I have a Carrier Gas Furnace that is blowing the 3 amp fuse for the secondary 24 Vac. I have replaced the fuse and after the heat is on for awhile the fuse will blow and then I have cool air blowing. Please advise on how to troubleshoot problem.Thanks

This doesn't make complete sense. Both the gas valve and blower relay depend on the 24 volts to their coil. When the fuse blows, the blower should stop as well as the gas valve shut. Either coil could be shorting out when it has had power for a while. Shut the power off. If you have an ohmmeter, measure the resistance of the gas valve coil. Them replace the fuse and let it run until the fuse blows. Immediately recheck the resistance of the gas valve. If it is much lower or 0, the gas valve is the problem.

Unfortunately it sounds like the blower relay is sticking too if it runs more than a few minutes after the heat quits. If it is on a circuit board, it is hard to check and replace. You may end up replacing the circuit board.

I have a carrier, and had to replace the original fan control board with a new one. It was an upgraded microprocessor control, with a fan relay on/off and another relay hi/low speed. Not like t he original low speed heat/ high speed cool relay setup. By default, when the new board's 3 amp fuse blows, it defaults to low speed fan on. This carier board is just like an ICM 271 board, and you can find it on the web and look at the specific instructoins and details.