My buddy has a payne heat pump system with a payne air handler and every time the heat is turned on at the termostat the fuse blows in the air handler cabinet. The fuse will not blow if emergency heat is turned on or the a/c what is the problem here? I have been working in hvac for six years but not on payne equipment. I hear there is a problem however with the circuit board. Please help!

High or low voltage fuse? If it is a high amp fuse youmay need to pull the heat bank out and make sure none of the elements have shorted to ground.

Low voltage 5 amp fuse. Fuse Only blows when you turn on heat. He said the board was replaced last year on the outdoor unit.

If it's a low voltage fuse blowing check at the heat pump. The defrost sensor,low pressure switch or high pressure switch is common to ground out on this unit. They will start to rub on the refrigerant lines short out and blow the low voltage fuse.

Thanks I will look at it tomorrow, I appreciate the help.