The heat pump is an air conditioning system that operates in reverse. An air conditioner takes heat from inside the house and gets rid of it outside, through the use of a substance called refrigerant. Most people know the refrigerant as freon. An air conditioner has two coils, an evaporator coil and an condenser coil. An evaporator coil is where the refrigerant absorbs heat from inside the house through the freon passing inside its coils. The freon is then pumped to the conderser coil by the compressor, where it gets rid of the heat. The freon flows back to the evaporator again to pick up more heat and the cycle starts all over again. When your system operates in heat - the cycle is reversed, the condenser becomes the evaporator and the evaporator becomes the condenser. Now the unit absorbs heat from outside and gets rid of it inside your home. Even though it may be cold out side, there is still heat, because the refrigerant has a very low boiling temperature I beleive that it boils around 45-70 degrese below zero. So even at thirty two degrese outside air temperature you will get some heat. Because there is moisture in the air outside and the coils have reversed and the evaporator is now outside, the moisture condenses on the evaprorator and on cooler days you will see frosting. However the frosting should be taken care of by the defrost cycle. If there is a full refrigerant charge in the system the defrost cyle takes the system out of heat cycle and puts it back into cooling the condenser becomes the condenser again and the evaporator becomes the evaporator again. The heat abosrbed in the house is used to defrost the coil outside. There is usually a thermostat outside that sends the unit into defrost, and there may be a circuit board that has a timer, about every hour to hour and a half the circuit board timer will look to see if the defrost switch is calling for defrost, if the switch is calling for defrost the unit will switch back to cooling to initiate the cooling cycle, this will defrost the coils. Usually defrost is terminated either by time or temperature, if the timed defrost takes the unit back out of cooling and again calls for heat the unit should be completely defrosted. Here is a list of things that can cause your problem, but please take note that on cooler days frost and ice may be normal, but the defrost cycle should defrost the coils completely if it does not-then I would suggest that you do have a problem.
1.Low refrigerant charge
2.plugged evaporator filter
3.Plugged evaporator coil
4.Bad defrost timer
5.Bad defrost circuit board
6.Bad defrost relay
7.Broken wire in the defrost circuit
8.Defrost timer set for too long inbeween defrost cycles
9.Bad indoor fan motor
10. Condenser fan motor is not taken off line when in defrost-this is the ouside motor-this motor may be taken out of the ciruit during defrost to aid the coil in defrosting - this may be due to improper wiring.
11.The indoor motor could be the wrong size and may not pick up enough heat in cooling cycle-this would be rare:
12.A bad reversing valve.
13.A bad reversing valve relay
SInce your unit was installed in july 2002 I would suggest that there is probably a low refrigerant charge or that the circuit board defrost timer is set too far inbetween if the frost does not disappear completely.