Staying Cool Efficiently

July 14, 2022 (Reston, Virginia)

It’s not unusual to have 40 or more days of 90°+ weather annually in the DC area. With rising electric rates, choosing an efficient central air conditioning (AC) system is a priority if you’re replacing an old system or choosing one for a new home.

There are three key measures for central AC:

• BTU, or British Thermal Unit, is a measure of how much heat an AC unit removes in one hour. It’s used to calculate what size air conditioner is needed.
• The Energy Efficient Ratio or EER, is determined by dividing a unit’s BTU rating by its wattage; the higher the EER, the more efficient the unit.
• The Seasonal Energy Efficiency Ratio or SEER, measures how well a unit will cool a home for an entire season; higher SEER = greater efficiency.

ENERGY STAR ratings are assigned to units that meet federal guidelines for energy efficiency. ENERGY STAR certified AC units use 8 percent less energy than conventional new models. However, energystar.gov cautions that central AC units also need blower motors that are usually part of the furnace; the only way to ensure a new central AC performs as rated is to replace the heating system at the same time—particularly if the furnace is 15 years old or older.

If you need cooling in a space without central AC (such as a room addition or a workshop above a garage), a heat pump/heat exchange unit (also known as a ductless mini-split), has a compressor and condenser outside the home that connects to one or more air handlers (evaporator and fan units) inside to heat and cool one or more rooms. While less efficient central AC, it’s more efficient and less obtrusive than a window unit.

System Design and Installation

Proper sizing and installation are extremely important; a poorly installed high-efficiency system can be as inefficient as an old system. Installation factors include:

• The size/cooling capacity of the system. A unit that’s too small will run constantly; one that’s too large will cycle on and off frequently, wearing the compressor.
• Adequate indoor space for system installation, maintenance, and repair.
• Properly sized ducts that are sealed and insulated to prevent loss of cool air.
• Enough supply registers to deliver cool air and return air registers to carry warm air back to the AC unit.
• Locating the condensing unit for unobstructed airflow.
• Locating thermostats on interior walls, away from drafts, windows, skylights, and internal heat sources.

Use and Maintenance

• Follow manufacturer recommendations for filter cleaning/replacement and professional servicing.
• Set the AC fan to “auto” mode. During periods of high humidity, temporarily set the fan on low to remove more moisture from the air.
• Trim foliage near the unit and keep the area around condenser free of dirt and debris.

Other Low-Cost Summer Comforts

• Block intense sunlight with awnings, solar shades/blinds, insulated drapes, or solar window film.
• Landscape for shade. Trees reduce surrounding air temperatures by up to 6° F. Deciduous trees planted south of a home provide maximum summer shade and let light in during colder months.
• Seal any air leaks around windows, doors, where the foundation meets brick or siding, in the attic, and AC ducts. For duct sealing, use mastic, metal tape or an aerosol sealant rather than duct tape, which doesn’t hold up.
• To disperse indoor heat when it’s cooler outside, run ceiling fans on the upper level and open windows on the lower level. For summer air circulation, set the fan blade rotation to force air straight down. Using a personal fan for work areas like a home office can keep you comfortable, while leaving the thermostat at an energy-saving temperature.
• If you need a new attic fan, consider a solar-powered model. Proper air circulation lowers attic temperatures and fights moisture.
• Use compact fluorescent light bulbs instead of incandescent; cut down on dryer use and indoor cooking or shift them to the evening when it’s cooler.
• Use cotton or humidity-fighting sheets, and pillows and mattress pads designed to be cooling.