I need to move/replace a couple of radiator supply pipes so I need to drain my heating system. Do I need to shut off the gas to my water heater and furnace? Once it starts draining, do I need to open the bleed valves on as many radiators as I can? And then when I refill the system, do I leave one bleed valve open at the highest point in the house (my third floor) to let the air escape?

Thanks for any help.