When summer weather turns tropical, air conditioners kick in across the country and utility bills can soar. (About 75 percent of homes have either central systems or portables.) We expect to keep cool no matter how hot it is outside, and the newest high-efficiency units do the job economically.
But most homes don’t have the latest, greatest and most expensive cooling equipment. A 10- to 15-year-old central system may have a SEER (seasonal energy efficiency ratio) of 6 or 7 as opposed to modern machines with SEERs of 14 or more. It’s a big difference–and unlike many energy-saving scenarios that save only pennies, annual AC costs involve big bucks. For instance, an older unit with a SEER of 6 might have annual operating costs of $600 or more, while a new unit rated at SEER 14 would cost about $250, saving $350 every year.
But if an expensive upgrade isn’t in the cards, consider this two-pronged approach for increasing cooling efficiency. First, reduce heat buildup in the house so the AC works less. Second, tend to basic maintenance so the AC, however old, works at peak efficiency.
Reducing heat loads
The easiest way to limit heat buildup is to keep out the sun, for instance, by drawing blinds during the day. If some windows are sun struck most of the day, you might install insulated blinds with reflective foil backs. To preserve a view, you could mount awnings outside.
Also reduce heat buildup by creating a better buffer between conditioned living space and unconditioned space like an attic or crawl space. Attic temperatures can soar to 150 degrees or more when it’s in the 90s outdoors. Increasing the amount of insulation in the attic floor will help.
But ventilation is even more important, particularly where AC ducts are routed through the attic–even if they are wrapped with insulating blankets. Without adequate insulation and venting, attic heat working through the ceiling can account for 30 percent of total cooling costs.
The best vent configuration is a balanced combination of inlets and outlets that creates thorough exchanges of air. Typical, gable-end vents at each end of a peaked roof can foster good cross-ventilation but only along the top of the attic. Use them, but include intake vents along the roof overhangs (the soffits) and outlet or exhaust vents along the ridge to pull fresh air up, through and out of the space.
Installing only one kind of venting–for example, adding a continuos ridge vent to exhaust air–isn’t productive. If you increase outlet square footage, also increase the inlet area by about the same amount.
A study by the Agricultural Extension Service at Texas A&M University found that a ventilation rate of one full air change per minute of 95-degree air will lower peak attic temperature from about 160 degrees to about 100 degrees. But half an air change per minute, which is much easier to achieve, will lower the temperature to about 106 degrees.
Contractors often use complex formulas to determine vent requirements. But in warm climates, a basic rule of thumb is to provide about one square inch of total vent area per square foot of attic floor space.
Basic appliance maintenance
On any AC system, be sure to disconnect the power before you start work. Also check the manufacturer’s recommendations for cleaning and observe cautions to prevent electric shock.
General maintenance always includes washing or replacing the air filter. As it gradually clogs with dust and dirt the filter blocks cool air, which makes the compressor work overtime to reach the thermostat setting.
Also clean the coils of pipes (inside and outside) where temperature is picked up and discharged. Where the pipes are embedded in thin strips of metal, use a vacuum cleaner or a soft brush, working gently so you don’t bend the fragile metal fins.
You could blast exterior units with a hose, but compressed air is even better, makes less of a mess and avoids clogs that can build up near the base of the coils when you use water. (Be sure that the machine is thoroughly dry before you run it.)
In any case, spray and clean from the inside to force out dust the way it came into the machine. If you haven’t handled this kind of basic maintenance before, consider calling in a contractor and following along step by step so you can handle it without a service call in the future.
