Hi. I have an American Standard Furnace. It was blowing the fuse on the igniton board when I swtched the thermastat to heat. I figured the bourd was bad so called a service company The tech replaced the transformer.The indicator light started flashing, and the external gas sensor powered up. But, when he called for heat the indicator light went out and a very small amount of smoke came from the board area. I then replaced the board myself because the tech wanted $600 bucks for a board I half suspected he just burnt up. So I have replaced the ignition board and the transformer and it is still blowing the fuse out evefytime I set the thermostat to heat. I disconnected the vent motor, and most of the safety swithes one at a time but the board still blows the fuse. The main blower motor while run on cool, just not heat. Something is shorted somewhere but I'm running out of options. Could the thermostat do this? Please help, its getting cold in Wyoming!

Remove the thermostat wire on "W" from the control board. Tape the door switch closed and use a jumper wire to jump from R to W. Will the furnace run? I'm trying to see if you may have a short in the thermostat wire or thermostat.