Central air conditioners and HVAC filters

BY DENNIS CREECH and ABRAHAM KRUGER

Q. I need to replace my central air conditioner, but I don’t know what size to go with. One contractor says I need a 3-ton unit and the other says I should go with a 4-ton. What should I do? Is bigger better?

A. Bigger is definitely not better. Determining the correct size of residential heating and cooling equipment is critical to achieving comfortable interior conditions—temperature and humidity—and saving on initial and operating costs. When equipment is oversized, initial costs are higher, efficiency is reduced, energy costs increase and comfort may be compromised. One Florida study showed a typical 9 percent increase in annual space-cooling electricity usage for units oversized by 50 percent or more.

Correctly sized equipment is especially important in humid climates, where short-cycling of air-conditioning equipment can lead to poor humidity control. Similar to an automobile in stop-and-go traffic, the overall efficiency of an air conditioner, furnace or heat pump is reduced by excessive cycling. More starts and stops accelerate wear and tear on the equipment, which can lead to premature failure. Oversized systems also use more fan power for the blower and often exhibit more duct leakage due to higher operating duct pressures.

When replacing an HVAC system, have the contractor determine the correct size using a procedure that follows the Air Conditioning Contractors Association’s “Manual J”—the industry standard for sizing heating and cooling equipment. A “Manual J” calculation considers the size of your home; window size, type and orientation; and the level of insulation for the ceiling, walls and floor. Once the measurements are made for your home, it takes only a few minutes for the contractor to get a computerized calculation of the best size heating and cooling system for your home.

Q. How often should I change my HVAC filter?

A. The filter for your air conditioner, furnace or heat pump helps keep dust and other debris from sticking to the blower. A dirty filter can block air flow, which can increase operating costs and damage the equipment.

How often you change the filter depends on its type and how much dust is in the air. For a standard 1-inch-thick panel filter, the type commonly sold in grocery and hardware stores, every few months is usually adequate. While these filters are inexpensive to buy, they are not efficient at trapping the small dust particles common in homes. More efficient filters, such as a pleated product, will last longer and trap more dust.

Dennis Creech is the executive director and Abraham Kruger is a residential energy expert at Southface—Responsible Solutions for Environmental Living in Atlanta, www.southface.org.