Hello: Ultimately, you want to follow closely what the manufacturer says. If they say to install a relief valve on the domestic side, than do it. Any inspector will follow the printed manufacturer’s instructions.
Here’s what I found: http://www.thermo2000.com/pdf/en-US/manu/manual_turbomax.pdf From what I read, they DO wand a T&P on the domestic side and the unit comes with a 30 psi relief valve on the boiler side. It’s pretty clear! 😎