I put one in last year, and according to my kill-a-watt meter, we're paying less than $10 a month for hot water even at our summer electricity pricing, which is 22 cents/kw. That said, the heart of the technology is basically the same as a $99 window air-conditioner, and the new units are made in China - so who knows what the durability will be. I paid $699 shipped, but apparently, these units were selling for $499 before the tax credit was announced, and it looks like the price was recently jacked to $799... The quality seems decent, but it does have a "buzzy" sound when it's running that would be really annoying if you had it near a sleeping or living area. The install was super-easy for me - I used a new AOSmith 50-gallon electric water heater as the tank (I hooked up the power supply to the standard heater, but turned off the breaker, so I can use the standard elements if the Airtap fails) Would I do it again? - MEH..I dunno - playing around with the pricing to steal the tax credit seems kind of sleazy, the warranty is really ambiguous - to qualify for the tax credit they had to offer a five-year warranty I believe, but their website now says two years - the new integrated units from GE and Rheem have ten years of coverage, and they have the benefit of being able to use both the heat pump and standard electric elements to give you super-fast recovery if you have guests over, or a huge garden tub to fill. A quick Ebay search shows the competitors are around $1500~ish, but if you haven't taken the tax credit yet, you could snag $500 in government cheese and nullify the cost difference with a better warranty. The technology is an excellent idea, and when the competition drives pricing to where it really belongs - (Until then... meh... Certainly it can pencil out, but the energy-saving payoff exceeds the warranty which makes me nervous. Solar really isn't any better, and tankless seems to be snake-oil when you look for real savings data, but to each his own...

Code requires that both the water heater PRV discharge line and the furnace condensate line must either terminate to free air outside the structure or into an approved "indirect waste receptor" The termination point of a PRV discharge line must point downward. When extending a pipe through a footer wall we must install a 90deg elbow on the end of the line to turn it downwards. (This is to minimize the risk of scalding water spraying out of the line and hitting someone standing near the point of discharge) Under the IRC when terminating outdoors the line must terminate within 6" of finished grade. Under the UPC it must terminate between 6" and 24" above finished grade. When terminating into the house DWV system or a storm drain system such as you have the PRV line must terminate a minimum of 2" above the flood level rim of the drain receptor, whether that is a floor drain, utility sink or a sump vessel. (The intended purpose of the 2" air gap is to prevent bacteria from entering the line and potentially going up the line and into the heater vessel.) In your case, you could install a tee on the storm drain line and stub a section of the storm drain up slightly above grade, then mount your PRV discharge line pointing down into the open standpipe. Just make sure that the bottom of the PRV line is 2" above the highest level of the riser from the tee. (Note, your 4" line is a storm drain therefore you are not required to install a P-trap below the riser.)

An interesting idea, but, for the reasons you have identified, it could be something you come to regret. It just seems like a lot of work and expense for something with modest benefits. In my view, it would be a pain to have them located in a crawl space where they won't be as easily observable for leaks, etc., so you'll have to remember to crawl in there to do regular inspections. Further, installation now and any future maintenance and/or changeout in a space like that would not be fun.

Depending on the configuration of the run, you may be able to use a simple electrical heat tape on the line that freezes. It won't help if the freezing is inside the wall, though (neither would the crawlspace radiator).