We have a gas furnace with 110 blower and it is not blowing hot air. We changed the thermostat and its still not working. When thermostat was placed in the off position it cont. to blow cold air so we shut off the breaker

Pressure switch failure. Looks like a little black hockey puck shaped thing normally held to the top of the furnce. Unscrew it, pull off the hose, and use a long pin to poke into the holes and make sure it's clear of any blockages. The small most insignificant things can cause this to plug up. Usually, a small clacium deposit. Pop a pin in on the switch, and on the furnace itself. Stick the little hose back onto the switch, and furnace, viola, problem should be solved.

Replace the hockey puck for 25 30 bucks.. just make certain you know where you got all the screws and put the wires back as they were.