A home A/C compressor is the engine that keeps coolant circulating through an air conditioning system. The compressor accomplishes this task by creating high and low pressure areas with the use of a moving piston in a sealed chamber. The high and low sides on a home A/C compressor are also known as its discharge and suction lines. These are the pipes in which cold and hot coolant is constantly flowing into and out of the compressor.
Open your A/C unit's access panel by either removing the screws holding it in place with a screwdriver or using your hands to open it, depending on the unit type. The compressor unit will be a single metal object, often cylindrical in shape with a rounded end. Some A/C units will allow access to the high/low lines without the need to access the compressor directly.
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The low side, or suction line, will be the line connected to the compressor from the top or higher position. It will be cold to the touch and may be wrapped with insulation. This is where freon enters the compressor as a gas.
The high side, or discharge line, will be the line connected to the compressor from the bottom or lower position. It will not be wrapped in insulation and will be warm to the touch. This is where freon exits the compressor as a liquid.