Procedure for Checking Air Temperature Drop (‘T) across Evaporator Coil 

1. Completely open all supply (discharge) air registers. 

2. Start the air conditioning unit and allow it to run for at least half an hour on high. The objective is to saturate the evaporator coil before running a temperature test. 

3. With a standard dial–type or digital thermometer, measure the temperature of the air immediately entering the return air grill of the air conditioner unit. This is the return air temperature. 

4. With a standard dial–type or digital thermometer, measure the temperature of the air immediately leaving the supply (discharge) louvres. This is the supply air temperature. 

5. Subtract the supply air temperature from the return air temperature. (If it is a ducted air conditioner unit, use the closest discharge register and make sure the temperature sensing device is measuring supply air temperature only.) 

6. A properly running air conditioner unit should have a nominal temperature difference of 20°F (11.1°C). 

NOTE: Slightly lower temperature differences are possible under extremely humid conditions. (The unit may have to run longer to remove moisture.) 

7. Temperature differences greater than 22°F (-5.5°C) are possible in warm, dry weather. Restricted air- flow over the evaporator coil or low fan speed may also cause greater than 22°F (-5.5°C) temperature differences. 

NOTE: When using two thermometers, one for intake and one for discharge, be certain they are calibrated to each other.