When it is cold outside and your heat pump is set in “heat” mode, it is normal for your heat pump to develop ice on the outdoor unit during wet and/or very cold conditions. The unit will defrost itself, however if the ice develops with significant thickness and the unit does not defrost itself, you will need to call for service. Only in an absolute emergency should you attempt to defrost the unit yourself by running it in cooling mode until it defrosts. This can cause damage to the unit and should be used in emergency only.
When using a heat pump in cooling mode, and the outdoor unit freezes, you should check the filter’s cleanliness, check the return grills for obstructions, and make sure the supply registers are open. If you find any of the above are the problem, you can turn the unit off with the fan on for 6-8 hours. After the unit is completely defrosted, you may resume normal operation. If the unit freezes again, you will need to call for service. Your serviceman will be unable to work on a frozen unit, so be sure it has defrosted before he arrives.