The answer to 'why is my air handler smells bad' is that a smelly air handler is often caused by mold, mildew, or bacterial growth within the unit. This can happen when moisture builds up in the air handler, providing a breeding ground for these microorganisms. Common symptoms include a musty, unpleasant odor coming from the vents or the air handler itself.
Some potential causes include a clogged condensate drain, leaks in the ductwork, or poor air circulation. Over time, these issues can allow moisture to accumulate, leading to the growth of mold and bacteria. The smell may also be a sign of a more serious problem, like a dead animal or pest infestation in the air handler.