My furnace is a bryant model-cacf030aza 5yrs old, natural gas,forced air the problem is that the air coming out of vents is cool air , we have central air, but we made sure that the thermostat was set to heat. In the furnace we do not see any fire at all. It is on and it blows but the air is cool not hot it do not look like it ignites. Please can you help me.

Sounds like an upper limit switch problem. The switch has gone bad and is signally the control board the heat exchanger area is overheated. It shuts off the gas and runs the blower to cool it down. You need to identify the switch and replace it. It will be in the heat exchanger and have 2 wires going to the control board. It may be easier to identify if the furnace has a wiring diagram.