I've searched all over the net, and I cannot seem to find anyone with the exact same problem. I have found similar problems, but this seems to be a little unique. I am not certain if my plumber crossed the lines on the install, but i was wondering if there was any other troubleshooting for this problem. Basically, when the shower is turned on, the pressure is just fine, but it does not call for hot water. I use a tankless water heater, and it does not fire up when the shower is turned on. Only ice cold water will come out, not matter where I set the thermostatic valve. After trying a thousand different things, we realized that if I manually cut off the cold supply to the shower (by closing the shut off valve), and leave only the hot running to it, then I will get scalding hot water only. I cannot seem to get it to mix at all. Is this a symptom of reversing the hot and cold lines running to the valve? Or could it be anything else? Thanks in advance for your help.

BTW, it is an Aquabrass 1/2" thermostatic valve