I recently bought the house and wanted to convert two old manual thermostats to a low voltage programmable type. When I pulled the cover off however, it looks like they used a single pole 22A-120/240V thermostats (to control the heat only). The wires are a thick gauge, but I only measured 27.66V with the multimeter @ the thermostat. The heating is oil (boiler) and it looks like they installed a separate programmable low voltage thermostat that only controls the AC unit.

In the boiler room, there’s a 24V transformer that has wires that looks to be spliced together with wire connectors. The wire from the transformer looks to be low voltage wire(maybe 18 or 20 ga) and it’s spliced together with thicker gauge wire that runs into the wall. The junction box where the transformer sits on then connects to the boiler with metal clad cable.

My guess is that whoever installed this may have used the wrong thermostats and wrong wires. Instead of running new low voltage wire to a low voltage thermostat, they just kept the line voltage thermostats with thicker gauge wires and spliced it into the 24v transformer. How can I go about fixing this? Can I use the same wires to connect to a new low voltage thermostat? Is it okay/safe to do so?

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My guess is that you are correct. If you want to verify the connection at the existing wires, disconnect those wire nuts and see if the voltage goes away. Since it's line voltage wiring, it is no doubt fastened in a few spots between the t-stat location and the boiler. Ideally you should replace the wire to avoid confusion in the future for anyone working on the unit. As far as it being safe? It is safe from an operation al standpoint but again the heavy wires may confuse someone in the future. It's wrong as it is. Personally I would fish new t-stat wire if that is at all possible and pull out the old wire... again if possible.