A couple of months ago, I wrote about how to “Add Fan Airflow to Static Pressure Readings in Four Simple Steps.” In that article, I made a comment about pairing larger-tonnage fans with smaller-tonnage outdoor units that has drawn a lot of interest. This problem is frequently found with gas furnace and cooling systems, but it also exists on heat pumps when a large air handler is installed. The two most frequent questions I received were “How do we look for this?” and “What can happen when this condition exists?” To answer these questions, let’s look at how we diagnosed and corrected a system with this problem a few years ago. It’s important to point out that the homeowners’ only criticisms of their five-year-old system were excessive noise and that it didn’t cool like they felt it should. They had come to accept its condition, even though they felt something wasn’t right. My buddy Vince DiFilippo of DiFilippo’s Service refers to one of the first steps to diagnosing system issues as “Walking the Ship.” When you walk the ship, you take the time up front to gather all of the necessary information before you test and offer solutions. A lot of issues appear when you slow down and per...