I have a Luxaire heat pump and noticed last week that the air handler fan motor is running constantly. I am an electrician (not an HVAC tech) and looked into the problem. I traced the fan motor wiring back to the control board which has the relay for the fan motor control. I disconnected all of the wiring on the control board that comes from the t-stat and the outdoor unit and left only the power feeding the board 240vac, the wiring from the board to power the xfmr primary, the secondary of the xfmr 24vac to the board, and the wiring from the board to the fan motor. Powered the board back up and the motor still ran constantly. After determining that the control wiring could not be the problem I thought that the board had a bad relay. I ordered a control board and installed it yesterday. Same results. What else could be the problem? Not sure where to go from here. Not much to the unit besides the board when all else is disconnected. Any suggestions?

I had this problem and the parts guy told me to replace the time delay relay for the fan motor in the air handler. It is the relay that runs the AHU fan motor a few minutes after the heat pump shuts off to get just a little more out of the coil while it is still hot or cold depending on the mode it was operating in. After years of service, the relay started sticking closed (actually arc'd and welded together) making the fan run constantly.