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Air Conditioning - For Comfort

Why is it so hard to please everyone when it comes to temperature?
Why can't we agree about some thing a simple as temperature?

WIFE: Did you turn the thermostat down? It seems to be getting chilly in here.
HUSBAND: Yes; I thought it was too warm.
WIFE: Well, I guess I can put on a sweater. But it seems kind of foolish, in July.
HUSBAND: Turn it back up, then. I can stand it.

How often is a scenario like this repeated in modern air-conditioned homes? In an earlier generation, everyone was subject to the temperature imposed by summer weather, and about all one could do was talk about it. But in today's controlled environments, the ability to choose the temperature often changes this subject from one of idle discussion into one of active dispute.

When people enjoy the freedom to choose their temperature, why do they so often differ in their preference? There are physiological reasons for this, some based on obvious differences between one individual and another. These differences are so common that we even find the Bible referring to some of them.

For one thing, there is a big difference between men and women. Women burn only about 72 percent as many calories as do men. That is why a man can be comfortable in a room where his wife is cold.) His higher rate of metabolism keeps him warm.

Another important factor is age. Our metabolism slows down as we get older. At 70 years, a person is burning 30 percent fewer calories than he did at 35. So, producing less heat inside, older people need a warmer room.... Not to be overlooked is the effect of one's body weight. Compared with a slender individual, a person who is overweight generates more heat in relation to the area of skin by which he dissipates it. So we often notice that a fat man prefers the room cooler than does a thin one.....

However, the fact that one person's preferred temperature is different from another's does not mean they cannot both be comfortable in the same room. For everyone there is a range of temperature, above and below his ideal preference, in which he is comfortable.

Take the man and the woman in the opening scenario. He might choose 75 degrees Fahrenheit (24° C) as the ideal temperature, but he would be comfortable anywhere between 70 and 80 degrees F. (21° and 27° C). His wife might prefer the room at 80 degrees F., but would find tolerable any temperature between 76 and 84 degrees F. (24° and 29° C). So both of them would be comfortable in a room kept between 76 and 80 degrees F.

It is this overlapping of individual comfort ranges that provides the solution to the problem. Imagine the happy ending to the scenario if we add these lines:

WIFE: Let's split the difference. I'll set it in between.
HUSBAND: Good! Then we'll both be satisfied.
The problem is usually this simple when only a couple or a family is involved. But when we get into a larger group, it gets more complex. If a dozen persons share a business office, or several dozen are dining in a restaurant, there are that many more individual preferences to take into account. Or suppose a hundred or more persons are present in a theater or a meeting place. Does this larger number of differing choices as to the ideal temperature make it impossible to satisfy everyone? Fortunately not.

Sometimes the one responsible for controlling the thermostat may feel frustrated when, in spite of his most earnest efforts to keep the room comfortable, he still gets complaints that it is too cold or too hot. He may conclude that it is just not possible to please a large group, no matter what he does. But there is a solution-not a perfect solution every time, but a solution that reduces to a minimum the number who are uncomfortable.

Although individual preferences differ, in a large group the choices cluster around an average temperature that is the same whether there are a dozen, a hundred, or a thousand. How do we determine this optimum comfort temperature, that is, the temperature at which the greatest number are comfortable?

This question has been the subject of investigation by air-conditioning engineers, and the result of such studies is summed up in a "comfort chart," shown in the illustration. Here the temperature is measured along the horizontal scale, and the percentage of people who are comfortable at a given temperature is shown along the vertical scale. The optimum temperature is where the curve reaches a peak.

This shows that at 78 degrees F. (26° C), 97 percent of the people in a large group will be comfortable. Notice that a degree up or down from this ideal temperature does not greatly change the percentage of those satisfied. But two degrees up will cause 15 percent to report that they are too warm; likewise, two degrees down will make 15 percent-a different 15 percent-of the occupants too cold. A deviation of more than four degrees either up or down will make the majority of persons uncomfortably warm or cold.

This comfort guide applies whenever the outside temperature is such that refrigerated cooling is needed. But when the weather is cold enough that heat is required, a different standard applies. The comfort chart has a similar shape, but the temperature scale is shifted down six degrees. The optimum temperature is then 72 degrees F. (22° C).

The comfort chart shows that at the optimum setting, there are usually a few, maybe three out of a hundred, that are too warm or too cold. Those who are too warm would be better pleased by dropping the temperature a degree or two, but by so doing we would be making a greater number of persons uncomfortable on the cold side. The optimum is the best temperature, in the sense that it balances the discomfort between those few who feel cold and an equal number who feel overwarm. So while it may be true that "you can't please everyone," yet the goal should be to please as many as possible. And 97 out of 100 is nearly everyone.

By this standard, most air-conditioned buildings have been kept too cold. Such overuse adversely affects both comfort and health, and wastes energy. A California utility company recommends that the thermostat be kept at 78 degrees F., admonishing: "Remember: air conditioning is not to make you cold-just comfortable."


But the temperature is not the only thing that affects comfort. Another important factor is the humidity. Theoretically, this varies from zero for perfectly dry air to 100 percent for air saturated with water vapor. This affects the comfort of the human body through its influence on the natural cooling mechanism, evaporation of perspiration from the skin. This goes on most rapidly when the humidity is low and causes a rapid cooling rate. At high humidity, evaporation is hindered, and when it becomes slower than the rate of perspiration, drops of sweat form on the skin, a source of discomfort.

So the effect of humidity is to make us feel too cool if it is abnormally low and too warm when it is high. There is a "comfort zone" between 76 and 80 degrees F. in which the most agreeable humidity range is between 40 percent and 50 percent. If the humidity gets too high, as it might in a room full of people without fresh air, then the upper temperature is no longer comfortable. The comfort zone is squeezed down toward the 76-degree F. limit. Conversely, too little moisture in the air would make the lower level too cold, and we are more comfortable toward the 80-degree F. limit. To provide comfort for an audience it is useful to monitor the humidity. A hygrometer is an instrument that indicates changes in relative humidity. Many commercial hygrometers do not give accurate readings, but they will serve to signal abnormal conditions and indicate the need for a one- or two-degree adjustment. So the preferred temperature shifts with changes in humidity. At 50 percent humidity, 78 degrees F. is the best temperature setting. But at 70 percent, the thermostat should be set at 77 degrees F. (25° C); at 30 percent, 79 degrees F. (26° C) would be better. At extremes of humidity, either high or low, it can be uncomfortable regardless of the temperature.

Air Movement

Now, temperature and humidity are the most important factors affecting comfort. But these alone are not enough. If there is no air movement, even ideal conditions of temperature and humidity will not be enough to keep us comfortable. In perfectly quiet air, the layer of air closest to our bodies becomes warmer and more humid. Circulation of the air serves to remove this invisible blanket. This aids the natural cooling of the body and replaces used air with fresh air. Some air movement is essential, but too much becomes a draft. The comfort chart on the preceding page is set up on a basis of air moving at 20 feet (6 m) per minute. This is a very gentle movement, hardly perceptible. A higher velocity has the effect of increasing the evaporation of moisture from the skin, so that a person feels cooler. A velocity of 100 feet (30 m) per minute cools the body so much that the thermostat would have to be set one degree higher to compensate. A simple way to estimate air velocity is by timing the motion of soap bubbles. Many modern structures are built without windows, for greater economy in heating and air conditioning. In these, air circulation must be provided from a central fan through a duct system. In such systems, the fan usually can be operated independently of the heater or air conditioner. This can be used to provide needed circulation when the temperature is between 72 and 78 degrees F., that is, when neither heating nor air conditioning is needed.

Air Distribution

An ideal air-conditioning system would introduce air into an auditorium uniformly over the whole ceiling. As a practical matter, it has to be brought in through diffusers mounted over the outlets of a small number of ducts. If these are too few, and if the design of the outlets sends the air directly down upon the occupants, they will feel a draft. What can be done to alleviate drafts? The best answer is in the design and installation of the ductwork. However, modifications can be made in an existing installation to improve an unsatisfactory condition. The diffusers on the ceiling outlets might be changed to a style that channels the effluent air horizontally along the ceiling, instead of vertically over the seats. Or one outlet in a corner of the hall might be blanked off to provide an area relatively free of drafts, for the benefit of those who are particularly susceptible to them. If the circulating air system is inherently drafty, it would give relief if you did not operate it continuously. Turn it off from time to time.

Be Comfortable with Air Conditioning

There are other things that you may need to do to get the best results from your air-conditioning system. The thermostat may need to be adjusted or calibrated, or it may be poorly located, giving inaccurate readings and control. A large, slow-moving ceiling fan could be put in to distribute the air and mix it more uniformly. At times it could be used instead of the air conditioner. But even with the equipment you have, you can get the most comfort. How? Simply by maintaining the temperature at 78 degrees F., adjusting this, if necessary, for abnormal conditions of humidity, and providing adequate fresh air, with protection against drafts.

97% are comfortable at 78°
85% are comfortable between 76° and 80°
More than half are uncomfortable above 82° or below 74°

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