These Rules of Thumb for performing a rough check for proper operating temperature of an air conditioning system include several procedures and describe the points at which you can take your air conditioning system's temperature as a step in evaluating its health.

• Temperatures at the AC refrigerant lines: some inspectors and air conditioner service technicians make a quick check of cooling system operation by looking-at and feeling the refrigerant lines. When the air conditioning system has been operating for at least 20 minutes we expect the (large-diameter) refrigerant suction line (returning refrigerant to the compressor) to feel cool or close to outdoor air temperatures at the compressor. (If the same condensate return line or suction line is covered with heavy frost right at the evaporator coil in the air handler, the refrigerant level in the system may be too low.)

  At the same time, also at the compressor/condenser unit, the high-pressure refrigerant line (refrigerant leaving the compressor to return to the indoor air handler and evaporator coil) will be hot. The little chart shown in the sketch here shows how the refrigerant suction line temperature varies as a function of the outdoor air temperature at the compressor. We don't suggest actually measuring these temperatures, but a quick look-at and careful touch-of the refrigerant lines can tell a lot about how the air conditioning system is working (or not). [Remember when medical doctors used to actually touch their patients?]

• Temperatures outside at the air conditioner compressor/condenser: if the air conditioning system has reached steady state and normal operation, the temperature of air blowing out of the outside compressor/condenser unit will feel [subjectively] warmer than the ambient outdoor air temperature. This is because a properly working system is transferring heat from the compressed refrigerant (perhaps 150 deg.F. inside the condenser coil) to the outside air (perhaps 85 to 90 degF ambient outdoor temperature).
• Temperature differences inside at the air conditioning system air handler's evaporator coil: if the A/C system is has reached steady state operation and normal operation, the temperature of air should be reduced as it moves across the evaporator coil. Typically this drop is 15 to 20 deg.F. [Carson Dunlop adds it may be as little as 14 degF. or as much as 22 degF.]. Look for this difference between the indoor air in the building as it reaches the coil and the air leaving the coil.
• Temperature leaving the air handler's evaporator coil: similar to above, measure the supply air temperature at a hole or crack in the supply plenum above (or "downstream from) the evaporator coil. It should be around 55 degF. Some experts argue that this is the only accurate way to examine the cooling system and that measuring temperatures across the evaporator coil is unreliable.
• Temperature of conditioned air at a supply register: If a measurement opening is not available at the evaporator coil at the air handler, measure air temperatures leaving the nearest supply register. There too, unless the supply duct is long, uninsulated, running in a hot space (such as an attic), the air temperature at the supply register should be around 55 degrees. As long as the building has not reached its thermostat's set point. the system should be blowing air that is noticeably cooler than the building's air.
• Too Low air conditioning output temperatures may indicate that there is a problem with air movement in the system. If air moves too slowly across the coil its temperature may be lower than the numbers specified above. Look for an evaporator coil that is blocked by dirt or is icing up. Too low a temperature can also be caused by improper metering of refrigerant into the evaporator coil (a bad TEV or thermal expansion valve), or by improper charging (too little refrigerant - more likely on commercial systems).
• Too high air conditioning output temperatures may indicate that the system is not working properly: perhaps the coil has become blocked solid with ice. Alternatively, high A/C outlet temperatures may be occurring, or even no cooling at all, due to other problems such as a loss of refrigerant. (Don't just add refrigerant, find and fix the leak. Otherwise you're just making the refrigerant delivery man rich.)