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A S H RA E J O U RN A L a s h r a e . or g F E B R U A R Y 2 0 1 52 8

TECHNICAL FEATURE | Fundamentals at Work

Kwang Woo Kim, Arch.D., is a professor of architecture at Seoul National University, Seoul, South Korea, and president of Architectural Institute of Korea. B jarne W. Olesen, Ph.D., isdirector, professor, International Centre for Indoor Environment and Energy, Technical University of Denmark in Lyngby, Denmark, and vice president of ASHRAE.

BY KWANG WOO KIM, ARCH.D., MEMBER ASH RAE; BJARNE W. OLESEN, PH.D., FELLOW ASHRAE

This two-part article describes basic knowledge of

radiant heating and cooling systems to give a principle

understanding of the design and operation of this

advantageous system including comfort, system load,

heating/cooling capacity, installation and application ofthe system with examples.

Embedded Radiant Heating and Cooling SystemEmbedded radiant systems are used in all types of

buildings. Due to the large surfaces needed for heat

transfer, the systems work with low water tempera-

ture for heating and high water temperature for

cooling. The water temperatures are operated at very

close to room temperature, and, depending on the

position of the piping, the system can take advan-

tage of the thermal storage capacity of the building

structure. Figure 1 shows the available types of embedded

hydronic radiant systems. The embedded radiant sys-

tems, except thermally active building systems (TABS),

are usually insulated from the main building structure

(floor, wall and ceiling), and the actual operation mode

(heating/cooling) of the systems depends on the heat

transfer between the water and the space.

This article was published in ASHRAE Journal, February 2015. Copyright 2015 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/ordistributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.

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F E B R U A R Y 2 0 1 5 a s h r a e . or g A S H RA E J O U RN A L 2 9

The radiant system is defined as a system where at

least 50% of the heat transfer takes place by radiation.

Figure 2 shows the total heat transfer coefficients between

a heated-cooled surface and a room. The radiant heat

transfer is, in all cases, 5.5 W/m²·K (0.97 Btu/h·ft²·°F). The convective heat transfer then varies between 0.5

and 5.5 W/m²·K (0.09 and 0.97 Btu/h·ft²·°F), depend-

ing on the surface type and on heating or cooling mode.

This shows that the radiant heat transfer varies between

50% and 90% of the total heat transfer. The heat trans-

fer coefficient for cold ceiling and warm floor will vary

between 9 and 11 W/m²·K (1.59 and 1.94 Btu/h·ft²·°F),

depending on the temperature difference between sur-

face and room.

The radiant heat transfer does not directly affect the

room air temperature. The long wave radiation heats or

cools the surrounding surfaces, which then indirectly

heats or cools the room air.

Standard for Radiant Heating and Cooling Systems As the heat transfer process between water and room

is quite different from conventional air systems, an

international standard on radiant heating and cooling

systems has been developed based on system design and

existing standards from different countries and was

published in 2012.ISO 11855, Building Environment Design—Design,

Dimensioning, Installation and Control of the Embedded Radiant

Heating And Cooling Systems,6–11

consists of six parts:

Part 1: Definition, symbols, and comfort criteria;

Part 2: Determination of the design and heating and

cooling capacity;

Part 3: Design and dimensioning;

Part 4: Dimensioning and calculation of the dynamic

heating and cooling capacity of thermo active building

systems;

Part 5: Installation; and

Part 6: Control.

ComfortOccupants’ thermal comfort is the primary objec-

tive in radiantly heated or cooled space. To provide an

acceptable thermal environment for the occupants, the

requirements for general thermal comfort shall be taken

into account by using the index of predicted mean vote

(PMV) or operative temperature, t o, and local thermal

comfort, e.g., surface temperature, vertical air tempera-

ture differences, radiant temperature asymmetry, draft,

etc.For radiant or convective systems the comfort require-

ments are the same when expressed by the PMV-PPD

index (–0.5 < PMV <+0.5) or expressed as an operative

temperature range corresponding to: 20°C to 24°C (68°F

to 75.2°F) for heating season and 23°C to 26°C (73.4°F

to 78.8°F) for cooling season in spaces with sedentary

activity.12,13

The operative temperature14,15

is the combined influ-

ence of air temperature and mean radiant temperature.

The operative temperature can be approximated with

FIGURE 1 Examples of water based radiant systems.3

Floor Ceiling Wall TABS

FIGURE 2 Heat transfer coefficients between heated/cooled surface and room.4,5

Btu/h·ft2·°F

2.03

1.85

1.671.50

1.32

1.14

0.97Heating

Cooling FloorCeiling

Wall

1.94

1.23

1.94

1.411.06

1.41

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the simple average of air and mean radiant temperature

in spaces with low air velocities (<0.2 m/s [39 fpm]), or

with a small difference between mean radiant tempera-

ture and air temperature (<4K, 7°F).

The operative temperature (t o) is in spaces with low air

velocities determined from the following expression:

t o = 0.5(t a + t r )

Where

t a

= air temperature °F (°C)

t r = mean radiant temperature °F (°C)

The occupants can maintain the same comfort level

with a lower air temperature in a radiantly heated

space, and the same comfort level with a higher air

temperature in a radiantly cooled space in comparison

to convective heating and cooling systems. Therefore,

reduction of the energy loss due to ventilation and infil-

tration is possible while maintaining the same comfort

level compared with conventional heating and cooling

systems.

As the reference temperature for the transmission heatloss is closer to the operative temperature than to the air

temperature, there will not be any significant difference

of transmission heat loss between radiantly heated or

cooled spaces.

Interestingly enough, the difference between air- and

mean radiant temperature is normally smaller in radi-

antly heated or cooled spaces. This is due to the fact

that in winter the windows will have a lower surface

temperature than the air temperature, which is com-

pensated by a higher surface temperature of the radiant

system and vice-versa in summer. With air systems the

colder window temperatures in winter will be com-

pensated by a higher air temperature, which will result

in an air temperature higher than the mean radiant

temperature.For rooms with sedentary and/or standing occupants,

the maximum permissible floor temperature for heating

is 29°C (84°F), and the minimum floor temperature for

cooling is 19°C (66°F). For spaces with occupants in bare

feet (bathrooms, swimming pools, dressing rooms, etc.),

the optimal floor temperature for comfort also depends

on the floor covering material.

For wall heating, a maximum surface temperature

range of 35°C to 45°C (95°F to 113°F) is recommended.

The maximum may depend on whether the occupants

FIGURE 3 Local thermal discomfort caused by vertical air temperature difference.6

0°F 3.6°F 7.2°F 10.8°F 14.4°F 18°F

80%60%

40%

20%

10%8%6%

4%

2%

1%

Y

X

X = Air Temperature Difference Between Head and Feet Y = Dissatisfied

0°F 9°F 18°F 27°F 36°F 45°F 54°F 63°FY

1 2

3 4

X

X = Radiant Temperature Asymmetry Y = Dissatisfied 1 = Warm Ceiling

2 = Cool Wall 3 = Cool Ceiling 4 = Warm Wall

FIGURE 4 Local thermal discomfort caused by radiant temperature asymmetry.6

80%60%

40%

20%

10%8%6%

4%

2%

1%

100

80

60

40

20

0

–1

–0.3

–0.2

–0.1

0.0

0.1

0.2

0.3

1.0

A c c e p t a b i l i t y

D i s s a t i s fi e d ( % )

20 30 40 50 60 70 80

Relative Humidity (%)

50.4°F

41.4°F

32.4°F

FIGURE 5 Human satisfaction with the IAQ depending on relative humidity and airtemperature.3

TECHNICAL FEATURE | Fundamentals a t Work

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Adverti sement formerly in this space.

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may easily get contact with the surface or whether occu-

pants are more sensitive persons such as children or the

elderly. Wall cooling is limited by the risk of condensa-tion and the development of a downdraft of cold air.

A vertical air temperature difference between head

and feet of less than 3K (5.4°F) is recommended. Most

heating and cooling systems will, in modern buildings,

normally have vertical air temperature differences

within this limit. In high ceiling spaces it is, for energy

reasons, important to avoid large vertical temperature

differences. This is why floor heating is especially rec-

ommended here (atrium, foyer, industrial space, etc.)

People are very sensitive to radiant temperature

asymmetry from a cold window and a warm ceiling.

Occupants may feel discomfort caused by a temperature

asymmetry of 5K (9°F) for warm ceiling, and a tempera-

ture asymmetry of 10K (18°F) for walls or windows ( Figure

4, Page 30). The critical factor at cold surfaces (windows,

walls) is, however, the risk of downdraft that may cause

discomfort.

The radiant heating and cooling system operates with

less dust transportation, as it is not a convective system,

and does not require the cleaning of heat emitters or

filters. With the radiant floor heating systems, carpetsare not necessary. Thus, the possible allergen sources of

emitting pollutants and a sink source can be eliminated.

The higher mean radiant temperature in radiantly

heated space means that the air temperature can be

kept lower than in convectively heated space. This has

the advantage that the relative humidity in winter may

be a little higher. Studies show that lower air tempera-

ture and lower air humidity have a significant effect

on perceived air quality ( Figure 5 3

). Due to the higher

heating surface temperatures, there is less chance

for condensation and mold growth. The relationship

between air temperature and humidity is one of impor-

tant comfort issues in radiantly cooled spaces. Wherethe humidity is not controlled by the air system, as in

naturally ventilated spaces, radiant cooling capacity will

be limited to avoid the forming of condensation on the

radiant surface (see section on control in Part 2 of this

article in next month’s Journal).

With air heating or cooling system more air has to

be circulated than the amount needed for providing

acceptable air quality. This may increase the noise level

in a space and also increase the risk for complaints

related to draft. When a part of sensible heating and/

or cooling load is taken care of by a water-based radi-

ant system, the ventilation system may have reduced

duct size and lower air velocity because it will only

treat the air renewal for required IAQ and, if needed,

dehumidification.

In buildings with thermally active building systems

(TABS) you will normally prefer to have free access to

the concrete surface to increase the heat transfer with

the room. This may require special solutions for the

acoustics. Acoustic panels on the ceilings and suspended

ceiling panels will reduce heat transfer. It will be moreefficient to hang down vertical acoustical panels.

16

The

application of the raised floor or the thermal/acoustic

insulation in floor will decrease the upper heat flow

from the TABS, which normally is much less than the

heat exchange from the ceiling.

Load Calculations and Heating/Cooling Capacity At a given average surface temperature and indoor

temperature (operative temperature, t o), a surface

will deliver the same amount of heat flux to a space

FIGURE 6 Embedded radiant system types.7

Type A and C Type B Type D Type G

1 = Floor Covering 2 = Weight Bearing and Thermal Diffusion Layer (Cement Screed, Anhydrite Screed, Asphalt Screed or Wood)

3 = Thermal Insulation 4 = Structural Base 5 = Heat Diffusion Device

1

2

3

4

1

2

3

4

5 Flooring Material

Joist

1

2

3

4


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Adverti sement formerly in this space.

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regardless of the embedded radiant system type.

Therefore, it is possible to establish basic formulas or

characteristic curves of heating and cooling for the all

heating and cooling surfaces independent of the embed-

ded system types. The heat transfer between the surface

and the space do, however, depend on the different sur-

face heat transfer coefficients ( Figure 2).

The heat transfer between the water and surface is differ-

ent for each system configuration. Therefore, the estima-

tion of heating/cooling capacity of systems is very important

for the proper system design. Two calculation methods

included in ISO 11855-27

are simplified calculation meth-

ods depending on the type of system, and finite element

method (FEM) or finite difference method (FDM). Given

system types are Types A and C, Type B, Types D and G.

The simplified calculation methods are specific for the

given system types within boundary conditions. Based

on the calculated average surface temperature at givenheat transfer medium temperature and space tempera-

ture, it is possible to determine the steady state heating

and cooling capacity. In case a simplified calculation

method is not applicable for the considered system,

either two- or three-dimensional finite element or finite

difference method, or laboratory testing may be applied.

The temperature distribution in floor cooling system,

calculated using FEM software, is shown in Figure 7.

Heat exchange coefficient is the parameter that

determines the amount of heat transferred between

surface and the space in relation with the system type.

Acceptable surface temperature is determined based on

comfort considerations and the risk of condensation.

Heating/cooling capacity of the systems is:7

Floor heating and ceiling cooling, q = 8.92 (t o − t S,m )1.1;

Wall heating and wall cooling, q = 8 (|t o − t S,m |);

Ceiling heating, q = 6 (|t o − t S,m |);

Floor cooling, q = 7 (|t o − t S,m |).

Where

t o (°C) is the operative temperature in the space

t S,m (°C) is the average surface temperature

The ceiling has the capacity up to 100 W/m² (31.7

Btu/h·ft²) for sensible cooling and 40 to 50 W/m² (12.7

to 15.9 Btu/h·ft²) for heating. The floor has the capac-

ity up to 100 W/m² (31.7 Btu/h·ft²) for heating and

40 W/m² (12.7 Btu/h·ft²) for sensible cooling. Whendirect sunlight strikes on the floor, the sensible cooling

capacity of the floor may be more than 100 W/m² (31.7

Btu/h·ft²). This is why floor cooling is often adopted in

spaces with large window area like airports, atria and

lobby halls.

For the thermally active building systems (TABS), the

steady-state heating/cooling capacity calculation is not

sufficient, and analysis with a dynamic computational

program that can predict the dynamic behavior and

performance of the system together with the building

FIGURE 7 Temperature distribution and cooling effect up and down for a floor system calculated by FEM software for a floor cooling system with 19°C (66.2°F) water tem-peratures and 26°C (78.8°F) room temperature.

Structure S4

Materialq = 0 q = 0

qup =112.6 Btu/h

qdown =22.3 Btu/h

Temperature (°F)

78.8

77

75.2

73.4

71.6

69.8

68

66.2

64.4

Floor Coveringλ = 0.08 Btu·in/h·ft2·°Fs = 0.6 in.

Screedλ = 0.40 Btu·in/h·ft2·°Fs = 2.4 in.

Thermal Insulationλ = 0.01 Btu·in/h·ft2·°Fs = 1.2 in.

Concreteλ = 0.70 Btu·in/h·ft2·°Fs = 7.1 in.


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is needed.17,1

Several programs exist such

Energy Plus, TRNSYS and IDA-ICE.

One of the main advantages of TABS are

reduced building height. For each story, you

may save 500 to 600 mm (1.8 to 2 ft) of build-

ing height, which for a seven-story build-

ing amounts to an entire story and related

building materials. As no suspended ceiling

is needed to cover air ducts, significant sav-

ing of building materials is possible. It is also

possible to operate the system at 30% to 50%

lowered peak loads allowing reduced plants’

sizes and possible operation of heating/cool-

ing systems with temperatures close to room

temperature, allowing increased plants’ effi-

ciency and use of renewable energy sources(ground heat exchanger, evaporative cool-

ing, etc.).

over 24 hours and by an air system normally over 8 to

10 hours. After the water circulation in the slab may

have been stopped during the day and will be started

again in the evening, there will be a high peak cool-

ing load between the heated slab and the cool water;

but this should not be used to size the chiller as it is a

very short peak and the capacity needed will after some

minutes decrease significantly. It can be somewhat com-

plicated to calculate the needed capacity on the water

side (chiller, heat pump); therefore, a dynamic building

simulation is recommended.

System DesignRadiant system design requires determining heat-

ing/cooling surface area, type, pipe size, pipe spacing,

supply temperature of the heat transfer medium, and

design medium flow rate. The design steps are as follows

(ISO 11855-38

):

1. Calculate the design heating and sensible cooling

load in accordance with a standard for heating and cool-

ing load calculation based on operative temperature.2. Determine the minimum supply air quantity

needed for ventilation and dehumidification. In cooling

application, calculate latent cooling and sensible cooling

available from supply air. Determine remaining sen-

sible cooling load to be satisfied by radiant system. Also,

designate or calculate the relative humidity and dew

point, because the cooling system should operate within

a surface temperature range above the dew point, which

shall be specified depending on the respective climate

conditions in the country. By limiting supply water

Thermally active building systems exploit the high

thermal inertia of the slab to perform peak shaving. The

peak shaving reduces the peak in the required cooling

power,7 so that it is possible to cool the structures of the

building during a period in which the occupants are

absent (during nighttime in office premises). This way,

the cooling can be delayed and lower nighttime electric-

ity rates can be used. At the same time, a reduction in

the size of heating/cooling system components (includ-

ing the chiller) is possible.

During daytime, the heat is extracted from the occu-

pied space by the ventilation system and stored in the

concrete slabs. Then, during nighttime, the level of ven-

tilation is reduced and the circulation of cool water in

the slabs will remove the stored heat.

For the conventional air system, the space load will

be the instantaneous system load, because all the heat

delivered to the space is immediately removed by the

air system. For the radiant system and especially fora TABS the calculated design space load should not be

used as system load. For both an air system and a TABS,

it is important that the room load over a 24-hour day

(Curve 1 in Figure 8) is removed within the 24 hours, else

the room will get warmer and warmer day by day if the

weather stays the same. The difference is that with a

TABS this load is removed from the space in three ways:

absorption in the concrete slab, removed by the ventila-

tion system and removed by the water circulating in the

slabs. Therefore, the load is removed by a slab system

C o o l i n g P o w e r ( B t u / h )

4

1

2

3

1 = Heat Gain 2 = Power Needed for Conditioning the Ventilation Air

3 = Power Needed on Water Side 4 = Peak Heat Gain Reduction

FIGURE 8 Example of peak-shaving (reducing the peak load) effect (time vs. cooling power).17

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temperature to be maintained above

dew point, the risk of condensation

can be easily avoided.

3. Determine the surface area for

radiant system, excluding any area

covered by objects immovable or

fixed to the building structure.

4. Establish a maximum permis-

sible surface temperature and a mini-

mum permissible surface tempera-

ture in consideration of the dew point.

5. Determine the design heat flux,

including the design heat flux of pe-

ripheral area and the design heat flux

of occupied area. For the design of the

cooling systems, determine the room with the maximum design heat flux.

6. Determine the radiant system

such as the pipe spacing and the cov-

ering type, and design heating and

cooling medium differential temper-

ature based on the maximum design

heat flux and the maximum and

minimum surface temperature from

the field of characteristic curves.

7. If the design heat flux cannot

be obtained by any pipe spacing

alternatives for the room of design, it

is recommended to provide supple-

mentary heating/cooling equipment.

In this case, the maximum design

heat flux for the embedded system

may now occur in another room.

8. Determine the thermal resis-

tance of backside insulating layer

and the design heating/cooling

medium flow rate.9. Estimate the total length of

circuit.

Hydronic radiant surface systems

are very often coupled with an air-

handling system. The air-handling

system usually operates only with

the amount of air needed for accept-

able indoor air quality, the required

IAQ standard, or amount of air

needed to remove latent heat from

TABLE 1 System design example for panel cooling.

STEP FIND EXAMPLE

0 A = 10 m2 (108 ft2), V = 30 m3 (1,059ft3) Room is Given to be Installed with

Radiant System and HVAC System1 Calculate Cooling Load Based on Opera-

tive TemperatureCooling Load (Latent)

Cooling Load (Sensible)

Qc,latent = 150 W (512 Btu/h)Qc,sensible = 1,000 W (3,416 Btu/h)

2 Determine Minimum Supply Air Quantity

Calculate Latent Cooling Available FromSupply Air

Sensible Cooling Available From SupplyAir

Design the Relative HumidityAnd Dew Point

Determine Remaining Sensible CoolingLoad to be Satisfied by Radiant System

V HVAC,min = 0.7 ACH = 21 CMH (12.4 CFM)

QHVAC,latent = Qc,latent = 150 W (512 Btu/h)

Assuming the SHF (Sensible Heat Factor) of HVAC, SHF =

(QHVAC,sensible / QHVAC,total ) = 0.7

QHVAC,total = QHVAC,latent / (1 - SHF)

= QHVAC,latent / 0.7 = 500 W (1,708 Btu/h)

Then, QHVAC,sensible = QHVAC,total – QHVAC,latent = 500 W – 150 W = 350 W

(1,196 Btu/h) is available from

supply air of HVAC

RH = 50 %, T dew = 14.8°C (58.6°F)

Remaining Qc,sensible = Qc,sensible - QHVAC,sensible

= 1,000W – 350 W

= 650 W (2,220 Btu/h)

3 Determine the Available Surface Area Aavailable = 5 m2 (53.8 ft2), 50% of ceiling area is availablefor radiant system

4 Establish a Minimum PermissibleSurface Temperature

T surf,min = 17°C (62.6°F) is acceptable for cooled ceiling* (which is higher than dew point temperature)

5 Determine Maximum Design Heat Flux Qc,max = 99 W/m2 (31.4 Btu/h·ft2) is allowed for cooledceiling*

6 Determine Radiant System

Pipe Spacing

Covering Type

Design Cooling Medium Differential

TemperatureDesign heating capacity of radiant

system,

Selected Radiant System has cooling capacity of 80 W/m2 (25.4 Btu/h·ft2).

mT = 0.2 m (8 in.)

punched aluminum sheet

T m = 2°C (3.6°F)

5 m2 = 400 W (1,366 Btu/h)

7 Select SupplementaryCooling Equipment

Required cooling capacity of SupplementaryCooling Equipment

Qout = Remaining Qc,sensible – Qdes

= 650 W – 400 W = 250 W (854 Btu/h)

8 Determine Thermal Resistance ofBackside Insulating Layer

Cooling Medium Flow Rate

R cover = 0.021 m2·K/W (0.12 h·ft2·°F/Btu)

m = 0.0478 kg/s (6.3 lb/min),

ensured of fully developed flow in pipe

If the resistance of backside insulation is high, the cooling

medium flow rate could be lowered.

9 Estimate Total Length Of Circuit Lcir = Aavailable / mT = 5 m2 / 0.2 m = 25 m (82 ft)

*ISO 11855-2: Building environment design – Design, dimensioning, installation and control of embedded radiantheating and cooling systems Part 2: Determination of the design heating and cooling capacity.


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the space and control air humidity level, while the hydronic

system supplies or removes the sensible heat depending on

the seasonal conditions. In the cooling mode, the air system

can play a key role in avoiding surface condensation.

Part 2 of this article will cover control, operation,

installation and application of the system.

Acknowledgments This article was supported by VELUX guest professorship, and a grant

from the National Research Foundation of Korea (NRF) funded by the

Korean government (MEST) (No. 2014-050381).

References1. Bean, R., Olesen, B.W., Kim, K. W. 2010. “History of Radiant

Heating & Cooling Systems, Part 1.” ASHRAE Journal (1):40–46.2. Bean, R., Olesen, B.W., Kim, K. W. 2010. “History of Radiant

Heating & Cooling Systems, Part 2.” ASHRAE Journal (2):50–55.

3. REHVA. 2007. “Guidebook No 7: Low Temperature Heating andHigh Temperature Cooling.”

4. Olesen, B.W. 1997. “Possibilities and limitations of radiant floorcooling.” ASHRAE Transactions 103(1):42–48.

5. Olesen, B.W., Michel, E., Bonnefoi, F., De Carli, M. 2000. “Heatexchange coefficient between floor surface and space by floor cool-ing: theory or a question of definition.” ASHRAE Transactions, Part I.

6. ISO 11855-1:2012, Building environment design - Design, dimension-

ing, installation and control of the embedded radiant heating and cooling

systems – Part 1: Definition, symbols, and comfort criteria.


ing, installation and control of the embedded radiant heating and cooling sys-

tems – Part 2: Determination of the design and heating and cooling capacity.

8. 8. ISO 11855-3:2012, Building environment design - Design, dimen-

sioning, installation and control of the embedded radiant heating and cooling

systems – Part 3: Design and dimensioning.9. ISO 11855-4:2012, Building environment design - Design, dimension-


systems – Part 4: Dimensioning and calculation of the dynamic heating and

cooling capacity of Thermo Active Building Systems (TABS).



systems – Part 5: Installation.



systems – Part 6: Control.

12. ASHRAE Standard 55-2010, Thermal Environmental Conditions for

Human Occupancy.

13. ISO EN 7730-2005, Moderate thermal environments—determination of

the PMV and PPD indices and specification of the conditions for thermal comfort.14. 2012 ASHRAE Handbook—HVAC Systems and Equipment.

15. ISO EN 7726-1998, Ergonomics of the thermal environment- Instru-

ments for measuring physical quantities.

16. Weitzmann, P., Pittarello, E., Olesen, B.W. 2008. “The coolingcapacity of the thermo active building system combined with acous-tic ceiling.” Presented at Nordic Symposium on Building Physics.

17. Olesen, B.W. 2012. “Thermo active building systems usingbuilding mass to heat and cool.” ASHRAE Journal 54(2):44–52.

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