DIY HVAC Air Freshener
Things You'll Need
Instructions
1Spread the cotton washcloth out on a flat surface. Add essential oils like peppermint, sandalwood, lavender or eucalyptus to the atomizer. Use one oil or create your own blend.
2
Spray each sides of the washcloth four times from 6 inches away. Access the air filter in the HVAC system. There may be a few screws to take out before the access panel can be removed.
3
Pin all four corners of the scented washcloth to the center of the filter with safety pins. Slide the filter back into position so the cover panel can be reinstalled. Turn on the whole house fan to test the potency of the air freshener.
4
Spray the air freshener with a two or three more squirts from the fragrance atomizer, if the scent is not detected after 15 minutes.
5
Remove, wash and replace the washcloth when it gets covered in dust.
Source...