I have a 1940's oil furnace with original radiator (built in fin tube with steel, radiator size covers) I want to switch to high efficiency natural gas. Are there any "Don'ts" that I should be aware of? I want to get a combination system and replace my electric water heater also. Is an instant hot water system like a Rinnai better than a system that uses a heat exchanger for the domestic hot water? Should I try to flush out the system before I Demo the old Furnace? What is that process? Is it OK to drain the system and leave it empty for a couple months? Brand name suggestions welcome