If the vent is not properly installed, the siphon can drain the water from the trap. If the vent is installed between the trap and the unit, air will be drawn into the vent. As the air flows back into the pan, it prevents the water from draining, causing it to overflow. If no vent is installed, the water flowing down the drain siphons the water from the trap, and a dry trap will allow air, and possibly sewer gasses, to be pulled into the unit, resulting in unpleasant odors. Remember, the vent must be installed after the trap.
Heat pump compressors failing too often, too fast.
When a heat pump compressor fails, the replacement compressor will very likely die a premature death if you don't determine the cause of the original failure. Let's examine the two closest components to the compressor, to see if either one is the cause of the original breakdown. First, consider the accumulator. Sludge build-up or debris in the accumulator can prevent the oil from returning to the compressor and may be the source of your problem. To find out, remove the accumulator and pour out the contents.