My gas furnace seems to be blowing cold air. I set the thermostat on 70, the furnace will run for hours blowing cold air. When I turn the power switch off at the furnace, then turn it back on, within about a minute the air coming out of the register gets a little warm and the exhaust pipe gets very hot. However within a short time, cold air starts coming out again and the exhaust pipe cools down drastically. Any advice?