Can old TH115 be switched out for nSpiration Series without any alterations to the system?
Switch out should be a very quick and easy process since the mounting is about the same. The electrician would simply need to dismount the old unit and disconnect all the wires (supply, load, and floor sensor). nSpiration Series Thermostats have clearly labeled and easy-to-use screw-terminals on the back for supply and load. The floor sensor needs to be connected to the front of the base, just like in the previous model.
Video showing the process: https://www.warmlyyours.com/en-US/video-media/upgrading-nspiration-thermostat
Wiring diagram for a new model: https://www.warmlyyours.com/en-US/publications/OJ-THERMOSTAT-GENERIC-WIRING-DIAGRAM-A