Churches can be difficult and costly to air condition. The large spaces, particularly overhead, use patterns ranging from packed congregations to small services, and window designs that may include large swaths of stained glass or no apertures at all present unique challenges. Additionally, the historic nature of many churches may preclude traditional venting and insulation aids. As in all air conditioning design, building size, weather conditions and interior air circulation patterns are critical factors in controlling costs and optimizing results.

Measure the length and width of the interior with a tape measure, other measuring device, or obtain from the building blueprints. Multiply the two numbers, and the sum will be your square footage. The U.S. Department of Energy's Energy Star website includes a handy chart to help you figure the number of BTUs your air conditioner will require along with providing a wealth of energy-saving tips.

Adjust your BTU requirements to make your estimate site specific. Add 10 percent in a particularly warm region. Subtract 10 percent in cool locales. Add 10 percent if you hold services that fill the building to capacity. Subtract 10 percent to well-shaded buildings. Add 20 percent to buildings with a substantial number of sealed windows in direct sun. Add 10 percent if such windows can be opened. Subtract 10 percent in buildings with a number of windows in shady areas that can be opened.

Heat rises, making pulpits, choir lofts and other raised areas a problem to cool, particularly given the exertions of singing, preaching and conducting services. The most cost-effective solution is to install silent fans in these places. This will avoid the need for dual thermostatic systems or the discomfort to the congregation or the choir that a single thermostat can produce.

Select a location for your air conditioner. For ease of maintenance and safety, it should be on the ground. This will also eliminate most concerns about noise. Be sure and pick a site at least 25 feet away from such pollution and odor sources as heavily trafficked roadways and kitchen vents.