Thermometer Trick for Vents

https://beausrhq666.juniperbrief.com/posts/warm-air-on-the-first-hot-day-means-north-fort-worth-ac-repair-and-a-call-to-comfort-experts-5

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