My master bathroom is the farthest from my electric water heater (traditional with a tank) on the other side of the house. All other faucets are much closer and receive quick hot water. During the bathroom remodel I've been spoiled using the guest bath that is directly adjacent to the hot water heater with instant hot water. I've looked into recirculating pumps but they seem to waste quite a bit of electricity having hot water in the pipes all the time. My pipes are mostly in the unconditioned crawlspace, however, they are insulated with fiberglass wrap or foam.
I was wondering if instead of a recirculating pump could I get a smaller model tankless water heater like the Rheem RTE 13 or the Ecosmart 13 and connect the inlet to my hot water line at the master bath. This would in theory have the tankless heater heat the water when it receives the cool water from the pipes and then stop supplying heat when it receives heated water from the tank. It seems like this is a similar arrangement to solar hot water backup, see Where can I find good resources on solar thermal backup options?.
One review of the Ecosmart says thermistor issues result in inconsistent heating when the inlet temperature is too high (> 120F). http://www.amazon.com/review/R2AH7NY585PJN3
Based on the calculators/ground water temps in Eastern Washington to supply my entire hot water demand for the bathroom would require a larger 18kW heater and it would be great to be able to get a away with a smaller model in the proposed back-up arrangement.
Does anyone have experience using a tankless heater in this way? Can it be done or should I simply go the recirculating route, although most evidence suggests this is a waste of electricity. Also, I would prefer not to loop my hot water down my cold water line so I'll probably run a separate PEX hot water line back to my hot water heater if necessary.