Thermal wellbeing in a climate controlled environment

Angel Sánchez de Vera Quintero. Cap del Departament de Serveis i Agricultura
Ahorro energético en el hogar

Analysing the level of environmental warmth we’re able to perceive in a closed space reduces itself to checking whether we’re comfortable or not. If there’s only one person in that space, the result requires no discussion; we can feel comfortable or uncomfortable and be 100% certain about it. But if more people live together in that space, different conditions mean that what some perceive as comfortable or uncomfortable others perceive as uncomfortable or comfortable. Under these determining factors, we’re going to analyse the thermal wellbeing of people in closed spaces which are moderated by climate control systems. 

People, as living beings, are similar to machines which take in fuel (food) and transform it into activity and heat (metabolism). It’s obvious that any food/energy which hasn’t been used up in doing activity/work must be dissipated/expelled to the exterior because our ability to accumulate/store energy is limited. The way of dissipating heat away from our bodies is in the form of sensible heat (through conduction, convection and radiation) and latent heat (by evaporation through respiration, transpiration through the skin, and sweat). In turn, the speed with which a person exchanges this heat with the exterior depends, fundamentally, on the temperature and humidity of that environment and on the level of insulation of the clothing being worn; sometimes, we might want to increase this insulation (winter) and at other times we might want to reduce it (summer) in order to achieve the best balance of heat exchange with the air that surrounds us. When the balance is neutral, we’ve achieved energy equilibrium, we’re comfortable and this will be our thermal wellbeing.

Explaining it practically, we experience it every day: For example, when a person gets hot through exertion or doing physical exercise, or when they are feverish, or eat a large meal (body parameters) and they counteract it, either by lightening their clothing and thereby reducing the insulation (clothing parameters) or by modifying the temperature and/or speed of the air that surrounds them (climate parameters). It’s specifically this final action - changing the climate parameters - that can make another person in the same climatized space uncomfortable because they are currently maintaining another energy balance specific to their metabolism and their level of clothing.

Why do people use climate control systems? To adjust the air that surrounds us in a closed space, making it warmer or cooler, and thus comfortably facilitate the thermal transfer between our body and the environment, to which we will give, or from which we will capture, heat depending on our activity and level of internal energy, reaching our thermal equilibrium without the need to sweat or feel cold.
 

Parameters of the human body. Production of metabolic energy through different activities

A first step has been to establish the quantity of heat generated by our bodies during a range of activities. Methods exist to estimate this and they have been brought together in standard UNE EN ISO 8996. (The fact that a UNE EN standard also has ISO certification indicates that its content has global acceptance).

The energy we release is expressed by the unit of measurement “met”, a unit based on the metabolism of a healthy person, sitting and not working. Given that this activity corresponds to an emission per unit area of 58.2 W/m2, the amount of energy that we emit to the exterior based on our activity, calculated in terms of sensible and latent heat and expressed in Watts, is scaled in the following table, assuming an average of 50% men and 50% women (a woman’s metabolism is 85% of a man's metabolism and that of a child is 75%).

Metabolic activity

sensible

latent

 

ACTIVITY

W

W

met

sleeping

50

25

0,76

lying down

55

30

0,86

sitting, without working

65

35

1,0

standing up, relaxed

75

55

1,3

strolling

75

70

1,5

walking

at 1,6 km/h

50

110

1,6

 

at 3,2 km/h

80

130

2,1

 

at 4,8 km/h

110

180

2,9

 

at 6,4 km/h

150

270

4,2

moderate dancing

90

160

2,5

working out in the gym (men)

210

315

5,0

team sports (men - average value)

290

430

6,9

working:

 

 

 

 

 

very light, seated

70

45

1,2

 

moderate (office; average value)

75

55

1,3

 

sedentary (restaurant, food included)

80

80

-

 

standing - light (light industry, shopping, etc.)

70

90

1,6

 

standing  - moderate (housework, shop assistant, etc.)

80

120

2,0

 

manual

80

140

2,1

 

light (in factories, men only)

110

185

2,8

 

heavy (in factories, men only)

170

255

4,0

 

very heavy(in factories, men only)

185

285

4,5

           
 

Source: UNE-EN ISO 8996 and Alberto Viti Corsi

 

The table shows not only the great variation in our heat emissions depending on the activity we’re doing at each moment, but also the significance that the amount of latent heat acquires with respect to the sensible heat in those different activities. Indeed, as well as the heat transfer function that blood performs at the superficial capillaries of our skin, our body is formed by a high percentage of water that also helps control our internal energy balance, migrating from our interior to the exterior and flowing and evaporating during respiration and through the skin during perspiration. In this way, the internal level of energy is regulated and any surplus is dissipated externally. 

The parameters in the table are very practical. Not only do they help us to understand and visualise what’s being expressed, but their quantification allows climate control systems to be designed which are appropriate to the power required for the activities which will be carried out in the location. It’s not the same to cool a cinema (where the power installed will need to be 100 W per spectator) and a gym (where the power will need to be 525 W per user), and all this regardless of the power required to overcome the thermal gains or losses through the doors and windows of those premises or through the ventilation air.

 

Clothing parameters. Thermal insulation of clothing

We are warm bodies, at a temperature of around 36.7 ºC depending on age, activity and even time of day. Maintaining that temperature makes us feel comfortable and we put on more clothes or fewer clothes depending not only on what we’re doing (activity) but also on where we find ourselves (ambient temperature).

The standard also tabulates a person’s levels of insulation depending on the clothing being worn. The unit of measurement here is the “clo” which defines the level of thermal insulation of clothing. The clo = 0 corresponds to a naked man and from there, by measuring the thermal resistance of the clothing in the laboratory, the clo for different combinations is calculated using the formula: 

Thermal resistance of the clothing = 0.155 x clo (m2 K/W)


In the following table, values of thermal insulation are established for clothing combinations, a more practical format for use in thermal wellbeing calculations.   

 

 

Type of clothing

Thermal insulation (clo)

Nude

0.0

In shorts

0.1

Tropical clothing: shorts, short-sleeved shirt and sandals

0.3

Light summer clothing: Light trousers, short-sleeved shirt, light socks and shoes

0.5

Work clothing.

0.7

Light winter clothing: Long-sleeved shirt, thick trousers, jersey, thick socks, shoes

1.0

Winter clothing

1.5

 

 

      Source: Standard UNE-EN ISO 9920 and Alberto Viti Corsi

 

This is another factor to be considered when determining the thermal wellbeing of an environment shared by several people. Although they might all do the same activity, the difference in the level of thermal insulation of their clothing may cause more discomfort for some than others at the same environmental temperature.

Let’s consider some practical examples. We experience this in summer in the office, where some people are wearing sandals or ballet shoes, without tights and with their arms or shoulders exposed to the air, whereas others are wearing shoes, socks, closed shirts and even jackets: some will want to turn up the temperature of the air conditioning system and others in the same space will want to turn it down. Or in winter, when some hospitals had to replace the thermostats which had been accessible in patient’s rooms (where the environment should be about 26°C for patients in bed with just a sheet) because when visitors arrived in winter clothing and coats, the first thing they did was complain about the heat and lower the temperature in the patient’s room.

 

Determining the most comfortable temperature

As we’ve seen, we’re all different depending on what we’ve eaten, what we’re doing at that particular moment and what we’re wearing. These three factors mean that our bodies have different requirements for the temperature of the environment which surrounds us in order to enable us to reach heat transfer equilibrium, giving out or taking in heat and thus achieving a comfortable temperature. 

As we said at the beginning, when it comes to choosing the temperature for an indoor climate control system, if there’s only one person, there’s no problem: they’ll either be comfortable or they won’t, and they’ll be 100% sure about it; so it just comes down to adjusting the thermostat to suit them.
The problem emerges when there are multiple users of one heating/cooling system. Whose requirements should be satisfied? What criteria should be used to decide on a single temperature for everyone? The logical solution would be to look for the greatest number of common points and satisfy this maximum number of people. This is what has been studied in detail and the methodology has been standardised in UNE-EN ISO 7730 Ergonomics of the Thermal Environment.

The analytical determination and interpretation of thermal wellbeing through the calculation of the PMV and PPD indices and the local criteria of thermal wellbeing. 

Specifically, the work contained in this standard is the result of having tested and collected the opinion of groups of people who were subjected, in a closed room, to variations in the thermal conditions of the environment. Their opinion on how they were feeling, on a scale of very hot, hot, comfortable, cold or very cold, generated what is known as the Predicted Mean Vote (PMV) which, in turn, has allowed the concept of the percentage of dissatisfied people to be established and the Predicted Percentage Dissatisfied (PPD) to be determined. 

These assessments allowed the development of an analytical calculation procedure that determines the percentage of dissatisfied people that would occur in a closed space based on variations in the parameters of the environment, the human body and clothing.

Although until now we have focused on two fundamental factors, the metabolism and the level of clothing insulation, because on the one hand they are the most important in determining the necessary conditions to be provided by the climate control system and, on the other hand, they are obviously subject to the discretion, degree of freedom or free will of the user of the facilities, the reality is that there are other influential parameters in the methodology for determining the thermal wellbeing of an environment, of which we can highlight not only the temperature of that environment but the relative humidity and the speed of the air that surrounds us.

To try to visualise this methodology practically, let’s analyse what would be the optimum temperature in an office (moderate work) in summer. To do this, we’ll select a metabolic activity which could be 1.0 met or 1.3 met and a clothing insulation level which could be 0.5 clo or 0.7 clo. 

We’re also going to set standard values for the climate control system: a humidity of 40% and a velocity of blown air of 0.1 m/s and we’re going to analyse the impact of the variation in the selected met and clo in determining the PPD through the application of the equations set out in the standard mentioned above.

 

Variation in PPD depending on the level of clothing insulation (clo)

In comparison with a clo of 0.5 that corresponds to a level of light summer clothing, what happens if someone is wearing something a little more “insulated”, for example a suit instead of a short-sleeved shirt, and has a clo of about 0.7?

The graph below shows the PPD at different temperatures as a function of the clo, with the other variables (metabolism, humidity and air velocity) remaining constant.

 

As show in the graph, a higher grade of clothing (higher clo) allows lower temperatures to be supported: for example, there’s a lower percentage of dissatisfaction at 22ºC, 5.7% of people with a clo of 0.7 are dissatisfied compared with 11.7% dissatisfied at this temperature with a lower grade of clothing (clo of 0.5). However, shoulders open to the air, for example, produces a higher level of satisfaction at higher temperatures and vice versa. As a result, many women have a shawl or cardigan in the office in summer to better deal with air conditioning temperatures set lower than 25ºC, while people wearing more clothes (jackets) start to feel warm above 24ºC.

In this specific example for this office, the temperature resulting in the lowest percentage of dissatisfaction would be 24ºC (people with activity of 1.3 met and dressed with a clo of between 0.5 and 0.7)

 

Variation in PPD depending on the variation in metabolism

But what happens if this office contains people doing different activities? Let’s consider someone who is working on the computer or doing a very light sitting job (met 1.1) compared with someone who moves around the office, participates in a meeting (met 1.3). Now let’s suppose that everyone is wearing clothing of the same clo, so that the metabolism is the only variable.

 

We can see from the figure that the degree of dissatisfaction at a lower temperature is very reduced if people “burn” 1.3 met in their activity, but it becomes an unsatisfactory environment, due to the feeling of cold, for those workers who barely use 1.1 met.

In this particular case for this office, the temperature that would provide the lowest percentage of dissatisfied people would be 25.7 °C (people with a clo of 0.5 and an activity between 1.1 and 1.3 met)

With all this, under cooling conditions, it can be deduced that a lower level of thermal insulation in the clothing or a more moderate activity means that the climate controlled temperature can be raised by a few degrees without risk of significantly increasing the PPD.

The analysis for heating is similar. In this case, an increase in clothing insulation will allow a reduction in the temperature of the premises without significantly affecting the PPD.

In view of all this, can a temperature be set for the climate control installations, either in the heat or in the cold? The Regulation of Thermal Installations in Buildings (RITE), in its revision of 2007, maintained the values of temperature and relative humidity established in previous versions as the internal design conditions:

 

Season

Operational temperature  ° C

Relative humidity %

Summer

23 a 25

45 - 60

Winter

21 a 23

40 - 50

 

 

But it introduced a clarification that these values would be for people with sedentary metabolic activity of 1.2 met, with a clothing grade of 0.5 clo in summer and 1 clo in winter and a PPD between 10% and 20%.

Speaking of thermal wellbeing, forgetting the climate control system and thinking only of our bodies and the need to achieve equilibrium, what forces us to change the temperature in a place between summer and winter, if we’re doing the same activity? The only difference is the clothing we’re wearing, which is not the same in summer as in winter as we have to go out into the street. Therefore, the RITE indicates that the installation moves in a range between 21ºC and 25ºC when the activity is 1.2 met and the clo varies between 0.5 and 1.

If this location didn’t also have to overcome the heat gains or losses due to its doors and windows and, being closed, require external air ventilation at street temperature, both of which cause work to be done in heating or cooling, the temperature of the room necessary for our thermal well-being would be a basic function of our met and our clo.

The clarification of the RITE 2007 was due to the fact that these values were considered to be applicable to all types of buildings and premises, while the reality is that we must differentiate between the different activities that might take place in them and act accordingly. For example, these cannot be the interior conditions of an indoor shopping centre in winter, where people come in wearing coats and are busily active, hurrying through it and carrying their purchases; or a heated pool, where the ambient temperature should be 2ºC above that of the water so that when wet we evaporate with less sensation of the cold. Or consider a gym where, according to nature of the sport, the comfortable temperature will be about 18ºC.

Ahorro energético en el hogar
Instalación solar térmica Hervidero de Cofrentes, Valencia