furnace won't shut off
The furnace is gas.
More info on the issue:
I had the thermostat shut off until the temp in my condo got down to 55 last night. The thermostat is programable, and in the evenings at 11:30pm it is set for 65 and then it goes up to 72 at 6:00am. When I turned the thermostat on at 11:45 the furnace kicked on and warmed the place to 65 and shut off. It worked correctly throughout the night and kept a constant 65 degree temperature. When the temp setting adjusted to 72 at 6:00 am, the furnace kicked on and went past the 72 degree mark and I had to shut it off manually at 75. So it seems that it works fine for temperatures below 70, but once the thermostat is set above 70, it will not shut off. Has anyone experienced this before? It doesn't make much sense to me. I am afraid to leave the furnace on at all (at any setting) when I leave for work because I fear it's going to kick on and keep running all day. The high is going to be in the mid 20's today, so I don't want to leave it off altogether, but I don't want to come home to 100+ temperatures again either.
Any suggestions? I am going to buy a new thermostat today, it seems like the only obvious answer. If there is anything else I am overlooking, please let me know. I will post the results after I install the new one.