Types of whole house units
There are four types of whole house units. The first type consists of a pan of water, a wheel with a pad on it, a control to turn it on and off, and a tiny motor to turn the wheel. These are called drum style humidifiers.
Some of the warm air from the outlet of the heating system is diverted to the inlet of the unit. When the furnace fan is running in the heat mode, the transformer for the humidifier is energized and 24 volt power is supplied to the humidistat.
When the humidistat senses that the relative humidity is below the setpoint, 24 volt power is supplied to a tiny motor in the unit. This motor spins a drum with a foam pad on it. The pad passes through the pan of water and absorbs some of the water. The warm air from the furnace outlet passes through the foam pad and picks up moisture through evaporation. The air then enters the furnace return duct and passes through the furnace and into the supply ducts where it is distributed throughout the home.
These require more maintenance than the flow through type to keep them working properly. The maintenance includes cleaning the water pan and replacing the pad. Also, if the system is not properly shutdown in the summer, the standing water in the pan can allow biological growth.
The main advantage of this type is that they typically use less water to maintain a given humidity level.
The next style uses a metal pad, a water valve, and a control to turn it on and off. This type is referred to as bypass flow through units. When the furnace fan is running in the heating mode, 24 volt power is supplied to the humidistat. When the humidistat senses that the relative humidity is below the setpoint, 24 volt power is supplied to the water valve. This opens the valve and allows water to enter the unit. The water passes through a metal pad where some of it is evaporated by the warm air from the furnace. The water that does not evaporate passes through the pad and enters a drain line.
The maintenance on these units includes cleaning/replacing the pad and cleaning the unit. They typically use more water than the drum type units to maintain a given humidity level.
The latest advancement in these units is called a pulsed flow through unit. This unit cycles the water solenoid valve open for a few seconds, closed for about 30 seconds and continues to repeat that cycle. This results in significantly less water usage.
The next type of unit is called a power humidifier. This type has it's own fan instead of using the furnace fan. The unit is mounted on the hvac system supply duct and does not require the furnace to be operating.
The final type of whole house unit is a steam humidifier. These units use a built in heater to heat a reservoir of water until steam is produced. This steam is then passed through a nozzle and into the duct system. These units usually have a higher capacity which is good for situations where the furnace does not operate very frequently.