Air Conditioning

The biggest sources of unwanted summer heat in homes are windows and walls (20 to 30 percent), internal gains from appliances and lights (15 to 25 percent), and through the roof (10 to 20 percent). In humid climates, damp outside air leaking into the house can also increase cooling load significantly.


Air conditioners are rated by their efficiency levels, Seasonal Energy Efficiency Rating (SEER - commonly pronounced SEAR). Ratings are shown on a yellow tag for room air conditioners and on fact sheets for central units. The SEER is the seasonal cooling output in Btus divided by the seasonal energy input in watt hours for an average U.S. climate. It takes into account the time the unit is not running. The higher the figure the better. A unit with a SEER of 12.0 costs half as much to operate as one with a SEER of 6.0. The higher initial cost of the higher SEER unit is normally paid back within a few years, making the more efficient equipment less expensive in the long run.

New residential central air conditioner standards went into effect on January 23, 2006. Air conditioners manufactured after January 26, 2006 must achieve a Seasonal Energy Efficiency Ratio (SEER) of 13 or higher. SEER 13 is 30% more efficient than the previous minimum SEER of 10. The standard applies only to appliances manufactured after January 23, 2006. Equipment with a rating less than SEER 13 manufactured before this date may still be sold and installed.

The ratings refer only to operating efficiency, or cost to operate, and have nothing to do with capacity, which is rated in Btus/hr. The Btus/hr figure indicates how much heat the air conditioner can remove from a room or house in an hour. Sometimes a tonnage figure is used instead of Btus/hr. One ton of air conditioning is the same as 12,000 Btus/hr.

Window Units Verse Central Unit

The buyer must make a basic decision - whether to use window units or a central system. Both have advantages.

The big plus for window units is that they allow for zoned cooling. This can save substantial amounts of electricity and money. Also, the actual purchase price of a window unit is less than that of a central unit; however, you will not have the convenience or comfort of whole-house cooling. The window units are also noisier because the compressor is in the unit within the living space. However, window units can be installed through a wall in an enclosed space and ducted to one or two rooms.

Central units provide whole-house air conditioning, which may be desirable if many rooms are used on a fairly constant basis, or it may simply be the choice of the homeowner. Central units, with their larger size and capacity, cost more to buy, install and run.

When replacing a condenser (outside unit), the evaporator coil (inside the house at the air handler) should also be replaced. If this is not done, the air conditioner will not have its high efficiency.


If you are installing or replacing a central unit, your contractor will perform the sizing calculations based on the size of the house, window exposure and orientation, construction materials, levels of insulation, air infiltration and lifestyle. In the past, it was standard practice to oversize the air conditioner by 10 percent to 50 percent. However, some researchers now believe that air conditioning systems undersized by 10 percent are more efficient and more effective in removing humidity. It is important not to oversize because such a unit, although it will cool the air, will not run for long enough periods to reduce the indoor humidity to a comfortable level. You may feel cool and clammy rather than cool and dry, a real comfort consideration in Missouri summers. Undersized air conditioning systems will also have lower first costs and longer equipment life due to less cycling.


Air conditioners remove moisture from the air by condensing water vapor as the air passes over cold coils. Water vapor condenses in the same way moisture from the air condenses on a glass of ice water on a hot, humid day.

Lowering the humidity makes you feel more comfortable, but it takes more energy, which reduces the efficiency of the air conditioner. One of the ways manufacturers have boosted air conditioner efficiency is by keeping the condenser coils somewhat warmer, which reduces condensation. Some of the new high-efficiency air conditioners do not dehumidify as effectively. Humidity can be reduced by including variable-speed or multi-speed blowers. Although there is no industry standard for rating the effectiveness of removing moisture, most literature does list water removal in pints per hour, which will help you compare one model to another. Some models have the fan speed controlled by a humidistat.

Placement and Maintenance

If possible, locate an outside compressor unit on the north side of the house. If that is not possible, try to position the compressor where it will be shaded as much as possible. Window units may not allow you the choice.

Outside compressors should be kept clean of leaves, twigs and grass cuttings so the compressor doesn't overheat. Mow grass so that cuttings are discharged away from the compressor unit, or brush or spray the cuttings off the compressor unit with a broom or a water hose.

Window units, if left in place during the winter, should be wrapped on the inside, and good weatherstripping should be used to block air infiltration around the unit. If wrapped on the outside, warm moist air from inside the home can condense and freeze inside the unit, possibly causing damage to the system.

For general seasonal maintenance, check the instruction manual. With central and window units, change the filters as often as once a month during the summer. Filters are inexpensive for what they give you - clean air, free of dust and pollen - and for what they do for the air conditioner - removing dirt or grit that wears out the moving parts prematurely, and producing a clear air flow for more efficient operation.

Operating Hints

When setting your thermostat, don't set the temperature colder than you want in hopes it will get cooler faster - it won't.

Experiment a bit to determine the highest temperature setting at which you can be comfortable. Try 78oF to start. Every degree higher will save about 4 percent in operating costs.

One way to be comfortable with higher settings is to run small fans that blow directly on your body.

If you have a whole-housing fan (attic fan), you can save substantially on your electric bill by using the fan at night when weather conditions permit - usually when the outside temperature falls below 78oF, and the humidity is not oppressive. But early in the morning, before the temperature begins to rise, turn off the fan and close the windows to capture the cool air. With this charge of cool air, the house can "coast" without the air conditioner until late morning or early afternoon. (The whole-house fan should be insulated over if it is not used during the air conditioning season.)