Humidifier, The Basics...

A humidifier adds water vapor to the home's air to improve its air quality. There are two general types of units (portable and whole house). The type of unit that is best for you will depend on the type of heating and air conditioning system that you have and whether you own or rent your home.

Portable units are a good option for renters as well as homeowners that do not have duct systems for heating and cooling. The disadvantages of this type is that they usually require you to fill a reservoir every day and they normally need maintenance more often.

Whole house units typically require maintenance two times per year and they automatically fill the water reservoir.

Normally, you set and adjust a humidistat and the system takes care of itself.

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.

Controls for the unit...

Most units use a manual humidity control. This control is usually mounted on the furnace return duct or on the wall beside the home's thermostat. They normally come with a chart that tells you what to set the control on for a given outdoor temperature range. The control has to be constantly manually adjusted to match outdoor conditions. This is typically not done and results in unsatisfactory operation.

For that reason, it is better to connect the unit to an automatic humidity control. These units combine an indoor humidity sensor with an outdoor temperature sensor. They automatically adjust the setpoint to the recommended level for the given outdoor temperature.

This feature is also included in some thermostats.

What size do you need?

There are different sizes of units, therefore you need to choose the right size for your home. The units are rated in gallons per day (gpd). This is the maximum amount the unit can put into the air in one day. The actual amount depends on how much time the furnace operates and the temperature of the air leaving the furnace.

The chart below will help you to select the proper size. Find the approximate size of your home (in ft2) and the type of construction (whether it is well sealed or not). The value where the two meet in the chart is the size of the humidifier you need in gallons per day output.

               1000 ft2      1500 ft2     2000 ft2     2500 ft2     3000 ft2   

TIGHT         4.3 GPD     6.4 GPD    8.5 GPD    10.6 GPD      12.7 GPD

AVERAGE     8.6 GPD     12.8 GPD   17 GPD     21.3 GPD     25.4 GPD

LOOSE        12.7 GPD    19.1 GPD   25.5 GPD   31.8 GPD     38.1 GPD

You can use our humidifier brand rankings to find the proper unit for your application and our installation guide to assist you in adding a unit to your home's hvac system. It is a pretty easy do it yourself project and can save you money on your heating bills.

Types of portable humidifiers...

There are basically four types of portable humidifiers. The first are called ultrasonic units. These units have a fan that pulls in air from the room. An ultrasonic transducer causes the water in a reservoir to vibrate and some of the water is absorbed by the air. This air with moisture mixed in it is delivered back into the room.

The next type is commonly referred to as cool mist units. These units typically have a fan to draw in air and an impeller that flings water droplets up into the airstream. Some of the water drops are absorbed and leave the machine in the air stream.

The next type are called warm mist units. These units work like the cool mist units except they have a built in heater to increase the amount of moisture added.

The final type is referred to as evaporative units. These units consist of a reservoir of water and a wick. This wick draws water from the tank and a fan blows air through the wick where the air picks up moisture. In general, this type of unit requires more maintenance than the other types and on most models you must periodically purchase replacement wicks.

Some portable units are operated by a simple on/off switch. These units can add too much moisture to the air if not closely monitored. Other units have a built in humidistat to cycle the unit and only add moisture as required.

Portable units have two major disadvantages. The first is that they require daily filling of the reservoir and more routine maintenance. The second is that to maintain proper humidity levels throughout the home more than one unit will probably be required.

In general, it is a lot better to install a whole house humidifier on the home's heating ducts if possible.