New Delhi, Nov. 18 -- When temperatures begin to fall, many people assume covering an outdoor AC unit will shield it from cold air, snow, and rain. It sounds like a protective step, but HVAC specialists say it can actually create several problems. A cover might trap moisture inside the unit, encourage rust and mould, and even attract small animals looking for a warm place to hide.

An AC unit needs space for air movement even when it is not running. Once a cover is placed on top, airflow drops sharply. This creates a damp, closed pocket inside the unit. Such conditions allow mould to grow and speed up corrosion on metal parts.

Another big concern is wildlife. A covered unit becomes a comfortable shelter for rodents, insects, and other ou...