To transition from an Application Engineer in HVAC to an HVAC Design Engineer, focus on gaining relevant experience and skills. Start by familiarizing yourself with HVAC design principles, software tools, and industry standards. Seek opportunities to collaborate with design teams on projects and take on responsibilities that involve system design. Pursue additional education or certifications in HVAC design to enhance your qualifications. Networking with professionals in the field can also provide insights and job leads. Ultimately, apply for design positions when you feel confident in your skills and experience.