The 2nd-century Chinese mechanical engineer and inventor Ding Huan of the Han Dynasty invented a rotary fan for air conditioning, with seven wheels 3 m (10 ft) in diameter and manually powered by prisoners of the time.[6] In 747, Emperor Xuanzong (r. 712–762) of the Tang Dynasty (618–907) had the Cool Hall (Liang Dian 涼殿) built in the imperial palace, which the Tang Yulin describes as having water-powered fan wheels for air conditioning as well as rising jet streams of water from fountains. During the subsequent Song Dynasty (960–1279), written sources mentioned the air conditioning rotary fan as even more widely used.[7]
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Free cooling systems can have very high efficiencies, and are sometimes combined with seasonal thermal energy storage so that the cold of winter can be used for summer air conditioning. Common storage mediums are deep aquifers or a natural underground rock mass accessed via a cluster of small-diameter, heat-exchanger-equipped boreholes. Some systems with small storages are hybrids, using free cooling early in the cooling season, and later employing a heat pump to chill the circulation coming from the storage. The heat pump is added-in because the storage acts as a heat sink when the system is in cooling (as opposed to charging) mode, causing the temperature to gradually increase during the cooling season.

We’ve found that many homes and businesses in Atlanta face significant indoor air quality issues as a result of the seasonal high humidity in our area, which creates conditions ripe for mold and mildew growth in both old and new HVAC systems. Our HVAC technicians can help you determine which strategies will work best for neutralizing airborne particulates in your home, which often involves a combination of filtration methods and ultraviolet light technology that together can help keep your system perpetually free of contaminants. We can also install whole-house dehumidifiers and humidifiers, which can make a huge difference in the amount of work your HVAC unit will have to do to be effective.
In 1758, Benjamin Franklin and John Hadley, a chemistry professor at Cambridge University, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object. Franklin and Hadley confirmed that evaporation of highly volatile liquids (such as alcohol and ether) could be used to drive down the temperature of an object past the freezing point of water. They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to speed up the evaporation. They lowered the temperature of the thermometer bulb down to −14 °C (7 °F) while the ambient temperature was 18 °C (64 °F). Franklin noted that, soon after they passed the freezing point of water 0 °C (32 °F), a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about 6 mm (1⁄4 in) thick when they stopped the experiment upon reaching −14 °C (7 °F). Franklin concluded: "From this experiment one may see the possibility of freezing a man to death on a warm summer's day."[9]
The contactor (relay) and start/run capacitor(s) (see illustration below) fail most often and are inexpensive. So it’s a safe bet to buy and install those parts right away, especially if your air conditioning service unit is older than five years. The condenser fan motor can also fail, but it runs about $150 — hold off buying that unless you’re sure that’s the culprit.
An important component of natural ventilation is air change rate or air changes per hour: the hourly rate of ventilation divided by the volume of the space. For example, six air changes per hour means an amount of new air, equal to the volume of the space, is added every ten minutes. For human comfort, a minimum of four air changes per hour is typical, though warehouses might have only two. Too high of an air change rate may be uncomfortable, akin to a wind tunnel which have thousands of changes per hour. The highest air change rates are for crowded spaces, bars, night clubs, commercial kitchens at around 30 to 50 air changes per hour.[17]
Evaporative coolers, sometimes called "swamp coolers", do not have a compressor or condenser. Liquid water is evaporated on the cooling fins, releasing the vapor into the cooled area. Evaporating water absorbs a significant amount of heat, the latent heat of vaporisation, cooling the air. Humans and animals use the same mechanism to cool themselves by sweating.
Air ducts come in two forms: cool air ducts (also known as “return ducts”) and warm air ducts (also known as “supply ducts”). Warm air from the furnace enters the home through the warm air registers. No more than 20: of these warm air ducts and registers should ever be closed off – this hinders air flow and could result in the overheating of the heat exchanger, causing costly damage to you home heating system.
The chill of winter can be a scary thought when your furnace needs repair. When the furnace in your home goes on the fritz, you need it fixed right away to keep your family safe and comfortable. Every BGE HOME technician is licensed and adheres to a stringent in-house certification program that guarantees their knowledge, affirms their commitment to the customer, and ensures excellence in completing every furnace repair with the highest level of quality.