Ok, so I've read through most of the posts here but was unable to locate my problem. Just had an ECO Smart ECO27 electric tankless installed 2 days ago in the lower level of my tri-level, which is where the previous hot water tank was located. Every faucet except the upstairs bath tub produces hot water. When the upstairs tub tap is turned on the output pipe from the tankless unit goes cold. I tried running hot water in the kitchen, which it was hot, then turned on the upstairs tub tap and the water cooled off.

Any help provided would be greatly appreciated