Rhem Heat Pump low voltage fuse keeps blowing
 

So it gets cold in the house and I realize I have not heat. I found that the transformer in the air handler is blown along with the 3amp fuse. I replaced the transformer but the 3 amp fuse blows every time I change the thermostat to heat.
If I set it to blower only it works fine.
If I set it to heat nothing happens.
If i set it to Em Heat everything works fine but when I switch it to regular heat. The fuse blows every time.
I eliminated the wiring from the Theromstat. I have an extra thermostat so I took it under the house with the air handler and wired it up. The fuse still blows.
I then eliminate the low voltage wiring from the air handler to the heat pump outside by installing new wiring.

I am not sure what to check next? It is COLD in here. I have called a technician but have not even received a call back yet.

Any ideas?

Post the schematic that is pasted on the inside of a furnace panel. At 24 vac and 3 amps you're looking for some resistance less than 8 ohms. If the fuse element is splattered it's more toward the low end of this range.

Look for a shorted wire in the heat pump. Any touching copper should be suspect. Mice like to get in and chew on the low voltage wires too.

I had the same problem with mine.. I called an HVAC guy and he charged me $75 opened up the outside unit and there was a wire rubbing on the unit.. He tapped it up and left.. A few months later it happened again.. This time i was ready, again a wire bare rubbing on the outside unit.