Have an old Maple Chase thermastate.. Husband had a recent stoke and cannot help me. I can do the heat but now that I want AC I put it on Cool and slide the Fan:Auto-On to the Auto-on position. I put 78 degrees on and then the thermastatre does not shut down. Runs all the time. What am I doing wrong?

Does not sound like you are doing anything wrong with the stat but many people kill the power to the AC unit during winter months. Any chance that was done? Check all the breakers and the disconnect switch outside by the compressor.

Do you have AC? I have a thermostat, no central air in the house. There's a cool option, but that doesn't mean AC.

During the summer my thermostat is off, no heat in the house at all. The thermostat control heat, unless you have central air, then the cooling option would turn on the AC. But you have to have central air for that option to work.

With the thermostat off in the spring and summer, the house normally gets to the temp that's outside. If it's 30 Celsius outside, it's at least that inside, and the thermostat shows that temp. But switching to the cool option, doesn't do a thing because we don't have central air.

So, do you have central air?

The fan switch is EITHER auto or on, not both. If you have the fan switch in the ON position it will run all the time. In auto, it will cycle with the AC demand. Make sure the fan switch is on Auto, let it run and see what happens.
Let us know.