I have an old battery operated thermostat right now with two wires (18/2) attached to it running to a boiler. No AC, fan, etc.-- just a boiler. I can easily replace the wire with 18/5 one, however I'm unsure how to later attach the wires inside the boiler and to then Honeywell wifi thermostat.
Wires inside the boiler:
The thermosat is a Honeywell RTH8580WF.