I have no heat or air coming from the vents in my house. But outside the fan is turning on the unit. Please Help??

All forced air heating systems have a blower controlled by a relay. Some blowers are belt driven. Many heating systems have limit switches or timers that do not start the blower until the air is warmed up to prevent a blast of cold air when it first starts up. It is also possible in a heat pump that the fan is running outside, but not the compressor, resulting in no heat. The compressor may also shut off if the temperature becomes too high because the blower isn't working. The motor for either the compressor or blower could be bad.

If your thermostat has a fan only setting, an easy first check is to select it. If you don't get any air out the vent, the problem is the blower or the relay for it. If air circulates, the problem is the compressor or some of the controls.

To get heat from a heat pump, you must have 3 motors running, the outside fan which is, the blower which isn't, and the compressor. The next check is to shut the system off long enough for the compressor to cool off, at least a half hour. Turn the system on and go to the outside unit. Try to decide if more than just the fan is running. Stay out there 5-10 minutes, and see if you hear the compressor stop, reducing the noise and vibration the unit. If so, likely that means problems with the limit switches or blower. If the fan is all that is running and nothing changes outside, likely it is the compressor.

Once you have some idea where the problem is, you will need to start the relays, switches, and for power to the motors. That means opening up panels and electrical boxes and checking with a meter, test light, or a nifty Ac sensor like my Santronics AC Sensor. Touch it to a hot wire, and the tip glows red without touching any bare wire or contact.

If you are not comfortable with that, you may have to call a repairman.

If you go ahead, post back with what you find, and I will try to guide you to the next step.