I installed an RTH2300 this winter to replace an old mercury thermostat. It worked fine for the heating season. I turned it to Cool for the first time today and my AC and furnace blower will not turn off. Keeps cooling even though I have it set at 80 degrees and the temperature reads 76.

Get out your install instructions,, go over it a few times if needed,, sounds like something not wired right

Or not programmed right