HVAC Scenting: Should I drill the injection tube into the Supply Duct or Return Duct?
Ah, the classic debate! Let’s clear the air with the official engineering recommendation.
For most professional cold-air nebulizers, inject the mist into the SUPPLY DUCT after the air handler. Injecting into the Return Duct forces the mist through the air filter, which traps the fragrance molecules. Drill on the supply plenum past the cooling coils, inserting the tubing 3-4 inches downward facing airflow, sealed with aluminum foil tape.