Success! After a long battle, we finally figured out the problem. It turns out, there was a bad check valve, near the recirculating pump that allowed the cold water to get into the system. the same check valve failed on both systems. We opened it up, and both of them were encrusted with calcium deposits and hard water debris not letting the valve close. Just for future reference to anybody with a similar problem here is how we tackled it: -We checked the dip tubes on both hot water heaters , and both of them were fine. -next, my plumbers isolated the hot water heater to make sure the heater was running properly. all checked out fine. -We turned off the radiant heat systems and bypassed them with the built-in valves. -My plumbers opened up each shower fixture and closed off the water valves to take those out of the equation. the hot water still ran cold. - Next, the plumbers removed the recirculating pump and checked the system. the water still ran cold. at this point it had to be the check valves, since nothing else would cause the cold water to come back into the system. - The plumbers then starting removing the check valves beginning with the one near the recirculating pump and discovered the broken valve. If this still didn't work, they would've kept going to check every check valve in the system(three total). the plumbers installed a temperature gauge to be able to see the change in water temperature. now the temperature holds solid and 120� when the hot water is turned on. I can't wait to take a piping hot long shower. Below is a picture of the plumbing system if you were curious. Two water heaters, one for each unit with the radiant heat manifolds. Radiant heat system by Warmfloors, Napa Ca Installation by AG Quality Plumbing San Francisco, CA(the best plumbers in San Francisco by far) Thanks all! Here is a link that might be useful:

I agree -- call installer. What's probably happening, although it shouldn't, is that somehow when the pump activates it either increases the cold water pressure or decreases the hot water pressure, perhaps through some sort of improper backflow, so you get more cold. The washing machine valve is garbage--that wouldn't affect anything unless the washer is on.

I recently installed a Delta single handle shower control, and the max setting for hot was accessible after removing the shower control trim. The piece that determines the max hot water position is is just two sets of toothed/geared wheels that fit together, and you can index them in either direction. This youtube video provides a good explanation of the hot setting for the Delta single handle controls. https://www.youtube.com/watch?v=n2_uZ7wgV1I My shower valve worked like the one shown at about 2:07 minutes into the video. Bruce

Our water sits in the ground for years to millennia, fully exposed to the environment, sits in the town water tanks for months, sits in pipes for days or weeks all without inducing an epidemic of Legionella pneumonia. I think (speaking as someone who has had a career in infectious disease) the primary risk comes from recirculating water or water continually condensing that gets atomized and inhaled, as in air conditioning systems. I'd worry much more about the hot tub with the bubbles than my hot water tank. If sitting contained were an issue you'd see a lot of disease associated with the use of bottled water. I'm not aware of any such data. If you're really concerned just flush the tank and then crank the temperature up to over 140 deg for a day, that will kill Legionella and most other bugs.