air handler issue

[acooke]

what happened to this 4 yo air handler...see pix

[Doug Edwards]

Most likely there is cold moist air leaking around the unit and plenum joint, meeting the warm moist air inside the air handler closet. These causes condensation to form on the surface of the plenum, mold begins to grow and viola...mold (mildew is a favorite name for this type mold) stains abound.

[wsiegel]

Ditto

[David A. Andersen]

The unit is located in an unconditioned or improperly conditioned space. Due to the high levels of humidity of the ambient air and reduced temperature of the air handler in general, condensation will occur when the temperature is below the dew point temperature. I see no indication of air leakage, just insufficient insulation of the unit and duct system. However, depending on the location the unit is placed, no amount of insulation may improve this condition. Insulation, if properly installed will prevent air (which contains moisture) from coming in contact with the unconditioned air.

Reduced airflow through the air handler will often increase condensation on the exterior. Though increasing airflow for the sole purpose of reducing condensation is not appropriate.