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.

Here is a quick update. I had a local plumber come out to try and help diagnose the problem. After about 30 minutes of troubleshooting, he said that it was not venting properly and not running right. After some investigating we discovered the pvc that vents to outside had a big belly in it, see pic, which in turn caused the condensation from the hot air to get trapped in the vent line and reduce the venting area enough in the pipe to cause the air to have to be expelled back to the area of least resistance (out the condensate port). I went to the middle of the pipe and shook it and it was full of of water. I raised it by pushing up on it, and a bunch of water drained back thru the condensate port. I need to eat crow and apologize for saying anything bad about Takagi since this was a plumbing install issue. In nearly 20 feet of pipe the original installer had one support and that was at the end where it 90’s up to the roof. Not the first problem I had with the original plumber, so I can’t say I am surprised. They installed my fireplace wrong, which I spent over a year messing with and another plumber discovered the problem finally. I am glad the issue is resolved. I am going to add some supports to the pipe this week to keep it from happening again. I am going to delete this thread in a couple days so as not to give Takagi a bad name when it wasn’t an issue with their heater. Thanks for all your help.

If this system works like a boiler, to control the temp it turns on the flame if water is below a low set point, and then cycles the heat off when it reaches the high set point. In addition, the water heater also uses the flow rate to determine when to turn on and heat water. The control units can malfunction in various ways, so you need to have some way to measure/view what is going on. For example, it may turn on and heat to the high set point, cycle off until the temp drops, but then fail to come back on when it reaches the low set point because of a relay/control issue. Or the control may be cycling the heat on and off at the wrong temps because a sensor is bad and its getting the wrong input. Does that far sink have cold water flowing when set to full hot, or does it not flow at all? Have you tried leaving a different sink on (where hot works), and then turning on hot water at the far sink? Bruce

@Karen Williams This is the control panel on my tankless water heater. Water Set Point ... the output temperature the unit feeds into the hot faucets ... can be set in 1°F increments between 50°F and 140°F. Temp Up and Temp Down buttons adjust the Water Set Point temperature. Then press Temp Set to make it the normal at-all-times temperature if desired ... otherwise the adjustment is temporary and will revert back after a number of minutes. I have 103°F as the "normal" temperature and raise it when needed for washing dishes or clothes, without pressing Temp Set, and it reverts back to 103°F after 30 minutes so I don't have to remember to do that. That's just how my unit works, they're not all the same. Setting the temperature "at the faucet" means I set the water heater to the temperature of heated water I want/need "at the faucet" ... such as 103°F for a shower, instead of 120°F or 135°F which is much hotter than needed for a shower ... and I turn on only the hot faucet (or set a single-lever faucet to full-hot with no cold mixed-in). Thus, the tankless unit has a high-enough flow rate passing through it for the duration of the shower to insure it doesn't shut off due to too-low flow. 120°F, 135°F, 140°F output temperature requires mixing in cold water and turning down the hot flow at the faucet to get a comfortable shower, which reduces the flow through tankless possibly below the required rate which can cause it to shut off completely and the shower goes cold. Remote control panel means exactly that ... some tankless water heaters can have an optional extra temperature control panel installed at a convenient location such as bathroom or kitchen so the homeowner doesn't have to go down to a basement or out to a garage or wherever the heater is located to adjust the temperature. My heater doesn't have a remote control but it's located in a broom closet in my laundry room behind the kitchen near the center of the house so is easy to access any time I want to change the setting.