building will experience from a specific point the

designer should be aware of both the temperature and the

exposure angle of all objects in question. [4]

A toe-kick

heater produces a high amount of heat but because it has

a lower angle of exposure the body experiences a lower

MRT. Walking towards the toe-kick heater will result in

an increased MRT because the exposure angle would

increase. A different example is that of a window on a

cold winter day, in which the window produces a lower

mean radiant temperature causing the human to

experience what would feel like a cold draft, especially

once the human approaches it. This is important because

in regards towards radiant ceiling panels, the temperature

of the conditioned water can be lower (for warming) or

higher (for cooling) than that of a more direct source

because its large covering on the ceiling will result in a

higher exposure angle, resulting in an adjusted MRT.

Traditional Cooling Systems

There are many ways to remove heat from a room. These

cooling systems all use a refrigeration machine, heat

rejection equipment and a heat sink to move heat from a

building. The big difference between the cooling systems

is how heat is transferred from the conditioned space to

the refrigeration machine. There are all-air systems that

pumps the room’s heat into a refrigeration machine and

then rejects the waste heat to a heat sink (either a

evaporative condenser or a cooling tower. The main

advantages of air cooling systems are that they can vent,

filter and dehumidify the air in a room. However, their

disadvantages include the large fan power needed in the

air handler unit and the massive space allocated to ducts

for the distribution on the supply air and return air in a

building. Another method of cooling a space is that of the

all-water systems in which water is chilled by mechanical

equipment like an evaporator coil and then delivered to

fan-coil units in each space. An advantage with this

system is that the pump requires much less energy than

the fans. However, the drawback of this system is that

venting and filtering of the air is often tough especially if

a room does not have any windows. A compromise can

be obtained when combination air-water systems are

used. A radiant cooling panel with supplementary air is

one of these systems. It uses water as the primary heat

convector and then utilizes a supplementary air system

(like a heat recovery ventilator) to control the ventilation

and humidity in the air.

“The big difference between all the cooling

systems is the process on how heat is

transferred from the conditioned space to the

refrigeration machine.”

Radiant vs Conventional Cooling

With climate change becoming a larger issue in the built

environment, the efficiency and effectiveness of HVAC

systems are being held to increased scrutiny. At peak

demand (during hot summer days when cooling in

building is almost unanimously needed) the conventional

HVAC system uses about 37.5% of its power on tasks

like the fan and motor for air distribution.[6]

The reminder

of the loads is accounted for by the chiller and

compressor units. The Department of Energy has stated

that, at peak load, a building uses about 10% to 20% of

the supply air as outside air. [6]

It is important to note that

only a fraction of this supply air is needed to properly

ventilate a building and maintain excellent air quality

levels. This realization is causing a lot of research in

developing new innovative technology involving

hydronic radiant systems which require less circulated air

and less wasted energy from fans and motors. The

radiation system provides it’s cooling by using water as

Figure 4 – The mean radiant temperature (MRT) is a metric used

in determining observed comfort. It is a relationship between the

surface temperature and the angle of exposure. [4]


the transport medium for removing heat from the

conditioned space. As discussed earlier, water has a

much higher heat carrying capacity allowing for

increased efficiency over all-air systems. A study done

by the Lawrence Berkeley National Laboratory has

shown a 42.3% increase in efficiency over a conventional

HVAC system during peak power times. Also noted in

the article is that the separation of cooling and ventilation

tasks not only improves comfort conditions, it also

improves indoor air quality as well as control and zoning

of the system. [6]

“This realization is causing a lot of new

research in developing innovative technology

involving hydronic radiant systems which

require less circulated air and less wasted

energy from fans.”

Additional benefits can occur when the area of the panels

is large enough for the mean radiation temperature to

decrease enough to where moderately cool panels (about

65 degrees) will suffice. [5]

This saves additional energy

by allowing less energy-intensive cooling to occur to

achieve the same level of comfort in the conditioned

spaces. When combined with sustainable primary

systems like geo-exchange heat pumps the total

efficiency of the cooling system can skyrocket from the

use of semi-cooled ground water (around 58-60 degrees)

in a closed or open loop exchange system. In addition,

using heat pumps with high coefficient-of-performance

(COP) values, night cooling, economizers or some

variation of these will allow for even further diminishing

energy costs.

A possible disadvantage can occur by condensation and

even indoor rain from the ceiling. This occurs from the

temperature falling below the dew point. A way to

prevent this problem is by having indoor humidity

controls and monitoring devices to make sure the

humidity is above the dew-point temperature of the air.

This is why it is critical to use a supplementary air

system for ventilation and humidity controls for when

they are needed. A second noteworthy disadvantage

occurs when there are too many occupants in the space

than it was designed for. The sizing of the system is

critical because the more people that are in a space the

more heat that will be given off. If this is a rather sudden

event then the heat removal of the radiant cooling will be

not enough to properly condition a space. A solution to

this problem is to avoid using this cooling system for

auditoriums or gathering spaces and focus on utilizing it

for more predictable usage spaces like offices.

Hydronic radiant cooling has an additional benefit of

requiring less space for HVAC than conventional all-air

duct based systems. This saves in additional space for the

building’s main use which could be substantial when

considering ductwork consists of about 6% to 9% of the

building in a typical conventional HVAC system. [4]

In

addition, money can be saved from not investing in

costly duct and air handling systems. Radiant cooling

systems save energy when compared to the amount of

leakage that occurs with typical all-air systems.

Radiant cooling systems are not all the same. There are

three main types of radiant cooling panels: concrete core

system, suspended ceiling panel system and the capillary

tube system. [4]

Cost, retrofitting conditions, location,

climate and building usage should be considered when

selecting the ideal system.

Concrete Core Systems

This system consists of a series of hydronic tubes

(usually oxygen barrier PEX) inside of a concrete slab.

The concrete core system is the cooling version of the

radiant slab heating method. However, it is often used in

ceiling scenarios because of the natural direction of heat

flow. In this layout, the concrete slab can be useful for its

thermal storage capabilities in consonance with the water

flowing in the tubes. The time lag effect can provide

great benefits in both summer and winter if used

correctly. The total building costs with a radiant concrete

core system have been proven to be around equal to that

Figure 5 – A comparison done by Lawrence Berkeley National

Laboratory on conventional HVAC vs Radiant cooling HVAC

systems during electrical peak power demand. [6]


of a conventional building approach with a conventional

all-air HVAC system. Some of the other advantages with

using a concrete core system include [7]

:

- High energy efficiency

- Integrated into building structure (not visible)

- Low maintenance

- Can install large areas at low capital costs

- Silent operation

- Very long operational life

- Provides stable space comfort conditions

Disadvantages of a concrete core system include [7]

:

- Harder to model temperatures in design phase

- Requires detailed coordination between architect,

mechanical engineer and structural engineer.

(Requires an integrated design team)

- Severe fire or seismic event will render the

system inoperable, with extensive repairs needed

- Aesthetical issues of having exposed slab finish

It is important to note that this concrete core system

among other radiant cooling systems requires that the

cooling system be kept at the ceiling. There is a great

decrease in effectiveness when it is used for floor

applications. [7]

Suspended Ceiling Panel System

This method is the most common and is built primarily

by a suspended ceiling consisting of a highly conductive

material like aluminum, however, other materials are

increasing in popularity. Above the panels lay the metal

tubes connected to the rear of the panel. The highly

conductive material allows for a fast response time to

changes in the room usage loads. This system is used in

tandem with many others to produce a highly efficient

cooling load in the space. It is perhaps more aesthetically

pleasing than having a concrete slab as a ceiling finish.

Also, if there ever is a problem with one of the panels, it

is far easier to remove one of them and make

replacements as necessary. Costs are also around the

same as the typical VAV (variable-air-volume) systems

but with additional lifelong energy savings. [8]

The users

of the building also get the pleasure of enjoying the

increased comfort with the minimal noise compared to

that of an all-air system. The most valid reason to use this

system as opposed to the concrete core system is its

ability to retrofit into an existing space. This allows for

many more applications of this method for the

marketplace.

Capillary Tube System

The final system discussed involves using cooling grids

made of capillary tubes placed close to each other. This

system is done by embedding the tubes in plaster or

gypsum board. [4]

Usually the system is done with a

backer board (or rigid insulation) on the underside of a

ceiling joist. This is different than the concrete core

system because there is no thermal mass involved and the

Figure 6 – The Concrete Core System is a type of radiant cooling

that can double as a structural support while providing cooling to a

space. [6]

Figure 7 – A Suspended Ceiling Panel radiant cooling system

being used in an office setting [8]


tube system is closer to the interior finish side. This

system provides temperature distribution to be evenly

spread around the ceiling of a space. An advantage of

this over other all-air systems is that there is less of a

need to ensure that there is not blockage in the

convection loop because with an increased surface area

virtually all areas of the space will experience an equal

level of cooling throughout. In addition, due to the tube’s

flexibility, the cooling grids can be effective decision in

retrofit scenarios where ceiling height may not allow for

a suspended ceiling panel system. While some

applications of this system have been used to cover the

walls, it is not needed as most of the cooling effect will

occur on the ceiling. Another consideration when using

capillary tubes in lower areas like on the wall or floor is

that the cooling effect is often less desirable on the lower

side of the body than it is at the head. This logic is

opposite of the conventional wisdom used when radiant

floor heating is used (in which, heating from the floor

makes greater sense). Location for the radiant cooling

system increased effectiveness when it is not obstructed

by furniture items and can also cool from convection. [4]

Due to the massive surface area of the system,

conditioned water can be kept at a much higher

temperature than it would be for a direct all-water system

like that of a fan-coil. This utilizes less energy, allowing

for more sustainable, yet effective buildings.

Dedicated Outdoor Air System

As mentioned earlier, all radiant cooling systems operate

best when they are combined with outside air for

ventilation and humidity controls. These ventilation

systems are also required by international building code

to meet a requirement for supply quantities for

introduced outdoor air. When combined with a heat

recovery ventilator, conditioned temperature can be

preserved at a much high efficiency while still

maintaining ventilation to the space. A dedicated outdoor

air system (DOAS) consists of two parallel systems: A

dedicated outdoor air ventilation system that handles

latent loads (fresh air) and a parallel system that handles

sensible loads (dehumidifying). [11]

A dedicated outdoor

air system uses minimum energy because it is often not

conditioning the air nor is there a lot of duct work

associated with it. However, in applications on internally

dominated buildings it can be comparable to a VAV

system with ducts reaching into the inner spaces.

A heat recovery ventilator is used to preserve the

temperature of the room while still allowing for new

outdoor air to enter the space. This system can reserve in

the summer and winter months and is used when code

requires a space to be mechanically vented. This system

should be used with radiant cooling systems to provide

fresh air in the building.

Figure 8 – A Capillary Tube System being installed before the

finishing of the plaster. Notice how close the tubes are placed to

each other. [8]

Figure 9 – A Dedicated Outdoor Air System (DOAS) is often used

with a radiant cooling system for ventilation and improved air

quality. Pictured above is a heat recovery ventilator (HRV) which is

used to conserve the temperature of the building while allowing for

supply air to enter [12]


A Side-by-Side Comparison

Infosys in Hyderabad, India is the first commercial

radiant cooled building in the country. The building is

among others on larger campus and is known for being

50% more efficient than any other building at Infosys. [13]

The reason this is such a fascinating case study is the

building is split into two symmetric halves. While one

half is cooled by a conventional VAV all-air conditioning

system the other half utilizes a concrete core radiant

cooling system. The area, location, envelope design,

orientation, lighting and number of occupants remain

constant for each side of the building allowing for an

ideal demonstration of the efficiency of radiant cooling

over VAV air conditioning. The building was awarded

LEED India “Platinum” rating by the Indian Green

Building Council. [13]

The building is world renowned for

being a high performance green building.

ASHRAE Journal in 2014 published in a feature on VAV

vs Radiant how the radiant cooling system allowed

architects and engineers to achieve such a high efficiency

with the method they used. Radiant cooling data showed

after two years a 34% reduction in operational costs

when compared to the VAV building. Along with other

remarkable advantages of a better comfort delivery

system and the best part, it was actually less expensive to

build then the underperforming VAV system. [14]

The

building was outfitted with many measuring devices

allowing building scientists a field day of data to study.

The building also utilizes a DOAS (Dedicated outdoor air

system) on the side of the building with radiant cooling.

This was to ensure that fresh air and humidity were

controlled in the space to facilitate a better indoor

environment and prevent indoor rain from occurring on

the radiant system. The designers combined this with a

energy recovery wheel to prevent the conditioned

temperature from being lost from the space. Another

aspect to the interior design was ceiling fans were used to

facilitate the cooling effects from the radiant panels

which can create a convection loop.

Infosys decided to divide the building’s HVAC systems

in half because at the time in India a large scale radiant

cooling system was not used before and they wanted to

compare the results with that of a typical system. [14]

They

also wanted to hedge their decision in case it backfired in

a negative result.

This case study in Hyderabad, India has some

astonishing conclusions. Radiant cooling may not only

costs less to maintain, but more importantly it also

provides a greater level of comfort to the people in the

building. A potential downside could be if some disaster

inhibited the use of the radiant cooling in the slab

maintenance could reverse all pervious savings, but for

the location of the building that may prove unlikely. It is

vital that architects and engineers realize that the boiler

plate all-air systems are not the only option for providing

cooling to a space.

Discussion

This article is not for the promotion of radiant cooling. It

is an encouragement to designers of buildings to think

outside the box when it comes to decision on the HVAC

comfort delivery method. New innovative solutions can

elevate the efficiency and comfort of the building if the

option is on the table. Relying solely on traditional

methods when more advanced and effective methods

Figure 10 – The Infosys building featured above is split in two

halves. Left side uses a VAV all-air system while the right side uses

a radiant cooling system. [13]

Figure 11 – Annual electricity consumption of the VAV system and

the radiant cooling system. Note the 35% decrease in energy used. [14]


exist is not moving in the right direction of progress. It is

paramount that designers focus on the advancement of

mankind and the idea that humans can reverse the causes

of climate change. The major player in this reversal

remains in the efficiency of the HVAC systems being

used in architecture.

The following list depicts the advantages of radiant

cooling over the conventional HVAC systems.

- Increased heat transfer when switching from

an air to water based system because of the stark

differences in thermal properties

- Decreased energy demands from removing the

fans, motors and air handlers of an all-air system

and switching to a more efficient hydronic heat

pump

- Moderately cool water will suffice because a

cool surface will achieve thermal comfort when

lowering the MRT. This benefit becomes

amplified when used on the ceiling and large

unobstructed coverings. Often chilled water can

be around 65 degrees as opposed to a much

lower temperature with fan-coil systems. This

allows for the use of geo-exchange heat pumps

and other systems to run more effectively with

not much additional energy required.

- Increased comfort when using a radiant cooling

system because typical all-air systems will

provide a dry, less comforting air than that of

radiant. While all-water systems often provide

more effective cooling comfort.

- Decreased heat loss from using hydronic lines

as opposed to using ducts and air handlers that

are more prone to leakage.

- Decreased capital costs when compared with

all-air systems because of the decreased need for

ductwork, expensive HVAC handlers and other

gadgets that go along with having a full scale air

system in a building. In addition maintenance of

these systems is less than that of other systems.

With that being said, there still are disadvantages when

using any all-water system. Radiant cooling is not alone

for many of those restrictions. The following are some

disadvantages when using radiant cooling systems and

some instances where they would not be recommended.

The following list depicts the disadvantages of radiant

cooling over the conventional HVAC systems.

- Not recommended in hot, humid climates because of the high humidity in the air, these

systems will not perform well at removing the

massive latently stored heat.

- A supplementary air system is often required

especially in internally dominated buildings with

no operable windows the dangers of stale

unventilated air will not be solved by using a

water-based system. Supplementary air will need

to be provided; however, often these systems like

the DOAS can provide clean air for a fraction of

the energy of a VAV or all-air system.

- Humidity concerns and indoor rain can occur

if the temperature reaches the dew point in which

condensation can occur on the bottom of panel

radiant systems. For this reason it is appropriate

that the space have a humidity control and

monitoring system to ensure the occupants don’t

need a rain coat indoors.

- Coordination required between architects,

mechanical and structural engineers to ensure

that the system is properly built. This is

especially true when using a concrete core

system because the HVAC equipment doubles as

the structural floor system of the building as

well. An integrated design strategy will be

required in these applications.

- Loss of effectiveness when occupants load rapidly increase because of the gradual cooling

of the space when there is a sudden change in

loads the cooling system will need to take more

time to remove the heat of additional unplanned

occupants. This makes this system less useful for

unpredictable changes in room use.

In conculsion, the use of innovated radiant cooling

systems can provide an asset to the building when used in

accordance with its depicted advanatges and for the

betterment of comfort delivery in architecture.

Figure 12 – Radiant

cooling on the ceiling can

provide a decreased MRT

allowing for more energy

efficiency. [4]


References

1: Sadik Kakaç, and Yaman Yener. Convective Heat

Transfer. 3rd ed. Vol. 1. Boca Raton, FL: CRC, 2014.

Print.

2: "Heat Energy Flows in Buildings | Sustainability

Workshop." Sustainability Workshop. Autodesk, n.d.

Web. 04 Oct. 2015.

3: "Psychrometric Charts | Sustainability Workshop."

Sustainability Workshop. Autodesk, n.d. Web. 04 Oct.

2015.

4: Lechner, Norbert. Heating, Cooling, Lighting.

Sustainable Design Methods for Architects. 3rd ed.

Hoboken, NJ: John Wiley & Sons, 2009. Print.

5: "Specific Heat." Hyperphysics.phy-astr.gsu.edu. N.p.,

n.d. Web. 04 Oct. 2015.

6: Feustel, Helmut. "Re: Hydronic Radiant Cooling

Systems." Weblog comment. CBS Newsletter. Indoor

Environment Program, 1994. Web. 04 Oct. 2015.

7: McDonell, Geoff, LEED AP. "Selecting Radiant

Ceiling Cooling and Heating Systems." Plant

Engineering. N.p., 2007. Web. 04 Oct. 2015.

8: Drake, Lawrence. "Radiant Cooling." Energy Design

Resources. Radiant Panel Association, n.d. Web. 4 Oct.

2015.

9: "Energy Design: Thermal Comfort in the Home." The

Challenge Series. N.p., n.d. Web. 04 Oct. 2015.

10: "Radiant Cooling." Energy.gov. United States

Goverment, n.d. Web. 04 Oct. 2015.

11: "Dedicated Outdoor Air Systems." Wikipedia. N.p.,

n.d. Web. 04 Oct. 2015.

12: "Radiant Cooling Systems." Energy Efficiency

Emerging Technologies Database. Washington State

University: Extension Energy Program, n.d. Web. 4 Oct.

2015.

13: "News." Hyderabad Radiant Cooling Building.

Infosys, n.d. Web. 04 Oct. 2015.

14: Sastry, Guruprakash, and Peter Rumsey. "VAV vs

Radiant Side-by-Side Comparison." ASHRAE Journal.

ASHRAE, May 2014. Web. 04 Oct. 2015.

Strategies in Architecture: Radiant Cooling Systems

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