Answer: I helped install and service a handful of the ThermaStor heat pump water heaters over a decade ago, so my comments may be a little dated. Take them for whatever they're worth. They were made by DEC of Madison, WI; and I recall that they offered both air-source and fluid-source models. The fluid-source ones impressed the heck out of me. They were used in dairy bulk tank applications, where the heat pump would extract heat from warm, "fresh from the cow" milk, chill it to prevent spoilage, and then dump the heat into its large water storage tank. This was at a time when most dairy farmers used air-cooled condensers to cool their bulk tanks, which just wasted all that "free" energy. I'm not sure about the economics of the bulk tank heat pumps, since I did not then have the skills or the desire to do a financial analysis. I do recall that one dairy farmer was able to use his bulk tank heat pump to supply all the domestic hot water needs of both his farm and his home. I have a slightly different take on the air-source ones. At least one of the DEC models I remember had a removable plate attached to the heat pump's air intake. You connected a pair of field-installed flex ducts to collars on this plate. During the summer, the ducts would suck warm air across the evaporator, remove its heat, and blow the cold air into the home. For winter operation, you would manually remove and flip the plate around so that the cold exhaust air would blow outside. If there was a demand for hot water that the heat pump couldn't meet (during a defrost cycle, for example) then traditional electric elements would heat the water. My main knock against the air-source heaters was the way they were positioned. They were (at the time) a challenge to troubleshoot because they were somewhat unusual and complicated... you'd need to be a fairly sharp tech with refrigeration experience to do justice to these systems. Yet because of codes and state laws at the time, I suspect these things were installed primarily by plumbers. The linesets even had Aeroquip-style couplings on them to keep things simple... but heaven help the plumber who actually had to troubleshoot the equipment. He would quickly be in way over his head. I'd be shocked if DEC's price structure could cover the costs of beefing up their in-house service staff and idiot-proofing the units while still saving enough energy to justify the high up-front costs. I'd also recommend that you perform a basic economic analysis of these units. It doesn't have to be fancy, but at least then you'll have a better idea whether you're doing your customers a favor by recommending these units... or just ramming snake oil down their throats. I'm also interested in seeing some of the other responses to your thread. I'd like to know how these products have evolved since my early plumbing days.