Our central unit out side frost over when the temp outside get below 35 degrees we have tried to just turn on the fan but you have to have the heat on for the fan to work can you tell what's going on

Hi,
Many heat pumps have a "defrost" cycle; i.e. at certain times during the heating cycle, it will supply heat to the unit itself, defrosting itself. This is controlled by a sensor, and is turned on automatically by a small relay part.
It could be that this small relay is bad, not allowing the defroster to come on.
Has it, in the past, stopped running outside, and defrosted itself? If so, and won't come on now, I would call a repair person to check it out.
Best of luck.

Which fan are you trying to get to run, the one on the outside compressor, or the one that circulates the air in the house? Most HVAC systems have a fan only option controlled by a switch on the thermostat. If your system has such a switch, but the fan won't work in the ''Fan on'' position, but does work with either the heat or cooling, the problem likely is with the switch or its wiring. Usually the switch switches the red wire from the transformer in the furnace to a green wire connected to the G terminal at both the thermostat and furnace. Power then flows to the coil of the same relay as powers the fan in cooling mode. Often heat mode uses a separate relay and a lower speed winding in the fan motor. In this case, it sounds like the switch is bad or a wire loose, which you should be able to find and easily fix yourself. Check it, and post back with the details if you need more help. If your system is a heat pump, they often use other wire colors and markings.

If the coil is freezing up outside on a heat pump, it may be low on Freon, or the valves that control the mode could have malfunctioned.

heat pump don't work very well when outside air temperature approach freezing ; very little heat (entropy) could be extract out of freezing air, that why the outside coil freez up... ( ** that is when the supplement heater kick on)