Our furnace just started this a few days ago... when the heat kicks on the furnace will heat up but the blower will not blow... does anyone know what could be causing this... we are thinking maybe thermostat?? WHen we turn the fan on then the furnace works properly... as long as we have the fan on... when the furnace kicks on... but it won't turn it on by itself...

Although the thermostat can turn the fan on, in the heating cycle it is controlled by the circuit board, or in older furnaces, a limit switch. Usually if the fan doesn't come on, it is the relay on the circuit board or the sensor that signals that the gas lit failed. On the older furnaces, the limit switch can fail. Usually fan on uses a higher speed winding in the blower motor controlled by a different relay than the heat. If you need more details, post back, and let us know if it is an older furnace. It is an older furnace if it looks like this:

http://tinypic.com/hvx6vm.jpg

It was installed with our home... we purchased our home in 1998... it is a 1999 model sectional home... we replaced the thermostat and it did correct the problem... my husband thought relay too, but upon looking our furnace doesn't have a relay?? But, anyway... thanks for your help!! Praise the Lord it was something easy to fix!! : :)