An air handler draws air from inside a building, optionally from a building space heating system, and expresses that air such that the air travels along the inside surface of a window glazing unit which is susceptible to condensation. The air can be expressed at ambient temperature or with a modest amount of supplemental heat. As the air is expressed onto the window, a convection curtain of relatively warmer air passes adjacent the inside surface of the glass, warming the glass enough that condensation does not form on the glass. The system can be controlled using a sensor, optionally controlled by a computer or other controller, which controls the air flow according to needs sensed at the window. Such computer can be used to provide individualized control of condensation on each of multiple windows. A blower may be used to achieve increased control of rate and persistence of air flow.