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Air Conditioning

The Refrigeration Cycle

The compressor compresses a low-pressure refrigerant vapor to a high-pressure refrigerant vapor. The compressor is located outside the home on residential systems and is part of the condensing unit. The high-pressure vapor passes through the condenser coil where it gives off heat to the atmosphere. The condenser coil usually surrounds the compressor. A fan blows air on the condenser coil to assist the heat transfer process. Residential systems use an air-cooled condenser coil. Some commercial systems use water to cool the condenser coil (figure #1).

Once the refrigerant leaves the condenser, it is now a high-pressure liquid. It passes through an expansion device and becomes a low pressure liquid. Now the refrigerant goes to the evaporator coil, in the air-handling unit (AHU). The AHU is usually located in the attic or basement (figure #2). Sometimes the AHU also has an oil or gas burner built into it so it can be used for heating as well. A fan in the AHU pushes warm air from the conditioned space past the evaporator coil, the air gets cooler and the refrigerant evaporates to a low-pressure vapor. Hence the name evaporator coil. The cool air travels to the conditioned space through ductwork, or directly into the space in the case of a through-wall or window air conditioning unit (figure #3).

The low-pressure refrigerant vapor returns to the compressor and the cycle starts again.

What's a Ton of Air Conditioning?

Before the era of mechanical refrigeration, ice was used for cooling. A "ton" of mechanical refrigeration equals the amount of heat a ton (2000 lbs) of ice will absorb in one hour.

144 BTU/Day x 2000 lbs = 288,000 BTU/Day / 24 Hrs = 12,000 BTU/Hr

12,000 BTU/Hr equals one "Ton" of refrigeration.

How Many Tons of Air Conditioning Do I Need?

A home of average construction will need a ton of air conditioning for every 400-450 square feet of conditioned space. Houses with many skylights may need more than that, and some well-insulated homes may need less.


Your air conditioning equipment should be serviced by a professional every two years, preferably prior to the cooling season. There is usually a replaceable filter in the air-handling unit (AHU). This should be checked every 4 to 6 weeks during operation and replaced if necessary. This is the most important thing you can do to extend the life of your compressor. The compressor is the heart or "pump" which drives your air conditioning system. It's the most expensive part of the system; a new one costs from $1500 to $2500, installed. In the northeast, air conditioning compressors generally last 8 to 12 years, depending upon the manufacturer, usage and maintenance.

Compressor Safety

Most manufacturers recommend that you need at least three consecutive days of temperatures above 65 degrees to start up your compressor in the springtime. Also, don't run your compressor if it is below 65 degrees as it can cause damage to the compressor. (Why would you need to anyway?) There must be power to your compressor for at least 3 hours before starting it. This allows an electric coil to warm the compressor, vaporizing the liquid refrigerant. Liquid refrigerant in an operating compressor will damage it.

Anything that blocks air flow to the condenser or evaporator coil will shorten the life of the compressor or may damage it, if the restriction is severe enough. Examples of things to avoid are: plants closer than one foot around or 3 feet above the condenser coil (it's around the compressor, outside the home); dirty filters at the air handling unit; or a dirty condenser or evaporator coils. Don't cover the supply or return air outlets with rugs, furniture, paintings or other articles for the same reason.

If you've been running your air conditioning and then shut it off, wait at least 5 minutes before restarting it to avoid damaging the compressor.

Other Issues

During the refrigeration cycle, moisture is also removed from the air in the conditioned space. This moisture drips off the evaporator coil in the Air Handling Unit (AHU). There is a drain pan to catch the moisture built into the AHU. The drain pan should be piped to the exterior of the building to get rid of the moisture. The drain line must have a trap on it (similar to a waste pipe trap under a sink). If a trap is not installed on this pipe, the suction of the fan in the air-handling unit will keep the moisture from draining properly. Water stains on the floor next to the air-handling unit may indicate the drain pan piping does not have a trap.

The drain pan piping is notorious for becoming clogged. For this reason, AHU's in an attic should have a secondary drain pan in case the primary drain pan overflows. The secondary drain pan is outside the air-handling unit. The secondary pan must be either piped to the exterior of the building or have an emergency cut-off float switch. The float switch will shut off the air conditioning unit if water fills the secondary drain pan. This protects the finished walls and ceilings below the air-handling unit.

Sometimes contractors will connect the drain piping from the primary and secondary drain pans into the waste piping system in the house because it is more convenient. This is not a good practice. There are sewer gases in the waste piping system. Normally, water in the traps of the drain pan piping would provide a seal keeping the sewer gases out of the home. However, this only works with water in the traps. When the air conditioning is off for any substantial time, the water in the traps evaporates and the sewer gases can enter the home. Sewer gases are mostly methane, which is toxic and potentially explosive under the wrong conditions.

Supply ductwork for the air conditioning system should be carefully constructed and sealed to prevent it from leaking the cool air you're paying for. The supply ductwork should also be insulated to keep it from "sweating" when the air conditioning system is operating. The large refrigerant piping should also be insulated to keep it from "sweating".

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