outside unit freezes over in heat mode

Unit heats interior of home but the outside unit coils freeze completely over with 3" to 4" ice while heating.
Can pour hot water over outside of unit and place the thermostat on cooling mode to reverse flow through coils and thaw out the unit and then the inside air flow is warmer when placed back into heat mode. Have been told by a.c. service tech this is normal. Unit is about 12 to 14 years old and only started doing this in the past 4 or 5 years. If this is normal, why did it not do this all the time? The previous unit never froze over no matter how cold it was outside.