Not sure what part of the site to post this in, but I figured I'd start here (please move as appropriate).

I'm looking for opinions/hang-ups on heat pump domestic water heating. It seems to make sense to me on the surface, just want to make sure I'm covering all the concerns.

Here's the situation. A Medical Office building (6 stories), with relatively low hot water usage (mostly just hand-washing), but quite a large recirc flow/losses. (Rough estimates based on the current system is somewhere around 50,000BTU/Hr losses to the recirc, with peak demand near 75,000 BTU/Hr. Currently, the demand is served by some AERCO steam heat exchangers, but the steam piping is running from a really inefficient boilers system across the campus, with quite a bit of transmission loss, and these heat exchangers are the only things on this steam line. It makes sense that separating it out and going with something else would have gains, but it's kind of a toss-up which way to go. The building has no gas in it, and would have to be run about 1200 feet across ground, and up 4 stories. The current system is in a penthouse at the 4th floor level.

Here are my thoughts:
- Condensing gas tank heaters - ideally something like 3 50-60MBTU, for redundancy, and future capability. Low operating costs, but high installation costs - due to the gas line requirement.
- Electric Tank - ridiculously high operating costs, far exceeding even the poor current situation.
- Electric Heat Pump - Middle of the road operating costs, middle-to-low installation costs (the heat pump is likely expensive).
- Solar Collectors - High installation, but very low operating costs. Still would require some backup source of heat.

What my first desire points to is to go with the Solar collectors, combined with a heat pump for backup-supplemental. Being in a large mechanical room, it is likely we could duct the cold-air discharge to the air handler intakes, and utilize that cooling when it is in use. Being in SoCal, we have pretty decent sun-hours, so I'm hoping we'd be able to get the tanks hot enough during the day to keep the demand through the night. Am I missing any problems with this setup?