My RUUD central ac and furnace blower run continuously even after I set my Durabrand automatic thermostat to OFF. It gets really cold in the house and I have to shut off the circuit breakers to both the AC and the Furnace to turn them off. Thermostat worked fine up until a week ago, new batteries installed. AC compressor and furnace fan runs continuously regardless of COOL-OFF-HEAT or temp setting.

Any clue as to what is going on?

Could be your contactor at the condenser is stuck closed. Shut the power off to unit and inspect it. It is not and expensive repair.

It is possible that in changing the batteries that you may have moved a wire at the termostat and it is shorted to another wire. If this happened it would probably be the red and the yellow wires that are touching.