Without a bit more detail, its hard to know how much piping you're bleeding or purging. If only a few fittings at the furnace, and there are no flames within 5', simply bleed it to the furnace area and wait 15 minutes. If more than that, consider temporarily connecting a garden hose and bleeding to the outside. It will take a hose thread x pipe thread adapter to connect the hose. Don't take any chances, better to be safe than sorry. If you bleed the line in the house be prepared to explain to the wife why she smells gas!what is the proper way to bleed all the gas out when switching fittings/pipes ? A gas furnace is the only thing connected in the gas line.