What is Thermal Comfort?

According to ASHRAE standard 55 thermal comfort is defined as a “condition of mind that expresses satisfaction with the thermal environment”. Simply speaking, it means that a person is neither too hot nor too cold. It is not easy to define thermal comfort distinctly, because of both physiologically and psychologically variation from one person to the other. Thermal sensitivity may vary depending on age, gender, dress, activity, habits, and many other factors. For example, in the cold room, one occupant that is doing some physical activity may feel too hot, while the other occupant sitting still may feel too cold. Nevertheless, there are some universal rules that can be used to describe thermal comfort.

Significance of thermal comfort

Thermal comfort is important for many reasons. One of them is of course the health of the occupants. But equally important is the well-being of occupants which determines how focused and effective we are at work, do we feel tired, distracted, or even stressed. Surprisingly, low thermal comfort in the shop may decide if the customers are willing to buy the products or not.

Factors influencing thermal comfort

There are six main factors that influence thermal comfort. We can split them into personal factors – related to the characteristic of each occupant, and environmental factors – related to conditions in the given environment.

The former factors are metabolic rate and clothing insulation, the latter are air temperature, radiant temperature, airspeed, and humidity. To make things more complicated we should remember that all these factors can vary in time, therefore during the calculation of thermal comfort parameters, the steady-state conditions are assumed.

Metabolic rate

Metabolic rate is defined as “the rate of transformation of chemical energy into heat and mechanical work by metabolic activities within an organism, usually expressed in terms of unit area of the total body surface.” (ASHRAE 55 Standard). In other words, it means that when consuming food, our body changes the chemical energy into heat necessary to keep our body warm and delivers energy to perform any kind of work. Of course, the metabolic rate strongly depends on the activity level and environmental conditions. A person doing some exercises transforms chemical energy much faster than a person laying on the bed.

Metabolic rate is expressed in met units, where 1 met = 58.2W/m2, which equals the energy produced per unit area of an average person seated at rest. The scale varies from 0 to 8 mets, depending on the activity level. A few examples of activities are given in Table 1.

Table 1. Metabolic rates for typical tasks. Source: ASHRAE Standard 55-2010

Metabolic rates are difficult to estimate for activities with values higher than 2-3 because they can be performed in many different ways. Therefore, there are several different methods that help to evaluate these values. One of them takes into account the rate of respiratory oxygen consumption and carbon dioxide production. Others are based on the heart rate.  Moreover. Food and drink habits influence metabolic rates, by affecting thermal processes in our body.

Clothing insulation

The amount of clothes directly influences the heat transfer between the body and surrounding air. It is expressed in clo units, where 1 = 0.155 m2 K/W. The definition takes into account heat transfer from the whole body, also not covered parts like head and hands. Clothing insulation is controlled directly by a person.

Air temperature

The air temperature represents the average temperature of the surrounding air taking into account the spatial average from the ankle, waist, and head levels – it varies for seating or standing occupants. It is measured with a dry-bulb thermometer.

Radiant temperature / Mean radiant temperature

Simply speaking, it is the temperature of the surfaces surrounding the occupant including solar radiation. It depends on the property of the material to absorb and emit heat – emissivity.

Airspeed

The airspeed is the velocity of the air that an occupant is exposed to. It is measured in m/s and it does not depend on the direction of the incoming air. The faster the air flows around the person, the higher the heat exchange is (the occupant feels colder).

Relative humidity

The relative humidity is defined as the ratio of the water vapor in the air to the maximum level of water vapor that air can hold at a given temperature and pressure. The higher the relative humidity, the more difficult it is to lose heat through the evaporation of sweat.

Controlling thermal comfort

Since thermal comfort depends on so many factors, there are also many ways of controlling it. The most efficient and basic one seems nowadays the environmental monitoring and control systems in the building, like heating, cooling, ventilation systems. Of course, at the same time, it is the most advanced and expensive method, which requires not only investment but also maintenance cost.

Another very simple but effective method is to adapt or change clothing whenever it is possible or necessary. Such a simple solution works well in a very hot climate. Similarly, flexible working hours can help avoid very uncomfortable conditions during the day.

Care should be taken in order to provide people a comfortable environment of work. On one hand, people should be separated from devices that may decrease thermal comfort like heat or cold sources. But at the same time, the equipment improving comfort, like desk fans, should be provided. Another example often used in tropical countries are ceiling fans used where air conditioning cannot be applied due to different reasons.

Predicting thermal comfort

There are many techniques for thermal comfort prediction, but standard EN ISO 7730 expresses thermal comfort in terms of:

• Predicted Mean Vote (PMV)
• Percentage People Dissatisfied (PPD)

PMV and PPD are models developed by Professor Ole Fanger at Kansas State University and Technical University of Denmark. The models are based on heat-balance equations and empirical studies to find out if people feel comfortable in different conditions.

The equations behind Predicted Mean Vote relate comfort sensation with:

• air temperature,
• radiant temperature,
• relative humidity,
• airspeed,
• metabolic rate,
• and clothing insulation.

PMV equals zero to represent thermal neutrality.

Since PMV predicts thermal sensation, it still does not tell us whether people feel comfortable or not. For that purpose, the Predicted Percentage of Dissatisfied (PPD) equation was developed to relate thermal sensation evaluated by PMV and occupant’s satisfaction. It was possible to relate both models by studies on people in a room with controlled indoor conditions.

Experience shows that the accuracy of the PMV/PPD model is low and can be estimated at the level of ~34%. Moreover, the accuracy strongly depends on ventilation strategies, building types, and climates. That is why, where non-uniform conditions exist, and the designers have to deal with complex environments, much more accurate predictions of thermal comfort can be achieved by Computational Fluid Dynamics (CFD) analysis.

Numerical simulations in SimFlow can easily be used for the evaluation of temperature, velocity, and comfort profiles for any given geometry, under specified conditions. Three-dimensional visualization of the flow helps to understand how a given ventilation system works, leading to better optimization of the entire system.

This article was based on the following sources:

1. ANSI/ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy
2. https://en.wikipedia.org/wiki/Thermal_comfort