Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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radiant floor heat

 

Table of Contents

Background Preparation

Introduction to Comfort
Main Comfort Factors
Other Comfort Factors

Units & Dimensions
Psychrometric Chart
Plotting On The Chart
Psychrometric Analysis
 

ASHRAE research and handbooks discuss the use of operative temperature for basis of establishing comfort. Operative temperature is the average of the MRT and dry bulb.

 

 

"As the heat transfer between the heated or cooled surfaces, the space and people in the space is mainly by radiation, it is important to use the operative temperature for specifying comfort conditions and for load calculations."

Source: Radiant heating and cooling by embedded water-based systems B.W. Olesen, PH.D.

  

Main Factors Which Influence Thermal Comfort
by Peyush Agarwal

There are six major factors that determine thermal comfort.

Environmental

Air temperature (dry bulb temperature or DBT)
Humidity (relative humidity RH)
Air movement (velocity v in m/s)
Radiation (Mean radiant temperature MRT)

Individual

Activity
Clothing

Air temperature (DBT)
The dry-bulb temperature is the temperature of the air around us (and is one of ) the important factors. The human body's primary response is towards the changes in (operative) temperatures (Dry Bulb + MRT) and it is this temperature that we attempt to keep within comfort conditions while designing structures for habitation. (see MRT below)

Humidity (RH%)
The effects of humidity (4938 bytes) Humidity of the atmosphere has little effect on thermal comfort sensation at or near- comfortable temperatures unless it is extremely low or extremely high. Of the various measures of humidity, the relative humidity (RH) is here the most immediately relevant, as it determines the possible evaporation rate. Moisture from the skin evaporates much more quickly in a dry than a humid atmosphere. At high temperatures, skin evaporation is the most important heat dissipation channel. Saturated air at 100% prevents any evaporative cooling.

Air movement (v)
 The air movement can produce different thermal effects at different air temperatures, in the following two ways:

  1. It increases convective heat loss, as long as the temperature of the moving air is less than the skin temperature. If this condition is not fulfilled, the air actually warms the skin.

  2. It accelerates evaporation, providing a physiological cooling. Its effect is insignificant at humidities lower than 30% , when there is an unrestricted evaporation even with still air, and humidities above 85%, when even air movement cannot help add vapour to the already highly saturated air. 

The effects of air movement (4424 bytes)

'Pleasant' ranges of air movements induce skin evaporation, more significantly in medium (40%-50%) humidities.

Thermal Radiation (MRT)
Next to air temperature, radiation has the greatest effect on thermal sensation. Radiation falling on the body surface activates the same sensory organs as the warmth of the air. Falling on an intervening surface, such as clothes, the radiant heat is converted to long-wave electromagnetic radiation causing sensible heat (molecular movement), which is then conducted through that material to the skin.

The above table represents combinations of Mean Radiant temperatures and dry-bulb temperatures that will give a thermal sensation of 21.11°C (70°F). MRT is the mean of thermal radiation readings from all material around us including walls, floors, other human bodies etc. Its the kind of radiation that comes from a heated stone. There can be a difference between MRT and the temperature of the air.

Activity (MET)
Generally speaking, the human body generates more heat when exerting physically in comparison to when at rest. A very important principle involved here is that of metabolic rate. The human body constantly produces heat, but at a varying rate. Metabolism is the term describing the biological processes within the body that lead to the production of heat.

Typical Metabolic Heat Generation for Various Activities
Activity Btu/(h.ft2) MET
Resting
Sleeping
Seated, quiet
13
18
0.7
1.0
Walking (on level surface)
2.9 ft/s (2 mph)
4.4 ft/s (3 mph)
5.9 ft/s (4 mph)
37
48
70
2.0
2.6
3.8
Office Activites
Writing
Typing
Filing, standing
Walking about
Lifting/ packing
18
20
26
31
39
1.0
1.1
1.4
1.7
2.1
Driving/ Flying
Car
Aircraft, routine
Aircraft, combat
Heavy vehicle
18-37
22
44
59
1.0-2.0
1.2
2.4
3.2
Miscellaneous Occupational Activities
Cooking
Housecleaning
Pick and Shovel work
29-37
37-44
74-88
1.6-2.0
2.0-2.4
4.0-4.8
Miscellaneous Leisure Activities
Dancing, social
Calisthenics/ exercise
Tennis, singles
Basketball
44-81
55-74
66-74
90-140
2.4-4.4
3.0-4.0
3.6-4.0
5.0-7.6
 

Clothing (CLO)
The individual can exert a considerable degree of control over most forms of heat exchanges between his body surface and the environment by choosing his/her clothes. Calculation of heat transmission through clothing would be extremely cumbersome, so the clo unit has been devised to simplify the handling of this insulating cover. This corresponds to an average U-value of 6.5 W/m² deg C over the whole of the body surface. Under still air conditions, when a person is engaged in a sedentary activity the variation of 1 clo would be compensated for by some 7 deg C temperature change. Under windy conditions, or if one is engaged in heavier work, the effect would be more pronounced.

Garment Insulation Value (CLO)

Description

Clo

Description

Clo

Underwear
Men's brief
Panties
Bra
T-shirt
Full slip
Long underwear top
Long underwear bottoms


0.04
0.03
0.01
0.08
0.16
0.20
0.15

Trousers and Coveralls
Walking shorts
Trousers
Sweatpants


0.15
0.24
0.30

Suit jackets and vests(lined)

Single breasted, thin-thick
Double breasted, thin-thick

0.36-0.44
0.42-0.48

Footwear

Sweaters

Ankle-high athletic socks
Calf-length socks
Panty hose
Sandals/thongs
Boots

0.02
0.03
0.02
0.02
0.10

Sleeveless thin-thick
Long-sleeve, thin-thick

0.13-0.22
0.25-0.36

Dresses and knee-length skirts

Skirt, thin-thick
Long-sleeve shirtdress, thin-thick
Sleeveless, scoopneck

0.14-0.23
0.33-0.47
0.23-0.27

Shirts and Blouses

Sleeveless, scoop-neck blouse

0.12

thin-thick

Short-sleeve, dress shirt
Long-sleeve, dress shirt
Long-sleeve, flannel shirt
Long-sleeve, sweat shirt

0.19
0.25
0.34
0.34

Sleepwear and Robes

Long-sleeve, long gown, thick
Long-sleeve pajamas, thick
Short-sleeve pajamas, thin

0.46
0.57
0.42


While the above factors are the most important, there are some subjective, largely non-quantifiable factors too, that an architect will benefit from the knowledge of, while in the design phase. Click to read about them.


 
 

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