Requirements for thermal comfort study material

1. Introduction – Thermal Comfort

Thermal comfort is an important aspect of the building design process, as the modern person spends most of the day indoors. Thermal comfort is defined as “a state of mind that expresses satisfaction with the thermal environment” (ASHRAE Standart 55-2004); the definition is easy to understand, but it is difficult to express it in physical parameters. There is extensive modeling and standardization for thermal comfort, which depends on both physical and physiological parameters as well as psychology. The thermal environment itself can be described as the properties of the environment that affect the exchange of heat between the human body and the environment. Research and practice in the field of thermal well-being is not a static field, on the contrary, since the emergence of air conditioning in the built environment, this field has expanded. One of the research highlights was the development of the PMV (Predicted Mean Vote) model by Fanger in the late 1960s (Fanger, 1970), which is used to assess indoor thermal comfort and forms the basis of current thermal comfort standards.

Thermal comfort is a very interdisciplinary field of study as it involves many aspects of different scientific disciplines: building science, physiology and psychology to name a few. This adds to the complexity of the matter. Advances in computing have led to an increased and improved ability to evaluate and model complex physical and physiological states. This meant not only more easily solving the non-linear equations on which the PMV model is based, but also performing complex performance simulations of buildings that were in the design phase to predict the comfort of future occupants.

2. Requirements

European standard EN 16 798-1 (with national translation STN EN 16 798-1) "Input parameters of the indoor environment for designing and evaluating the energy efficiency of buildings - addressing indoor air quality, thermal environment, lighting and acoustics" specifies how design criteria can be established and is used for sizing systems. It defines how to establish and define the main parameters to be used as input for building energy calculation and long-term assessment of the indoor environment. It also identifies parameters for monitoring and displaying the indoor environment as recommended in the directive on the energy performance of buildings (EN 16 798-1).

The purpose of the international standard EN ISO 7730 "Moderate thermal environment - Determination of PMV and PPD indices and specification of conditions for a thermal environment" is to present a method for predicting the thermal sensation and degree of discomfort (thermal dissatisfaction) of people exposed to a moderate thermal environment and to specify acceptable environmental conditions for comfort. It applies to healthy men and women and was originally based on studies of North American and European subjects, but also agrees well with recent studies of Japanese subjects and is expected to apply with good approximation to most parts of the world. It refers to people exposed to indoor environments where the goal is to achieve thermal comfort, or indoor environments where there are slight deviations from comfort.

European Technical Report CEN CR 1752 "Ventilation of buildings - Design criteria for indoor environments" covers criteria for indoor air quality, ventilation, thermal comfort and noise.

Critical issues such as adaptation, effect of increased air velocity, humidity, type of indoor pollution sources, etc. are still being debated, but in general these standards can be used worldwide. Nevertheless, it is important to consider people's clothing related to regional traditions and the season (Olesen, 2004).Criteria for an acceptable thermal climate are specified as requirements for general thermal comfort (PMV‐PPD index or operative temperature, air velocity, humidity) and for local thermal discomfort (draught, vertical air temperature differences, radiant temperature asymmetry, surface temperature of the floor). Such requirements can be found in standards and guidelines such as EN ISO 7730, CR 1752 and ASHRAE 55.

Table 1: Acceptable thermal environment for general comfort (ASHRAE Handbook, EN ISO 7730)

An equation in EN ISO 7730 calculates the PMV index based on the six factors clothing, activity, air and mean radiant temperature, air velocity and humidity. Even if a PMV value of 0 is obtained, there will still be at least 5% of the occupants who will be dissatisfied with the thermal environment (EN ISO 7730).

Table 2: Example design criteria for spaces in various types of building (EN ISO 7730)

Different labeling is used in the new CEN standard STN EN 16798-1. There are four categories indicated by numbers from I to IV. The categories I, II and III correspond to categories A, B and C in tables above. The category IV accounts for short deviations (for example exceeding the noise level when opening window for a short period of time, etc.) This category can be accepted only certain period of time. Moreover, unlike in previous standards, STN EN 16798-1 sets values also for residential buildings/spaces. The four categories are described in the following Table 4.

Table 3: Description of applicability of individual categories in STN EN 16798-1

Recommended values for the acceptable range of the indoor temperature for heating and cooling are based on a range for the PMV‐index. An example is shown in Table 5.

Table 4: Default design values of the indoor operative temperature STN EN 16798-1:2019

Indoor environmental parameters, which directly affect the heat balance of human body and are directly related to the thermal comfort:

• Insulation of clothing
  
• Metabolic rate
  
• Air temperature
  
• Mean radiant temperature
• Relative air velocity
• Relative humidity

No uniformity of the thermal environment (vertical air temperature differences, radiant temperature asymmetry, warm or cold floors and draft) may cause local thermal discomfort. The limits for vertical air difference and radiant temperature asymmetry corresponds to approximately 5% dissatisfied, while the limits for floor temperature correspond to 10% dissatisfied and for draft to 15% dissatisfied. The higher percentage of dissatisfied is allowed for draft, since air movement is most difficult to control in practice. The percentages of dissatisfied are not additive, as it may be the same people who are feeling discomfort due to draft, radiant asymmetry, air temperature differences, and floor temperatures (Ashrae, 1997). The people with higher activities are able to accept also higher values.

2.1. Clothing and metabolic rate

Fanger (1970) showed that the overall thermal sensation can be predicted by “the comfort equation” (PMV). Equation connects six variables that have a large influence on comfort: activity level (met) thermal resistance of the clothing (clo), air temperature, mean radiant temperature, relative air velocity, and water vapor pressure in the ambient air. Fanger’s work constitutes the basis for comfort standard EN ISO 7730 and ASHRAE 55.

Clothing, through its insulative properties, is an important modifier of body heat loss and comfort. Clothing unit represents the thermal resistance of the clothing (1 clothing unit = 1 clo = 0,155 m2K/W) and the activity level (metabolic rate) means the released heat from human body per unit skin area (1 metabolic unit = 1 met = 58,2 W/m2, an average person is 1,7 m2) (ISO 7730).

Figure 2.21 shows the predicted percentage of dissatisfied, based on this comfort equation, for sedentary office work and different types of clothing. A total acceptance (0% of dissatisfied) for a given room climate cannot be reached. EN ISO 7730 thus specifies three comfort categories A, B and C with a maximum permissible rate of dissatisfied of 6, 10 and 15%, respectively. For office spaces, the categories A and B are appropriate.

Figure 1 Predicted percentage of dissatisfied (PPD) for a range of room temperatures (operative temperatures) and different clothing (sedentary activity). Comfort ranges for winter/transitional period and summer. (EN ISO 7730)

Clothing or garment insulation is quantified in clo units. Value Icl is used to describe the insulation provided by clothing ensembles. The contribution of an individual garment to overall insulation is expressed in terms of its effective insulations – Iclu. Clothing worn by people indoors is modified to a great extent by the season and outdoor weather conditions. During the summer months, typical clothing in commercial establishments consist of lightweight dresses, lightweight trousers, short- or long-sleeved shirts and blouses, and occasionally a suit jacket or sweater. These ensembles have clothing insulation values (Icl) rating from 0,35 to 0,6 clo. During the winter season, people wear garments constructed of thicker, heavier (i.e., warmer) fabrics and often add more garment layers to an ensemble. A typical indoor winter ensemble would have an Icl value rating from 0,8 to 1,2 clo (Ashrae, 2010).

2.2. Metabolic rate

Humans produce heat from approximately 80 to 1000 W depending on the activity level.
For sedentary activity, as is usually used by administrative buildings, value of metabolic rate is 1.2 met = 126 W (ISO 7730). The heat generated by a person can be exchanged with the space in different ways: radiation to surrounding surfaces, convection to the ambient air, conduction, evaporation, respiration and excretion. Radiation has the highest heat transfer coefficient, followed by convection and conduction (ISO 7730). Metabolic rates for typical tasks are shown in table 6.

2.3. Operative temperature

There are two parameters that the operative temperature describes – air temperature and mean radiant temperature. The combined influence of these two temperatures is expressed as the operative temperature. For low air velocities (<0,2 m/s), or where the difference between mean. radiant temperature and air temperature is small (<4°C), the operative temperature can be approximated with the simple average of air and mean radiant temperature (ECBSC Annex 37).

This means that the air temperature and the mean radiant temperature are equally important for the level of comfort in a space (Babiak, 2007). Simmonds (1996) found that the traditional design criteria such as dry-bulb temperature and operative temperature were not always sufficient, while mean radiant temperature has an important influence on the comfort results.

Table 7: Optimum and acceptable ranges of operative temperature for people during light, primarily sedentary activity ( ≤ 1,2 met) at 50% relative humidity and mean air velocity ≤ 0,15 m/s (Ashrae, 1992)