THINGS TO CHECK IF THE UNIT WILL NOT DEFROST:
Defrost controls vary in type and design. Some older ones are as much mechanical as they are electrical and will have a remote tube with a gas filled bulb on the end that will sense for the ice build up on the outside coils and an internal (or external) relay to do the electrical switching. The bulb will be strapped strategically on the coils. The defrost mechanism has a built-in timer that will periodically initiate a defrost. This will only occur if the remote bulb is sensing ice on the coils. If not, it will just do nothing and revert to the standard heating mode. If it does sense there is ice on the bulb, then it will turn off the outside unit fan motor and reverse the flow of the refrigerant via the 4 way valve. The compressor continues to operate with the hot gas now routed through the outside coil which will defrost the ice. Usually this process is set to run until the ice is defrosted and is sensed by the bulb as it is warmed or the defrost will continue for a maximum of 10 minutes or so. The above process is normally enough to defrost the ice build-up, but occasionally during periods where the temperature is near or at freezing with rain, the ice build-up will be excessive and defrost will terminate before removing all the ice. I highly recommend you should not be running the heat pump during these conditions anyway. At temperature below about 35 or 37°F (4°C), you should have switched the thermostat to the EMER. HT position which disables the heat pump and will supply quite adequate heating from the auxiliary source which is usually electric heat strips or gas or oil heat from an inside unit. Besides, the heat pump has become inefficient at these temperatures and the indoor air feels drafty. Yes, the cost of operation will be slightly greater, but what do you want when it has gotten cold outside: warmth or draftiness? What's more, likely as not, the heat pump will have not been supplying enough heat at this juncture to maintain the temperature you have the thermostat set to operate and periodically the indoor heat will have been running to keep the house warm. The problem here is that most thermostats will control the auxiliary heat to shut off three degrees below the setpoint (where you are running the heat pump). When you switch to EMER. HT, most thermostats will switch control of the auxiliary heat to the primary control and will maintain the temperature you set it to hold. Now you have comfort again. Isn't this the objective? In any event, if you continually get icing of the coils, then just replace the defrost mechanism. There is little or no repair possible on the devices. TIP: The outside unit will probably continue to ice more and precaution should be taken before you turn back to the heat pump mode as the weather warms again. You will have to manually defrost the outside unit. Be especially careful not to energize the unit in heat pump mode with ice in and on the fan blades. Newer units have printed circuit boards (with relays onboard) and remote thermistor sensors. Thermistors sense temperature very accurately and will send resistive circuits back to the microprocessors for interpreting. Some manufacturers will use sensors with tiny contact points that will open or close when a thermal disc senses the ice build-up. Many of the PCBs have the sensor wires and sensor as part of the board. You will most likely have to replace this printed circuit board and sensors if you continually have icing. 4-way valves sometimes will stick in one position or the other and will not respond for energizing demand or the release of demand. Check to see if the heating mode requires the 4-way valve coil to energize or to be de-energized. You may have to give the model and make to a dealer and ask which is the usage. If it isn't reversing when it needs to, then it will not defrost and the unit will ice badly. Replacing the valve will require the services of a professional with all the fancy equipment for reclaiming the refrigerant and a very skilled use of an oxygen/acetylene torch. Don't try it yourself! Don't even think about it. It's a bear.
Low refrigerant can sometimes cause the coils to ice and this is one concern. Measure the line temperature of the hot gas line as it comes out of the unit and it should be approximately 100°F (38°C) above the ambient (outdoor) temperature when correctly charged and running in the heat pump mode. Dirty outside coils, which in the winter time have become the cooling coils, will not allow enough conductivity of heat transfer and the coils will ice. Just wash the coils until nothing else will come through them but clear water. Be careful not to bend the fins. Of course, this will have to be done at temperatures above freezing.