Quick answer
Consider duct cleaning if you see mold growth inside ducts, signs of pests, or heavy debris coming from registers. Consider duct sealing if some rooms are always hot or cold, your attic is dusty around ducts, or bills seem high. Consider replacement if ducts are crushed, torn, disconnected, or the insulation is falling apart.
Cleaning vs sealing
The EPA notes that duct cleaning hasn't been shown to prevent health problems in most homes, but it can make sense when there is visible mold, vermin, or excessive debris. Leaky ducts, on the other hand, pull in attic dust and waste conditioned air, and sealing them often has a larger effect on comfort.
How leakage is measured
A duct leakage test pressurizes the system and measures how much air escapes. This is the same type of test HERS raters use for Title 24 compliance when equipment is replaced.
Signs of failing ductwork
Flexible ducts that sag, kink, or are flattened; foil tape that has peeled off; disconnected boots at registers; and brittle insulation are common in older Los Angeles attics.
