Why does a heat pump freeze up when you run the AC?

Air conditioner will sometimes freeze up when the thermostat is not cutting off and allowing the coil to ice up or you can have the thermostat turned down too low and the unit does not have emough air flow to keep it from freezing up. == New answer== It could also be that your system is low on refrigerant. If that is the case you probably have a leak in your system and need to have it checked. A dirty filter could be the culprit, check to see if it is clean.