Which method is typically NOT used to join metal sheets in HVAC?

The method of joining metal sheets in HVAC applications generally involves techniques that provide strong and reliable connections suited for the dynamics of heating, ventilation, and air conditioning systems. The use of welded pipes is typically associated with plumbing and piping systems, where permanent joints are necessary to handle fluids under pressure. This method is not usually applied to the joining of flat metal sheets, as HVAC ductwork and similar components require flexibility and the ability to disassemble for maintenance purposes.

In contrast, rivets, screws, and adhesive seams are commonly utilized in HVAC. Rivets and screws provide mechanical fastening that allows for easy assembly and disassembly, which is crucial in HVAC installations. Adhesive seams offer an alternative that can work well in applications where sealing is more critical than mechanical strength.

Thus, welded pipes do not align with the standard practices for joining metal sheets specifically in HVAC work, making this the correct choice for the question.