I got a stumper for ya. I installed a 119 Ultra with a thermostatic mix valve to the domestic hot and life was good. Then I added a grundfos UP10-16 recirc. pump (with internal flow check)on a dedicated return line and connected it into the domestic cold inlet on the Ultra tank through a tee. On the bull of the tee is the incoming domestic cold which I installed a spring check. When the pump is on the cold water at the tap gets roasting hot as hot as the hot side. I checked the spring check, internal check in the recirc. pump and replaced the thermostatic mix valve that has plastic check valves on the hot and cold side in the part and all seem to operate fine. Am I missing something. jb