If you could do just one thing to your duct design that would make it more energy efficient and less expensive, would you do it?

Spiral Pipe Symbols - Spiral Pipe of Texas

You might recognize the two symbols above. The one on the left is used to denote that a duct will be round. The one on the right denotes that a duct will be flat oval. Most of the time, for a typical HVAC system, they indicate you want spiral duct.