Just bought a new home last fall and noticed that the AC fan was running constantly. Replaced the old thermostat with a programmable version. Didn't really use the air much after that but it has been working the heat pump all winter with no problems. Now we are starting to use the AC again and are running into the same issue. Fan kicks on with the condenser but will not turn off. I can hear the relays opening in the thermostat but the only way I can turn the fan off is by flipping the breakers off for about a minute. Then it seems to reset itself. Any suggestions?

When you say AC fan is it the fan motor outside on the heat pump or the indoor blower motor?

Sorry, it's the internal fan. I took the thermostat apart last night just to double check my connections. While the thermostat was disconnected, I turned the fan on with a jumper wire and even then it stayed running. It was a nice evening last night so I didn't reconnect the thermostat until earlier today, and now it seems to be working fine for the moment. But I know the problem will return just as it did last season. By process of elimination I'm guessing the problem is on the AC unit itself. Is there anything I can check on it or should this be left to a professional?

There is likely a control board in the air handler near the filters. The relay on it for the fan is sticking. Usually they get worse. If you are very sure you have located the problem, you can change the board. You must be very confident before coughing up for the board. Just be very to get all the wires back where they were.

I agree with labman sounds like a blower relay is sticking. How old is the unit?