A/C Basics

Connecting Air Conditioning manifold

The gauges will have a high pressure (red) and low pressure (blue). The easiest way to remember which hose goes to the correct line is to think about a fluid going through the system. A smaller pipe size would need more pressure to move the same amount of fluid than a larger pipe size. So that would mean the smaller pipe will have the high pressure (red) hose from your manifold gauge set. While the larger pipe will have the lower pressure (blue) hose from your manifold gauge set. Yellow hose is for adding charge to you system, big black hose on 4-line gauge set is for vacuuming system down to 500 microns.

Approach Temperature method

This method can verify refrigerant charge without connecting a refrigerant gauge set. Before you begin the evaporator (inside coil above furnace) and condenser (outside coil) must be clean. You will find the approach temperature chart either in the manufacturers installation instructions or on the panel for the condenser. This chart will be specific to the condenser you are working on and will differ from other units.

Measure temperature of the air entering the condenser in a few places to get the average temperature being careful that the sun does not affect the measurements. Then measure the liquid line temperature (small line) with an accurate clamp thermometer. Determine the temperature difference between the liquid line temperature and the outdoor air temperature. The temperature difference is the approach temperature.

Adding refrigerant will decrease the approach and removing refrigerant will increase approach. Allow 10-15 minutes of operation for the approach to stabilize after refrigerant is added or removed. Insure you use proper recovery containers when removing refrigerant since it is against the law to release into atmosphere and comes with a big fine if caught.

Liquid line temp – ambient temp = approach temp


Measuring Superheat tells you how effective the evaporator is working; normally best 10 to 15 superheat for air conditioning.

Superheat is the amount of heat added after the gas changes from a liquid to a vapor. Proper Superheat set point is important to an air conditioning system for these reasons

Low Superheat can cause liquid to get back to the compressor breaking internal parts.

High superheat will reduce the effective cooling, which happens best when liquid refrigerant changes state from a liquid to a vapor, also if too much superheat the compressor can overheat.

To measure superheat use the low pressure (blue) line of the refrigerant gauge set and clamp on thermistor to measure temperature at the suction line as close to the evaporator as possible. Suction line as close to evaporator outlet – saturation temperature of the refrigerant= Superheat


Subcooling tells you how effective the condenser is working normally you want 10 degrees subcooling but should always follow manufactures recommendations. 
Subcooling is the amount of heat removed by the condenser past the point of change from vapor to a liquid (saturation point). Proper Subcooling set point is important to an air conditioning system for these reasons.

Subcooling insures a solid column of liquid to the metering device and if set too low you could get flash gas, which will affect the meter device.

High subcooling will cause the refrigerant to stack up in the condensing unit bringing up the head pressure in the compressor. With higher pressures the compressor will need to work harder using more electricity and wearing it out faster.

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