The Rheem ECO180XP3 is an outdoor unit, so it's fastened to an exterior wall and directly vents to the atmosphere.
The unit doesn't connect to a metered natural gas line; the fuel source is dual 20-lb propane cylinders, feeding into a regulator with a flip switch so that you can change tanks if one reuns empty. The outlet connection from the regulator is 3/4" steel pipe 4' long to the unit. And yes, the unit is made for propane and not naturral gas.
This is a direct replacement for an old Paloma unit that worked fine until it finally crapped out. So there's no question about the adequacy of the gas volume and flow. I haven't changed the layout at all, so it seems to me I should have been able to just slap the Rheem unit in, flip it on and have a hot shower.
Then I got a new regulator, but that didn't help.
No, I've got ball valves on both cold water and hot water lines. Thanks for your help, but I finally got through to Rheem tech support. They ran through a diagnostic procedure and it turns out that the impeller that senses water flow was defective and wasn't turning, so even though I had full water flow the machine thought it was zero. I returned the unit to Home Depot and got another one. All is well now.