The dispatcher calls Bob and explains that a new customer has called and complained that their heat pump is blowing cold air and not heating the house very well. It is the first cold weather of the season.
Bob thinks he knows what the problem is and is thinking it over as he rides to the job. He thinks that the electric heat is not coming on during the defrost cycle. Some manufacturers do not operate the electric heat during defrost to improve the unit's seasonal energy efficiency ratio (SEER). When the system is in defrost, the unit is actually in the cooling mode for that time span and if the customer is standing in the airflow it will be cool.