Fans and Natural Cooling
Breezes and Fans Can Keep You Cool
Bird's eye view
Alternatives to central air conditioning
Central air conditioning is a fact of life in many U.S. homes, especially in the humid South and desert Southwest. But there are simpler and less expensive approaches to keeping a house cool. Alternatives include ceiling fans, natural ventilation and evaporative coolers. Insulation, window placement and reflective roofing materials also make an impact.
Choose the best fan available
The performance of poorly engineered equipment is likely to be disappointing, so choose high quality equipment. A house without air conditioning can afford to choose the best available fans.
The best available ceiling fan is the Gossamer Wind fan developed by engineers from the Florida Solar Energy Center.
The best available whole-house fan is the Tamarack HV1000.
To learn more, see "About Fans and Natural Ventilation" and "More About Fans and Natural Ventilation" below.
Be aware of prevailing winds, vegetation
In a hot climate, a house can be oriented to maximize exposure to prevailing winds. (In cold climates, on the other hand, it's usually better to shelter the house from prevailing winds.) Trees, shrubs, and outbuildings can be arranged to channel breezes toward the house. Casement windows should be arranged on either side of the house to capture the prevailing breeze and encourage cross ventilation. In most cases equally sized windows on windward and leeward sides work well, but when breezes are minimal smaller, low openings on the windward side and higher, large openings on the leeward side accelerate air flow.
Choosing windows, ceiling grilles
Natural ventilation: Unlike double-hung windows, which can be no more than half open, casement windows open fully, allowing more effective natural ventilation.
Evaporative coolers: To address the whole-house pressurization caused by an evaporative cooler, builders should install ceiling-mounted pressure-relief grilles to allow indoor air to be exhausted to the attic.
Control Solar Gain: Educating clients about daytime solar heating can go a long way toward reducing cooling loads. Shades or awnings should be used to reduce solar gain during the day. Electronic controls are available that open and close shades on a schedule or when temperatures rise above a pre-set limit.
Operable windows are required
The International Residential Code includes requirements for natural ventilation in Section R303. All habitable rooms need natural ventilation through windows, doors, or louvers, with a minimum openable area no less than 4% of the floor area being ventilated. Exceptions include rooms with adequate mechanical ventilation.
The International Residential Code includes requirements for evaporative coolers in Section M1413. Among the requirements: "Evaporative coolers shall be installed on a level platform or base not less than 3 inches above the adjoining ground and secured to prevent displacement."
Illustration: Code Check HVAC 2nd Edition. click to buy
ABOUT NATURAL & ALTERNATIVE COOLING
The principles of staying cool haven't changed
For thousands of years before residential air conditioning became popular people relied on a variety of strategies, including natural ventilation, to keep their homes cool.
These time-tested strategies still work. In temperate climates, they should be enough to handle all cooling and ventilation needs. In warmer regions, natural cooling can reduce reliance on refrigerated air conditioning equipment, which is expensive to install and operate.
Strategically placed windows capture prevailing winds and direct fresh air inside. Window selection also helps: casements make better wind scoops than double-hung or sliding windows.
In the evening, when it's cooler outside than inside, convection can expel warm air from vents or windows at the top of the house. Cooler air will be drawn in to replace it.
Using natural convection. A light monitor or a solar chimney creates a stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. that helps cool a house without electricity.
A solar chimney is a vertical shaft designed to encourage air flow. Solar chimneys usually have dark, heat-absorbing material on the inner wall and glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. on the south-facing outer wall; these features readily heat the air in the chimney, causing it to rise.
As air rises and escapes at the top of the chimney, fresh air is drawn in at lower levels.
Fans make us feel cooler. Fans move air without lowering its temperature, yet they can make us feel cooler. At night, whole-house fans can be used to flush hot air out of the house and replace it with cooler outdoor air.
Evaporative coolers save energy. Suitable for some, but not all, climates, they are less complicated than a full-blown air conditioning system. These alternatives to central air conditioning use evaporation to lower the temperature of ventilation air.
Fans are a low-cost cooling option
Fans run the gamut, from simple tabletop units to whole-house fans mounted in the attic. They use much less electricity than air conditioners, so operating costs, as well as maintenance, should be substantially lower.
Ceiling Fans. In most tropical countries, homes are more likely to use ceiling fans than air conditioners. Ceiling fans make a house feel cooler even if the windows are closed because moving air induces heat loss by convection and evaporative cooling.
A gentle breeze of 100 ft. per minute can extend the comfort range by 2° F to 4° F. Most people find air movement of 200 ft. per minute to be at the upper limit of comfort.
But air moving at that velocity can increase the comfortable temperature range by 10° F.
Whole-house fans. A whole-house fan installed in the attic speeds up the effects of natural convection. These high-volume fans pull hot air out of the house, which is replaced by cool outside air that comes in through open windows.
Whole-house fans are ineffective during the day when outdoor temperatures are higher than indoor temperatures. But nighttime cooling may be enough to let the house coast through the hottest part of the day.
The best whole-house fans come with gasketed, insulated shutters, such as the Tamarack HV1000 whole-house fan. Its motorized shutters are insulated to R-22.
For more information on whole-house fans, see Fans in the Attic: Do They Help or Do They Hurt?
ABOUT EVAPORATIVE COOLERS
Swamp coolers not appropriate in all climates
Evaporative coolers, also known as swamp coolers, use a fan to blow outside air over pads that have been soaked with water, causing the water to evaporate and the air to drop in temperature. As long as they are installed in a location with a dry climate (in the U.S., that generally means west of the Rocky Mountains), they will use far less electricity to cool a house than central air conditioning.
Air can be routed into a central location in the house so several rooms can benefit, or into ductwork that distributes it around the house.
Low-tech but effective. These coolers are sized by the volume of air they move, typically between 3,000 and 25,000 cubic feet per minute. As cool air enters the house, warm air is expelled through open windows or through ducts venting into the attic.
Temperature and humidity can be regulated by opening and closing windows: simple and effective.
Suited to dry climates. If outdoor air is dry, an evaporative cooler can lower the temperature of incoming air between 15 F° and 40 F°.
Evaporative coolers add moisture to the air, a plus in arid regions. In regions where relative humidity is high, evaporative coolers can't cool the air effectively because the air is too damp for further evaporation.
Houses in climates where the outdoor wet-bulb temperature exceeds 70° F for no more than 1% of the cooling season can be comfortably cooled with an evaporative cooler. That area includes most of the Western U.S. Exceptions include regions that regularly experience several weeks of hot, humid weather.
As outdoor relative humidity (RH) drops, the wet-bulb temperature drops with respect to the dry-bulb temperature. For example, if the outdoor temperature is 90° F at 10% RH, the wet-bulb temperature is only 58° F.
Three types to choose from. Direct evaporative coolers — so-called swamp coolers — have been around for years. Two new types also are available: indirect and two-stage units.
Indirect coolers use the evaporation process to lower the temperature of a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.. Outdoor air passing through the heat exchanger falls in temperature but doesn't pick up any additional moisture.
The best known indirect evaporative cooler is the Coolerado.
Two-stage evaporative coolers first cool the air in an indirect evaporative cooler. After that, the air passes through a conventional direct evaporative cooler, further lowering the temperature of the air while raising its moisture content.
The best known two-stage evaporative cooler is the OASys.
- Don Mannes/Fine Homebuilding
- Chuck Lockhart/Fine Homebuilding
Mar 23, 2010 7:43 AM ET
Oct 5, 2009 7:29 AM ET