Have ac unit which has a 5amp fuse between the transformer and the thermostat-i turn the thermostat to fan and it blows air-but when I turn the heat or ac on it blows the fuse everytime-replaced capacitor and contactor on outside unit and still doing the same.

Disconnect low voltage wires, insulate at compressot contactor and set to cool, If it still blows fuse, you have a problem with control wiring.

Disconnect low voltage wires, insulate at compressot contactor and set to cool, If it still blows fuse, you have a problem with control wiring.

Disconnected low voltage wires and nothing happened to fuse-replaced thermostat and wired accordingly-connected low voltage outside and still blew the fuse again