[KimnKim]

Thank you Michael, Wally and Richard for all the great info and tips.

The valve on the coolant hose leading from the engine to the front of the bus was the turning point leading to the succesful conclusion of the water heater install. Apparently, as this coach resided in Texas (the Midland area) for the last ?? years and that owner must have spent some time living in it (judging from the old water heater - a conventional model purchased from Lowes with no heat exchanger), circumstances were such that no devices forward of the engine that rely on warm coolant were required, so that tap was completely closed. It might have taken a very long time to solve all my heat problems without that heads up.

So thanks again to you all.