If your unit supports low temp operation (not all dehumidifiers do), the first answer above applies and you'll see the evaporator coil freeze up, the unit should detect the freezing coils and eventually run through a defrost cycle to melt the ice and drain off the water. When the surrounding air is cold enough to freeze up your evaporator coil (typically around the mid to upper 60's), this process continues in a repetitive cycle.
If your unit's defrost cycle is broken or if your unit just doesn't support low temp operation, when the surrounding air temp gets around the 60's or below, the coils will freeze and never thaw out until you turn the unit off for a while (not really usable in lower temp environments...).
David, January 2007
