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Air Conditioner Basics, A Guide To Home Cooling

Your air conditioner is designed to automatically remove unwanted heat from your home. By understanding some basics, you can help it do this efficiently. This will in turn allow you to keep more of your hard earned money. There are many different types of units including ductless mini split systems, geothermal, window units, ptac, portable air conditioners, and evaporative (swamp) coolers but we will look at the basic ducted split system here. These systems consist of an evaporator coil, a condensing unit, and refrigerant lines. These parts work with the fan and control system in the furnace/air handler to cool your home.

Basic Operating Sequence

The cooling cycle starts when the thermostat senses that the home's air temperature is above the thermostat setpoint. Contacts in the thermostat are closed and control voltage (24 vac) is supplied to the furnace control board y and g terminals. This causes the blower on the furnace/air handler to start. The furnace/air handler control board then supplies control voltage to the outdoor part of the system which is called the condensing unit. This control voltage causes a device called a contactor to close its contacts. This supplies power to a fan and to a pump that is referred to as a compressor. This pump raises the pressure and temperature of the refrigerant gas in the system. This high temperature and high pressure refrigerant passes through the outdoor coil. This coil which is called the condenser coil works the same way as the radiator on a car except it uses refrigerant instead of water. In the condenser, this high pressure refrigerant is cooled and condenses from a gas to a liquid. It is usually cooled to a temperature about 20 degrees below the saturation temperature and is called subcooling. This high pressure/low temperature liquid refrigerant is pumped to the indoor section of the system. Then the refrigerant passes through a metering device which regulates the flow to the evaporator coil. In smaller and older systems this device consisted of small copper tubes called capillary tubes which lowered the flow and pressure. Others use specialized flow restrictor devices. Modern high efficiency systems use a device called a thermostatic expansion valve. This device senses the refrigerant temperature at the outlet of the evaporator and automatically adjusts a valve to regulate refrigerant flow. These devices cause the refrigerant pressure to go down. Air is circulated over and through the evaporator coil by the fan on the furnace/air handler. Heat from the home's air is transferred to the refrigerant. This heat raises the temperature of the refrigerant above the saturation point and it changes back into a gas. The flow of refrigerant is regulated so that extra heat is added above the saturation temperature. This is called superheat and is added to ensure that the refrigerant stays in the form of a gas as it goes through the compressor. This refrigerant gas is then pumped back to the condenser where the process repeats itself over and over. When the thermostat senses that the home's temperature is below the setpoint, the contacts are opened and control voltage is no longer supplied to the furnace or the air conditioner.

Air Conditioner Size & Efficiency?

The size of an air conditioner is expressed as tons of cooling capacity. (The process of finding the proper size unit for your home is called a load calculation.) A ton is equal to the ability of the system to remove 12000 btu of heat per hour. Btu stands for British Thermal Unit and is just a standard measure of heat. The relative measurement of a unit's energy use to perform this cooling is it's SEER rating. SEER stands for Seasonal Energy Efficiency Ratio and is a ratio of the cooling output divided by the power input. The minimum efficiency of these units that can be manufactured in the U.S.A. is mandated by federal law and is a SEER of 13. In general terms, the higher a unit's SEER number, the less electricity it requires to produce the rated amount of cooling.

Premium Features...

Higher efficiency units typically have some special features. They usually have two stages (speeds) of operation. In some cases this is accomplished by having two compressors in the unit. One is ran on low speed and both are energized for high speed. In other cases this is achieved by having one compressor with two motors inside it. One motor is energized for low speed and the other is energized for high speed operation. On some high efficiency units the condenser fan motor can operate in low or high speed too. The most advanced units have a variable speed fan motor to precisely balance the airflow and limit the noise production. The highest efficiency air conditioner uses an advanced compressor called an inverter drive. This compressor has a motor that runs on direct current(DC) instead of the normal alternating current(AC). The unit is supplied with the normal AC power and a device called an inverter changes it to DC. This DC power can be easily varied to provide almost an endless number of speeds. This allows the unit to precisely match the cooling requirements of the home and can save a lot of energy.