I ran into something I have never seen before.....I got a service call for a old floor furnace. Customer wanted cleaned and also wanted to know why there was a power cord coming from t - stat. huh????? So when I get to the call I see the cord coming from the stat. I plug it in and set the stat to call for heat and a Christmas light in the stat comes on along with the furnace. Now keep in mind this is a old mercury stat (Robert Shaw). So my question is have you seen this before. and why? I removed the power cord and light. Then wired the stat up normal and the heater would not fire up. I re- install the power cord minus the light and the heater fires up. Soooooo weird. I saved this call for the end of the day and did not have time to crawl under house to see how this thing is wired up. I don't work on very many floor furnaces but I do know that most do not need power and run on millivolts.