Do you ever get the feeling there are some questions you should never have asked? Well ... that applies to my last entry on here!
I've had a flood of replies, and I'll answer them all individually ... eventually, but guys, don't get upset if it takes me a while, it's almost like writing a novel!
The general consensus of the replies, is that combi systems (by combi system I mean combined hot water heating, and hot water central heating; not quite the same as on-demand hot water!) are not common in the US, and that you're well behind the times, mostly due to inertia, and lack of foresight of course. Just because something has always been done a particular way, doesn't mean it should always be done that way!
Over here, we see the use of immersion heaters to heat our hot water in tanks as incredibly expensive, and wasteful, and many households are moving to solar hot water heating to supplement that. It's also increasingly common to see Photo-Voltaic Arrays on south facing roofs nowadays. I'd have thought both of those systems would be de-rigueur in places like Texas! (That generally held belief in the US that it's always raining in the UK is a load of bollocks!)
I know housing is geographically less dense in the US, but those areas that don't have mains gas could use oil, or LPG, fired combi-boilers. I'm no expert, but I would guess the type of systems used in the UK would be eminently suitable for the colder, and more temperate areas, like the Pacific North West, but not so suitable where you need air conditioning more than you need heating, in those situations I guess HVAC rules.
What I do know is, that if I was ever to emigrate to the Southern US, I'd want on-demand hot water for my shower! :o)