ALthough Harold has his points, I don't necessarily 'agree' with the reasoning.
In the cases I've seen where this is a problem, usually it stems from older systems that required water to 'sit' in a sort of 'pan' all the time. For example, I've seen a system that was made up of thin brass wheels that 'spun' in a pan of water, thus, allowing the warm (forced hot) air moving over it to evaporate the moisture for 'use' in the house. Yes, it was on the Plenum side of the system BUT the pan was always filled with water. This system had a 'recommended draining' of once per month, plus a bi-annual cleaning for mold/mildew. Today, the newer systems only 'deliver' water when the system needs it. In other words, there's no 'advanced storage' of water that 'sits' in any pans. They're kinda like a car's radiator, where water is dripped over coils and warm air passes through the coils for delivery throughout the home. When the HVAC system shuts off, the water stops dripping. Of course, the downside to this type of system is that water is somewhat 'wasted' because there is a means to 'catch' excess water that's NOT evaporated. The water that happens to 'drip through' the coils is captured and drained away. So, in a sense, if the system feeds 8 gal per hour, 7 gal per hour may be drained into La-La-Land. This is the tradeoff to having water 'sit' in a pan as I initially described.
Other than that, perhaps others may have something to offer. I would believe the best way to deliver moisture is through a forced hot air system. If you don't have one, then you'll need humidifiers in strategic 'places' in the home.