Q: Where would be the best place to put my supply and return probes?
A: Probes need to be placed where they can get adequate air mixture, such as more than 3 feet away from the evaporator. The return grill in the occupied space can be used for the return probe. The supply probe can be placed in the 1st available register off the plenum. Probes should not be placed too close to the evaporator, since this will cause a false low air flow indication.
(answer provided by your expert Eric Urrutia.)