Also, check the procedure for connecting a thermostat to your boiler (or other compatible system) using its user manual:

• 1. Identify the power wires and terminals on your boiler (or compatible system): live (typically dark, black, brown, red, etc.) and neutral (typically blue). These terminals are sometimes labeled L-N / P-N.
• 2. Identify the control terminals on the boiler (or compatible system). If replacing a thermostat, simply locate the wires it was connected to. For a first-time installation, the terminals may be labeled Ls-Lr, TA, RT, 3-4 (or OT for OpenTherm-compatible boilers).