I have an old fuel oil furnace that will shut down after running for a bit. If you set the t-stat to say 75 and let it satisfy it the furnace will not come back on. If you turn the stat down before it reaches 75 it will start again on its own. The furnace power would need to be turned off to reset the furnace but now you need to hit the reset button to get it to resume operation. What does the reset switch accually reset. Is the a limit swith on this furnace. I know how to fix gas fired furnaces but have not worked on fuel oil. 75 is just the number I used, it will happen with any setting on the stat.