Piping, Ductwork, and Insulation
This domain rewards attention to details that show up in field failures. For piping, know material compatibility, support spacing, expansion and contraction, and the difference between schedule 40 and thinner-wall pipe. In hydronics, understand that air is the enemy - air elimination, proper pump placement (pumping away from the expansion tank), and minimum pipe sizing for velocity and noise. For refrigerant lines, know that suction lines are insulated, liquid lines generally are not, and that line sizing affects pressure drop and oil return.
Ductwork is about pressure, leakage, and insulation. Know the difference between total, static, and velocity pressure; the concept of equivalent length for fittings; and why elbows, transitions, and flex duct add resistance. Manual D sizing and Manual J load calculation are the design references. Duct sealing (mastic, not duct tape) and duct leakage testing matter for efficiency and for not depressurizing a house. Insulation is rated by R-value; in Alaska, vapor barriers go on the warm side, and duct insulation must resist condensation in cooling and heat loss in heating.
Traps: confusing R-value with thickness without accounting for material; putting a vapor barrier on the cold side (a moisture disaster in cold climates); and ignoring that flex duct compressed into a tight run loses capacity fast.
Worked example: a duct run of 30 feet with four 90° elbows. If each elbow counts as roughly 10 equivalent feet, total equivalent length is about 70 feet - size the duct for that, not the straight-line 30 feet, or you will undersize it.