The answer to 'why is my heat pump freezing up' is that a frozen heat pump is a common issue, often caused by a problem with the defrost cycle or airflow issues. Symptoms include ice buildup on the outdoor unit, reduced heating output, and the system shutting off.
Some common causes include low refrigerant levels, a malfunctioning defrost control board, blocked airflow around the outdoor unit, or a problem with the reversing valve that switches between heating and cooling modes. In many cases, this can be fixed with a simple repair, but it's important to address the root cause to prevent the problem from recurring.