Shop for Heating, Venting & Cooling at The Home Depot. Master Flow Metal Pipe is available in a variety of common lengths, diameters and gauges to help make your next HVAC project a success. Round pipe can be used as a main trunk line or branch line, as well as in return air situations. It maintains the most efficient characteristics for airflow and is common throughout the country. A simple snap lock design and sturdy galvanized steel construction on Master Flow pipe creates a quick and secure installation that you can rely on.