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One of the most important ways we can affect the air quality in our homes is by adjusting the humidity. In summer or in humid climates, we usually want to bring the humidity down. But if you live in a very dry climate or winter air is drying out your home, you might want to add moisture to your house’s air with the aid of a humidifier.

A common method of increasing household humidity is through the use of small, portable humidifiers that can add moisture to the air in the room where you need it most. They are inexpensive but not ideal if you want to humidify your entire house. For that, you need a whole house humidifier, sometimes also known as a central humidifier. These humidifiers attach to your house’s HVAC system and provide a constant flow of moisture to every room in the house. In 2011, the Environmental Protection Agency has estimated there are 350,000 whole house humidifiers installed every year in the United States, compared to 8 million portable humidifiers.

How does a whole house humidifier work?

There are three types of whole house humidifiers. They all work by introducing water vapor into the ducts of your home’s HVAC system, but they each accomplish this in different ways.

1. Steam humidifiers

These humidifiers connect to your home’s water supply and electrical system. They heat water until it evaporates into steam, then injects the steam into your home’s heating ducts, where it is spread throughout your home.

2. Drum humidifiers

A drum humidifier also requires electrical power and connection to the water supply. The water fills a tray, and a drum rotates partially in the tray. The bottom of the drum rests slightly in the water. As the drum turns, the water wicks through the sponge and emerges into the center of the drum as water vapor. Hot air from the furnace passes through the center of the drum, introducing the moisture to the air that is spread throughout your home.

3. Flow-through or moisture pad humidifiers

This type of humidifier connects to the water supply, but has no moving parts and requires no electricity. The water supply moistens an evaporator pad. Hot air from the furnace flows over the pad, where it picks up the moisture and adds it to the stream of air flowing throughout your home. These humidifiers require an additional drainage outlet, usually to a floor drain, utility sink or sump pump.

Both drum and flow-through humidifiers use bypass ducts. They divert some of the hot air from your heating system into a separate section of duct, where the humidifier is installed, and then the air re-enters the primary duct system. A steam system can inject the steam directly into the primary ducts. All three types can be installed into the cold air return portion of an HVAC system if necessary. However, it is not the preferred installation method, since hot air more efficiently evaporates and carries the water vapor.

If your home does not have a forced air HVAC system with ductwork running through the house, there are stand-alone humidifiers that use fan units to disperse humidity throughout your home.

Whole house humidifier installation

If you excel at do-it-yourself (DIY) projects, you may be able to install any type of whole house humidifier. However, it does require time and the right tools. You should be prepared to do plumbing and wiring work. It also requires specialized skills and tools for cutting and attaching ductwork, which makes it one of the more difficult DIY projects a homeowner may attempt. Steam humidifiers tend to be more complicated than the other two types. Professional installation is a better option if you are not sure you can do it yourself.

Drum and flow-through humidifiers cost between $150 and $500, and installation will add $100 to $300 to that. Steam humidifiers are much more expensive, with units costing $1,000 or more, and professional installation will cost $500 and up.

Whole house humidifier maintenance

One of the biggest drawbacks of a portable single room humidifier is the maintenance. These humidifiers need to be dried and cleaned out regularly to avoid the formation of mold or bacteria. According to the EPA, “Microorganisms often grow in humidifiers which are equipped with tanks containing standing water. Breathing mist containing these pollutants has been implicated as causing a certain type of inflammation of the lungs.”

Portable humidifiers also have filters that need to be changed frequently for the same reason. If not properly maintained, a moldy humidifier will be sending mold spores directly into the air, making your air quality much worse instead of making it better.

Whole house humidifiers are remarkably easy to maintain. Steam humidifiers have water collection mechanisms that prevent liquid water from remaining in the ducts, directing it back into the humidifier. The evaporator pads of flow-through and drum humidifiers are dried out by the hot air passing through them (as long as they are set up correctly so that water only flows to the pad when the heat is running). This means that they do not allow mold to grow in any part of the system. Flow-through humidifiers even flush the evaporator pad clean of mineral deposits from tap water, so they only require cleaning and replacing the pad once per year. Drum humidifiers need monthly maintenance, so they are not as hands-off as a flow-through design.

How to use a whole house humidifier correctly

There are several benefits to increasing the humidity in your house properly:

  • Excessively low humidity is anything below 30 to 40 percent relative humidity. According to a study in Environmental Health Perspectives, “The majority of adverse health effects caused by relative humidity would be minimized by maintaining indoor levels between 40 and 60%. This would require humidification during winter in areas with cold winter climates.” (“Indirect health effects of relative humidity in indoor environments,” Environmental Health Perspectives, March 1986, Arundel, et al).
  • At low humidity levels, water vapor tends to stay suspended in the air. Between 40 and 60 percent relative humidity, larger water droplets tend to precipitate out of the air onto surfaces, which can then be cleaned. Airborne bacteria tend to attach themselves to water droplets, so higher humidity levels have the effect of reducing airborne bacteria. Hospitals that maintained 40 to 60 percent relative humidity showed a significant decrease in patient infections compared to hospitals with relative humidity below 30 percent (“Is low indoor humidity a driver for healthcare associated infections?” Taylor & Hugentobler).
  • Low humidity can damage wood furniture and floors, causing it to shrink and eventually crack.
  • Low humidity can lead to dry skin and dry, scratchy throats and nasal passages.

The humidity level on a whole house humidifier must be set correctly according to the temperature. Often, the ductwork in a home passes through areas that are outside the insulation, such as a portion of the basement or attic. That means the ducts are cold, which will cause moisture in them to condense inside the ducts. Cold weather also makes windows cold, which can lead to condensation if the humidity levels in the house are too high. Since people who live in cold climates tend to use whole house humidifiers specifically to keep winter air from getting too dry, there is a high probability that improperly set humidity levels will cause condensation. Condensation leads to mold, rust and rot.

Modern whole house humidifiers have sensors that detect temperature and humidity in the house and automatically activate or deactivate the humidifier as needed. This helps to avoid the “incorrect humidity level” problem.

Even under ideal conditions, a whole house humidifier can cause problems. Introducing moisture to the air and specifically to the ductwork in your house can cause plenty of problems, from mold in the ductwork itself to excess humidity in poorly ventilated areas. Choosing whether to use a portable, single-room humidifier or a whole house humidifier will depend on how serious your dry air problem is and your willingness to keep a portable humidifier clean so that it does not become contaminated.

If one of your major purchasing decision on a whole house humidifier is due to ongoing symptoms such as allergies and asthma or air quality issues such as mold, you may also want to consider expanding the solution to air purifiers for select rooms. For more information on air purifier for mold, click here. Conversely, if you are actually looking to reduce humidity in your home, you are actually looking for a dehumidifier. You can read more about how dehumidifier works here.

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