Thermometer Trick for Vents

https://zukoski-group.chbe.illinois.edu/index/index.php/Getting_Your_AC_Ready_for_Summer_With_a_Comfort_Experts_Plan_to_Avoid_North_Fort_Worth_AC_Repair_Calls

Thermometer Trick for Vents Quick tip from our house: put a cheap kitchen thermometer in a supply vent after the AC runs 15 minutes, then check the air at the return. We see about an 18 degree drop when things are healthy