The largest quantity of HVAC leads come from referrals, so consider looking into ways to increase your referrals systematically. Ensure you have a sticker on the items you sell, and ensure it's well designed and has a clear website + number to call. Referral leads are also the easiest to close. The second largest chunk of leads comes from being visible on Google (through ads, or finding ways to get higher in the 'organic' listing.) While mailers, and directories are still important to at least be somewhat present on, online marketing will be one of the largest broad categories of advertising you should vigorously pursue.