Thermometer Trick for Vents

https://paxtonumsa058.northstarcolumn.com/posts/comfort-experts-on-replacing-your-ac-when-north-fort-worth-ac-repair-no-longer-makes-sense-2

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