Most thermostats use 24vac, but some could control 120vac or
240vac. Then, without knowing if your system has multiple zone
control capabilities, you may not be able to add another
thermostat without changing the controls at the boiler. Exactly
what does the thermostat turn on/off? A zone valve, a pump, or
other? In its simplist form, a thermostat is just like a
temperature controlled on/off switch. There's power on one lead,
when the thermostat determines heat is required, it closes and
applies that power signal on the other lead - just like a wall
switch to turn on a light. It depends on what that switch is
trying to turn on. Depending on the voltage on the wires, and
the distance of the run, would determine the size and type of
wire required. So, without knowing more about the system, it
isn't safe to say. Often this is something like 18g bell wire,
but it could require larger or smaller, or some other type
depending on what the system design.