We have a Carrier Weathermaker furnace approx 20 years old. The blower will work if we go and hit the switch off and on but no heat is coming out of vents. No problem till now. Checked filter and it does need replaced but there was still no heat when filter was removed. Not sure what to check. Any suggestions before I phone for repair? It is 4 degress this morning! Thanks!

At 4 degrees now is not the time to be a do it yourselfer.