All About Air Conditioning

An efficient system and well-sealed ducts are critical in keeping a home cool. See how a well-planned HVAC system can optimize comfort.
Air Conditioner Repair

Air Conditioner Repair

While heating can be done through several different systems, cooling can only be done through an air-conditioning unit (also known as a "condensing unit").

Photo by: © Moore

© Moore

By: Jeff Wilson
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Central, or whole-home, air-conditioning (AC) was once a luxury. These days, it’s standard equipment on many homes. A good AC system will not only cool a home, but help control humidity as well. It’s important for homeowners to understand their AC system components, how they work, and how to make sure contractors install the correctly sized system. “An ounce of prevention is definitely worth a pound of cure when it comes to AC systems,” says Dave Moody, an AC industry pro with Service Experts, “Regular maintenance can make the system more efficient and make it last longer, keeping you comfortable for less.”

AC System Components

Your AC is part of the heating, ventilating, and air-conditioning system (HVAC). It is either paired with a furnace or it is part of the cooling cycle of a heat pump unit. As a matter of fact, a heat pump is simply an air-conditioner that cools in the summer but then runs in reverse in the winter, heating the home. Still, both furnace-paired and heat pump systems will have similar components. Some components are installed inside the home while others are placed outside the home.

The key ingredient in any AC system is refrigerant (more commonly known by its trade name, Freon) which is compressed by a compressor into a high pressure gas where it’s released into a closed loop of copper tubing. Refrigerant has the unique ability to absorb and release heat energy rapidly. The condenser coil and compressor are typically mounted on a pad outside the home. The evaporator and blower (also called air handler) unit are located inside the home, usually in a basement, attic, or closet. The blower, which serves both the AC and the furnace, is attached to ducts which function as pathways to move air throughout the house. The system is controlled by the thermostat.

The compressor/condenser is located outside the home. The evaporator/blower is located inside the home.

The compressor/condenser is located outside the home. The evaporator/blower is located inside the home.

How It Works

When the thermostat signals that the home warmer than the set temperature, the AC system turns on. Outside, the compressor begins to compress the refrigerant into a high pressure gas. The compressor pumps the high pressure gas through a radiator-like condenser coil of copper (or sometimes aluminum) tubing and aluminum fins where a large fan transfers heat from the gas to the outdoors. This is all accomplished in the outdoor component of your air-conditioning equipment.

Next, the cooled, compressed refrigerant, now a liquid, is pumped into the house via the copper tubing, arriving at the evaporator coil (also radiator-like). There, under less pressure, it vaporizes from liquid to gas once again. A natural property of this change is that the refrigerant absorbs heat rapidly from the air being blown across the evaporator by the blower. The cooled air is circulated through the home via the ducts.

At the same time, humidity in the air condenses on the cold evaporator surface into liquid water, eventually dripping into a drain pan and down a drain. This way, the air is simultaneously cooled and dried, making the home more comfortable for the occupants.

The refrigerant gas then heads back outside to the compressor, where the cycle starts over again. Once the temperature in the home reaches the thermostat set-point, the system automatically turns off.

Planning for a New System

Whether you’re having an AC system installed for the first time or replacing an old one, it’s important to use a knowledgeable expert HVAC contractor. AC units should be neither too large nor too small, but optimally sized using detailed calculations (called “Manual J” calculations). These take into account information about a home’s size, insulation rating, the number and types of windows and doors, and more. Moody advises, “An optimally sized system will run more efficiently, keep your home more comfortable, and last longer than one that’s too large or too small. Never allow an HVAC contractor to suggest a system size without doing these very important calculations.” This is true even when replacing an old system – there’s no guarantee that the old system was properly sized in the first place. Also, if the home has been altered through renovation since the old system was originally installed, its cooling requirements may have changed.

If ducts are being installed, the contractor must do detailed Manual D calculations as well. This ensures that ducts will deliver the right amount of heated or cooled air to all parts of the home as needed in each different room. A poorly planned duct system will compromise even the best, most efficient AC unit, so be sure to insist that these calculations are done thoroughly. Ducts should not be run through unheated parts of the home, like exterior wall cavities or vented attics. If they are, they should be properly insulated. Hard-sided duct material (as opposed to flexible duct material) is preferred, since it offers less resistance to the air traveling through it. The joints in all ductwork should be carefully sealed with duct mastic, not tape, since duct air leakage can lower the efficiency of any system.

Once the size of the unit is determined by the Manual J calculations, you’ll have several options in purchasing a system. First, consider the efficiency rating. The Energy Efficiency Rating (EER) measures an AC’s operating efficiency at a given temperature. The Seasonal Energy Efficiency Rating (SEER) measures an AC’s efficiency over an entire cooling season. With either rating, a higher number indicates higher efficiency and lower energy costs over the life of the unit. Use yellow EnergyGuide labels to compare the efficiency of different units. An easy rule of thumb to get high efficiency in any appliance is to look for the EnergyStar label, which is awarded to high-efficiency appliances by the Department of Energy. Also, check multiple consumer ratings sites to determine manufacturers’ overall reliability and unit lifespan. A modern central AC unit should last about 10 to 15 years.

Use EnergyGuide and EnergyStar labels to find the most efficient central air-conditioners.

Use EnergyGuide and EnergyStar labels to find the most efficient central air-conditioners.

Controls for a modern central AC system should include a programmable thermostat. By raising the temperature by 10°F when you are away from the home for at least 8 hours, you can save 5%-15% on cooling costs. Many higher-end systems will be equipped to control humidity as well. This type of system often includes a multi-stage compressor along with a variable-speed blower. The thermostat will include a humidistat, allowing the homeowner to set a preferred humidity in the home. When the home is cool but the humidity is too high for comfort, the AC system will run at a very low level to act as a dehumidifier. This option can save money, since the unit can operate at a lower level. It can also improve comfort, since AC without this feature may have to “over-cool” the home to remove enough humidity.

A programmable thermostat can save money.

A programmable thermostat can save money.

Once installed, it’s important to regularly change the filter in the blower system. “A dirty filter can impede airflow, reducing efficiency and resulting in poor comfort and possibly premature system failure,” says Service Experts’ Dave Moody. “Change the filter once before the cooling season and once before the heating season. In particularly dusty areas, change the filters every three months.”

Also, while some maintenance can be done by a homeowner, there are some steps a homeowner can’t accomplish. For example, a system with an undetected slow refrigerant leak can result in the compressor failing. Likewise, if a non-expert over fills the refrigerant, the compressor can be damaged as well. That’s why it’s important to have a trained expert service technician check and adjust the system just before each cooling system.

AC End of Life

Refrigerants used in AC units (as well as all heat pumps, window-mount AC, refrigerators, etc.) are controlled chemicals which can negatively impact the environment if released into the atmosphere. Older units use refrigerants that are powerful greenhouse gases, so they must not be allowed to escape when an old unit is being removed or serviced. Only a trained expert HVAC service technician should remove an old AC system by first capturing the refrigerant before removing the unit.

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