Why do I have a high voltage wire here?

[Hands007]

I have a 4 zone system and all of the zones are wired with two wires to the thermostats. This is a heat only system, gas boiler with hot water radiators. One of the zones has a much thicker wire to the thermostat and was wired to a high voltage thermostat. Can you think of a reason why this should be a high voltage wire but not a high voltage system?

This is a a large house that was illegally converted to and SRO and back so there is a lot of questionable work being removed.
The zone valve is the same as all of the others. I suspect they just had some wire lying around or had electric baseboards in the basement at one time and replaced them with hot water radiators. The wire would be too thick to go into a standard low voltage thermostat.

 

[Reach4]

I think your two proposed reasons are the right ones.

 

[Sponsor]

 

[Jadnashua]

Some low-voltage controls have an end switch that can be used to turn a circulator on, and that may be 120vac...that would have required thicker wires than used for a thermostat. Hard to tell without seeing the whole picture or some history.