Thermal ComfortWhat is meant by thermal comfort? It means that a person feels neither too cold nor too warm. Thermal comfort is important for health and well-being as well as productivity. A lack of thermal comfort causes stress among building occupants. When they are too warm, people can feel tired; when too cold, they will be restless and distracted.
As you may guess, thermal comfort has to do with more than the temperature. It can be achieved only when the air temperature, humidity and the movement of the air are in proper balance with each other. Adding to the complication, it is obvious that one person’s thermal comfort zone is not the same as another’s. Temperature preferences vary greatly among individuals, and it may not be possible to satisfy everyone in a group. Thermal comfort can also be a matter of perception – how do you feel when imagining a sun-soaked tropical beach, or a frozen lake?
Let’s start with temperature. There is no one ideal temperature for all times any more than there is one for all people. Generally, it is recommended that indoor temperatures be kept between 69 and 73 degrees F. The outdoor weather is a factor, however – when outdoor temperatures are hotter, it may be advisable to keep air-conditioned spaces warmer to minimize the temperature difference between indoors and outdoors; reducing the shock on people entering and leaving the building.
So-called radiant heat is also an important consideration. One will certainly feel warmer sitting in direct sun, than sitting in the shade at the same air temperature. Building occupants near sun-exposed windows will feel warmer in the same space than others further away.
“It’s not the heat, it’s the humidity!” This oft-repeated refrain can be quite true. Humidity is an important element of thermal comfort, and humidity control is a major (and energy-intensive) function of building HVAC systems. Excessive humidity makes occupants feel warmer, and the air seem “stuffy”. It is also unhealthful because it can lead to the development of bacterial and fungal growth.
Too little humidity causes discomfort by drying out people’s throats and sinuses, and contributing to skin rashes. Dry conditions also lead to the familiar electrostatic discharge when people touch each other or different surfaces. Depending on regional climate and building design, HVAC systems may be capable of either humidifying the air supplied to the building interior, dehumidifying it, or both.
Air movement is also important. A simple fan can provide a cooling breeze in warm weather; but the same breeze in a colder situation would be an uncomfortable and unhealthful “draft”. A small amount of air movement is always necessary, if only to ventilate a space – introducing fresh air and removing stale air. In fact, the power to drive the fans that move air around a building can be the largest job – energy-wise – of the HVAC system.
Fully climate-controlled buildings have energy-consuming equipment that will heat and cool the interior spaces as well as add or remove moisture from the air, as appropriate. Fans will move air for various purposes. Automatic shading systems can regulate solar radiant heat to maintain comfort, as well.
Other buildings may be much simpler. Consider a house with radiators to provide heat, and that is all. A radiator heats the air that is already in the room, and natural air currents are set in motion as warm air rises. Hot-air heating systems blow heated air into a space. Other than that, we may have windows and doors that open, shades to admit or exclude the sun’s rays, fans – built-in or portable -- and no “air conditioning”. In older homes and buildings without central heating systems, fireplaces and stoves were the sole source of warmth. Buildings in tropical climates may have no heating at all.
Green buildings may be designed to be responsive to regional climate, and use less active strategies to help provide an acceptable range of thermal comfort through the seasons. Passive solar heating will maximize the warming benefit of the sun in heating season. Passive cooling will use proper shading and natural ventilation to reduce indoor temperatures, and provide air movement to increase comfort. The timing of ventilation is important as well. Passively cooled buildings will be set up to maximize outdoor air intake at night, when it is cool. The “stack effect” set up by rising warm air, vented near the top of a space, will serve to draw in cooler air down low.
A good example of a simple but effective piece of equipment that helps with thermal comfort is the familiar ceiling fan. They are quite necessary in tropical climates, in places where air-conditioning is not normal, but also very useful in almost all other situations as well. In winter, especially in spaces with high ceilings (gymnasiums, certain factories, etc.) fans will keep the air well mixed, and keep more heat nearer the floor, where the occupants are located. Hot air rises, and without proper air circulation, most of the heat will end up near the ceiling, while occupants at floor level may be cold, and calling for even more heat. The fans increase comfort and save significant heating energy. In summer, ceiling fans can reduce the need for air-conditioning by providing simple cooling air movement.