I have an old Lennox boiler which doesn't appear to have a PCB board. Originally the old honeywell thermostat had a 2 pair wire attached which included Red and White Wires. I snaked an 18/5 thermostat wire and replaced the thermostat with a google nest thermostat. When I tried using just the red and white wire it prompts me to attach the C wire, but I'm not sure where to attach it on the boiler. It also prompts me that it detects the white wire, but doesn't detect the red wire. I'm not 100% certain whether it is not detecting it solely because the C wire isn't currently attached. I can confirm that there is definitely power coming from the switch to the boiler. The wires coming from the switch are marked with L1 and L2 and I tested it with a voltmeter. When I test the voltmeter against the white and red wire I get about 26 volts so I'm not sure why the smart thermostat isn't detecting it.
If i'm missing any pertinent information guys please let me know and I'll include it.