Who wants to learn about problems after the job is completed? Residential HVAC system design is a foundational step in the process to deliver predictable comfort, and estimating the heat loss and gain of a dwelling (Manual J) is the cornerstone of that process. Approaching a new equipment installation with a few guesses could lead you into a trap. But on the other hand, a few minutes spent on a survey and data entry will make you look like a professional and save you from uncomfortable conversations later. You can follow the procedure and have confidence or guess and find out the hard way. Doing it right in our industry is the mark of strength.