Hi: I have noticed that two upstairs showers (both have single faucet dial and are the two furthest from the water heater which is located in the basement, suddenly have almost no cold water. I installed a new electric marathon water heater in the basement about two years ago. The plumber did install some recirculation systems under a number of the upstairs sinks which were supposed to decrease the time for hot water to get to them. I suspect that one of these may be the problem but I cannot figure out how to go about finding the source of the problem. The toilets are cold and all faucets that have separate hot and cold dials are fine. Any ideas on how to diagnose the problem? Thanks.