Nest Learning Thermostat 3rd Generation Stuck On Schritt Nr.2 Verlauf

+	[title] The Solution
+	[* black] The solution is to add a C wire, also called a common or ground wire.
+	[* black] The Nest Thermostat and Base can use power between R and C and not affect the W (heating) wire.
+	[* black] Fortunately, in my installation, the wire used actually had three conductors, so I just had to strip and connect the third wire.
+	[* black] Also fortunately, the Caleffi ZSR-106 has a terminal for C, it just wasn't wired up, so it was a simple matter to strip and connect the third wire ony this side too.