I want to replace an existing thermostat with a Salus RT300RF. The existing stat is connected to a Siemens controller by only two wires, presumable acting as a simple SPST switch. The Salus appears to need permanent 230v line current to function. I assume that, in addition to connecting the existing 2 wires to the Salus's Line and HeatOn terminals, I need to provide a third wire to function as a neutral to allow the stat receiver to have permanent available voltage. Can someone confirm this analysis? Thanks