Came home to our house at 88 f with the thermostat set at 66! This happened yesterday as well but thought it was a one off, I was thinking it got stuck or something. I turned the furnace on over night ( once we cooled the house down) and when I left at lunch time it was running just fine.
What do you think, is it the thermostat or the furnace?

Melting in Ontario,
Thanks

At this point we can not tell. You ned to ride shotgun and be in the home when it starts to overheat. When that happens disconnect the stat and see if the heat keeps rising, the most likely condition is a bad stat. can you tell us brand and model?