Central air conditioning

Each part of a central air conditioner has one job, and together they work as a loop. The compressor, in the outdoor unit, pushes refrigerant, the fluid that moves the heat, around the system and raises its pressure. In the condenser, the outdoor coil, a fan blows outside air over the hot refrigerant, which gives up its heat and turns back into a liquid. The compressor works as a pump that moves refrigerant between the condenser and the indoor coil, called the evaporator, to chill your indoor air. Inside, the blower, a large fan, moves your home's air over the evaporator and sends the chilled air out. That indoor coil takes both heat and humidity out of the air.

Not every central system is split in two. Systems that put the evaporator, condenser and compressor in one cabinet are called packaged systems. An air conditioner with more than one factory-made assembly, such as an indoor and an outdoor unit, is commonly called a split system. The cycle that lets refrigerant carry heat outdoors has its own page if you want the full picture.

Source: ASHRAE · U.S. Department of Energy

A working air conditioner needs a steady flow of air across its indoor coil. Air conditioners are typically designed to move 350 to 400 cfm (cubic feet per minute) of air across the indoor coil for each ton of cooling. A ton of cooling means removing 12,000 Btu of heat per hour. That is why a clean filter matters so much.

Water is a normal part of cooling. Moisture forms on and near the cooling coil because the coil is colder than the dew point of the air around it. Your central air conditioner has a condensate drain to take that water away, and a plugged drain can cause water damage and affect your home's humidity. Water belongs in the drain, not on the floor.

Source: U.S. Department of Energy · U.S. EPA · ENERGY STAR

Short cycling means the system turns on and off rapidly instead of running steady cycles. Oversized units tend to short-cycle, which puts extra wear on the compressor and tends to shorten their lives. Oversized equipment does not run long enough to remove humidity from the air. A unit that is too big will cycle on and off often and remove moisture poorly, especially where the air is humid. Noisy equipment, rooms that cool unevenly, extra dust and frequent cycling may all point to a system that is not working efficiently.

A technician looks at more than size. A check-up includes the starting cycle, to make sure the system starts, runs and shuts off properly. Rules of thumb are too often used to size systems, and they lead to equipment far bigger than needed. If you are replacing a system, ask how its size was worked out.

Source: ENERGY STAR · U.S. EPA · U.S. Department of Energy

Ice on the indoor coil in summer is a sign worth taking seriously. Too little air across the coil can make it ice up. Very low airflow, below 200 cfm per ton, can lead to icing of the indoor coil and can shorten the equipment's life. A dirty coil cuts airflow, will eventually freeze over, and may damage the compressor. Too little refrigerant can also drop the coil's temperature low enough for ice to form, and that ice then blocks even more air.

The first airflow check is yours to do. Inspect, clean or change the air filter in your central air conditioner once a month. Make sure no filter is missing and that air cannot slip around it through gaps in the holder. A technician can confirm that the refrigerant charge and the airflow across the indoor coil match the maker's recommendation. If you see ice, tell us what you see and when it started.

Refrigerant work belongs to a certified technician. If your air conditioner leaks, ask the technician to find and repair the leak instead of just topping it off. Major leaks rarely develop in units that were properly installed and are regularly maintained. Recharging a system requires Section 608 certification, and refrigerant for home systems is only sold to certified technicians or the companies that employ them.

Source: U.S. Department of Energy · ENERGY STAR · U.S. EPA

Most water problems start where the condensed water is supposed to leave. Keep air conditioning drip pans clean and the drain lines clear and flowing. If pans do not drain, or are not cleaned often, water can get into the system and dirty the ducts. Drain pans should be checked routinely, because they can become a home for mold and bacteria if they are not installed and kept up properly. The condensate line should be clear and should drain to a safe place where it will not cause damage, mold or a slip hazard.

Pipes can drip too. Rust on a drain pipe shows that condensation forms on it, and such a pipe should be insulated. If you find water near your indoor unit, a technician can trace where it comes from.

Source: U.S. EPA · U.S. Department of Energy

When an air conditioner will not start, the simplest place to look is the thermostat. Check that its settings keep you comfortable while you are home. Frequent breaker trips, blown fuses or dimming lights are telltale signs of an electrical problem. Equipment can start up unexpectedly or release stored energy while it is being serviced, which can seriously injure the person working on it. That is why the panels on the outdoor unit are for a technician to open.

A yearly check-up covers the parts you cannot see. Have a contractor do a check-up before each cooling season. Air conditioners should be inspected in the spring. The technician tightens electrical connections and measures voltage and current on the motors, because faulty connections can make the system run unsafely and shorten the life of major parts. Dirty coils make the system run longer and cool less, which raises costs and shortens the equipment's life. Too much or too little refrigerant makes the system less efficient and shortens its life.

Keep the outdoor unit clean and clear of leaves and other debris. You can keep repairing an existing system, including replacing a major part such as a condensing unit or compressor. ENERGY STAR lists an air conditioner more than 10 years old as one sign it may be time to think about replacing it. The repair-or-replace page walks through that choice.

Source: ENERGY STAR · U.S. Consumer Product Safety Commission · OSHA · U.S. Department of Energy · U.S. EPA