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Central Humidifiers for Forced Air Heating Systems

If you are looking for a way to add moisture to your house without than having to routinely refill and clean a portable humidifier, you should consider installing a central humidifying system. This article discusses systems for homes that have forced air heating systems.

The basic concept of a central humidifier is simple. A small water line that is connected to the household plumbing carries water to a humidifying unit located somewhere in the ductwork of the forced air furnace. Inside the humidifying unit, the water evaporates into the passing air and thereby increases the air's humidity level. Where central humidifiers differ, however, is in how the evaporation is accomplished, what happens to any excess water, and what controls are available to regulate the humidification:

  • The most common method of converting incoming water to airborne moisture is to allow the air to flow through some form of pad, screen, or grillwork that is kept constantly damp by water to trickling through it. The greater the surface area and the greater the flow of passing air, the greater the amount of evaporation. Some designs use a gravity grill in which water is distributed at the top and allowed to drip down the grill as air passes through it. Others use a pad wrapped around a drum that is kept wet by rotating through a small pool of water. To boost the quantity of air that passes over the moist surfaces, some humidifiers include their own fans instead of relying upon the air movement created by the blower in the furnace system. The most elaborate (and most effective but most expensive) means to achieve evaporation is to heat the incoming water to the point that it turns into steam that mixes with the air flow, much like a tea kettle in your furnace ductwork.
  • Ideally, all the water flowing into the humidifying unit would evaporate, but in reality most systems have to contend with leftover water. The usual solution is to provide a drain tube, but this means that the humidifier must have access to a drain pipe. Some designs retain and recycle the incoming water until it all evaporates, but the trade-off for this approach is the reduction in fresh water can be used to carry away any mineral deposits and potential mold build up. Thus, while drainless designs conserve water and do not need drains, they require more frequent cleaning and maintenance.
  • The basic control for humidifiers is a humidistat, which turns the humidifier on or off according to what it senses as the level of air humidity--just as a common household thermostat controls temperature. However, if the humidifier relies on the furnace blower to force air through it, the unit will only work when the furnace is running. Since a forced air furnace cycles on and off
    according to temperature, there can be times when the humidifier should be on but the furnace is off. Humidifiers that contain their own fans can independently (and thus better) regulate the humidity.

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