The answer to 'air handler smells bad' is that a foul odor coming from the air handler is often caused by mold, mildew, or bacteria growth within the unit. This can happen when moisture builds up inside the air handler, providing a breeding ground for these microorganisms.
Some common symptoms of a smelly air handler include a musty, damp, or rotten egg-like smell coming from the vents. This odor may be more noticeable when the system is first turned on or running. In some cases, you may also see visible mold or mildew growth inside the unit.