Types of Furnace Humidifiers
There are only a few general types of furnace humidifiers on the market. The most common are traditional drum & sponge units, flow-through (flow-thru) units, and mist/steam units. In recent years, a new drum style humidifier by Desert Spring Products, with a unique rotary disc evaporative surface has emerged on the market.
There are various options for point-of-use (countertop and console) style humidifiers, however, they are typically regarded as troublesome to maintain and costly to operate as they need to be tended to on a daily basis for refills, etc. Furthermore, they are designed only to cover single rooms, not an entire home. Accordingly, this analysis focuses on whole house furnace mounted central humidifiers.
Rotary Disc Humidifier (Desert Spring)
The simplicity and effectiveness of a drum-style humidifier, the cleanliness of a flow-through, 100% water efficient, and low maintenance - this is what the market has been looking for. Desert Spring released their unique "rotary-disc" humidifier in the mid 90's. It incorporates a wheel assembly composed of about 40 textured plastic discs that rotate through a reservoir of water similar to a drum-style humidifier. The interesting thing about this product is that it does not suffer the same degradation in humidity output due to mineral build-up, thus it maintains its efficiency and water cannot stagnate and become a problem. While some minerals will adhere to the discs, the majority of the minerals are deposited in the basin due to the unique design of the discs and their vertical presentation to the water reservoir. Best yet, it doesn't waste water and generally only requires a simple annual cleaning. There are no pads or screens to replace and it really does work! Desert Spring also recently released an automatic flushing kit called the AutoFlush which can be programmed to regularly flush the basin of its water, further reducing mineral build-up and maintenance and providing extra protection against water stagnation. We recommend it for homes with hard water.
Drum humidifiers with a foam or sponge pad
We are all familiar with these old relics. Foam pad or drum whole house humidifiers are common evaporative units for forced-air systems (furnace mounted whole house humidifier). A simple sponge attached to a rotating wheel dips into a reservoir of water (controlled by a float) and warm air from the furnace is passed through the sponge and picks up moisture which is then delivered to the duct work and out to your home. They put as much moisture into the air as it will absorb and are generally controlled by a humidistat.
The problem with this style of humidifier is obvious. Within a short period of time, the sponge becomes encrusted with sediments and minerals and the pad turns hard and abrasive. The minerals eventually block or reduce the flow of air through the media resulting in a loss of evaporative capacity and little or no humidity is added to the air. The water then stagnates in the reservoir creating a potential breeding ground for microbiological contaminants such as mold spores, bacteria, and viruses.
Before settling on my current unit (not a drum/pad style humidifier), I had the nasty experience of living in a home with 2 different models of drum humidifiers with evaporative sponges/pads. Both proved to be an utter waste of time and money even though they were relatively cheap to purchase. I guess you get what you pay for...
Flow-Through (Flow-thru) Humidifiers
The introduction of flow-through humidifiers was a huge improvement to the market. These units are generally hooked up to a nearby floor drain and allow a steady light stream of water to drip down a metal or plastic screen. Air blows through the screen, picking up moisture and delivering to the home via the central heating system. Sounds great right? Well unfortunately, this style of humidifier has been shown to waste an enormous amount of water. Some statistics suggest that many models waste an average of 5 to 8 gallons of water for every gallon of moisture added to the air. With the average output of a home humidifier in the range of 7 to 8 gallons per day, that equates to wastage of almost 2,000 gallons of water per month! This has prompted several states, provinces, and municipalities to consider heavy restrictions on the use of these units. In the summer of 2002, the EPA (the United States Environmental Protection Agency) began to investigate ways to enhance the market for water-efficient products as a potential program to respond to the growing demands placed on America's water supplies and water infrastructure systems - one of the items under consideration was the wastage caused by flow-through humidifiers!
Further, while these units are certainly superior to the old drum-style units, they still require replacement evaporative screens every heating season, resulting in added cost and inconvenience.
Atomizing or steam humidifiers
There are limited numbers of atomizing or steam emitting furnace humidifiers on the market today. For the most part, these units are expensive energy hogs. They consume large amounts of power and in the case of atomizing units in particular, may allow unevaporated water droplets to enter the duct work and cause condensation, rusting and other problems. These units are mechanically susceptible to mineral problems as well.