1. Make sure the diffuser is mounted up right.
2. Make sure you do not install the diffuser in the attic where there's heat during the summer. This will mess up the oil scent over time. The diffuser needs to be installed in room temperature. You will need to extend the 3/8 hose up to the attic where the HVAC is installed.
3. Make sure to use silicon to seal up the hole in the duct work.
4. Ensure the hole you make is on the outflow where air is sent out of the vents.