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Humidifiers

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Published
July 10, 2009
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Does humidity become a concern with warm/hot air heating systems?

Many people think that warm-air heating systems dry out the air by removing moisture from it, but this isn't true.

Then why does my skin sometimes feel dry when my heating system has been operating?

The air does feel drier, and your skin may crack, but this isn't because the air has lost its moisture. What has changed is the relative humidity, which drops, as the air gets warm.

What's “relative” humidity?

It's a term that compares the amount of moisture in an air sample to the total amount of moisture that the air sample can hold. As we heat air, it has the ability to hold more moisture. The opposite happens when we cool air. Here’s an example. Let’s take one cubic foot of air at 70°F. We'll say that this air sample contains 12 grains of moisture. Let’s also say that this one cubic foot of air has the ability to hold 24 grains of moisture at that same temperature. The relative humidity of the air sample is 50%, since the air can hold a maximum of 24 grains, but is only holding one-half that amount. Make sense?

Now, if we heat that same air sample to 110°F, the air can now hold a maximum of, say, 36 grains of moisture. But the actual amount of moisture in that sample is still only 12 grains. The air, at a higher temperature, has the ability to hold more moisture, so the relative humidity of this sample is now only 33% because the amount of air in the sample is one-third of the total amount of moisture that the air can hold. That's why you feel dry.

And by the way, it takes 7,000 grains of moisture to make up one pound of water.

What does the term “absolute” humidity mean?

Absolute humidity is the actual amount of moisture contained in an air sample. So, if a cubic foot of air at 70°F contains 12 grains of moisture that will be its absolute humidity. If we heat that air sample to 120°F, the amount of moisture in the air sample will still be 12 grains, but the relative humidity will have dropped. Get it?

Do I need a humidifier on my heating system?

It's a good idea. A humidifier will add moisture to the air, increasing its absolute humidity, and by doing this; the relative humidity will also be higher. It's good to keep the relative humidity level in your home between 40% and 60%, with 50% being ideal. At a relative-humidity level below 40%, your skin and furniture may start cracking. At a relative humidity level above 60%, mold may start growing.

A humidistat controls the humidifier, starting and stopping it, depending on the actual relative humidity within the space. If the relative humidity is too low, the humidifier will cycle on. If the humidity is too high, it will cycle off.

Are there different types of humidifiers on the market?

Yes, many types, but they break down into two categories, based on how they operate. You'll have either an evaporative humidifier, or an atomizing humidifier.

Evaporative humidifiers use a moist media and expose this media to moving air. Since the moist media is very wet, it will readily transfer some of its water to the drier air that passes over or through it. As the drier air passes over the media, some moisture will evaporate from its surface and flow with the moving air. You'll find evaporative humidifiers in either the supply- or return duct and they may be the bypass type, which has no fan or blower, and relies on the pressure difference between the air in the supply duct and the air in the return duct to operate. This pressure difference causes air to flow through the humidifier.

You can also mount an evaporative humidifier on the underside of the duct. This type of humidifier uses a rotating media that dips into a water-filled reservoir and then rises into the path of the air moving through the duct. It's like a little Ferris wheel.

Some humidifiers have standing water in them and others work as flow-through devices. The under-duct-mounted humidifier, for example, maintains water at a certain level, so that the rotating media will always be wet. The bypass filter is typically a flow-through device, where water continuously flows through the media, carrying particulates and minerals away with it. There should be no water accumulation in a bypass humidifier.

Atomizing humidifiers spray tiny droplets of water either directly into the occupied space or into the ductwork of the heating system. These droplets evaporate into vapor and join the air passing through the duct. Some atomizing humidifiers use propellers to speed the process.

Which type of humidifier is the best?

If properly maintained, either type will provide adequate humidification.The atomizing humidifier requires more maintenance because water contains minerals that can come out of solution and clog the nozzles that atomize the water. If the residence has excessively "hard" water (meaning the water has a high concentration of minerals), it’s best to go with the evaporative humidifier.

If I have air conditioning (cooling) as part of my heating system, does the humidifier operate year-round, even in the cooling mode?

The air conditioning (cooling) system cools the air and dehumidifies it. Since the humidifier increases the humidity in the air, it would work against the cooling system. Wire the humidifier so that it can work only when the system is in the heating mode, and the relative humidity of the air is below the desired set point.

How do I maintain my humidifier?

Disable and clean it during the warmer summer months. Here's a list of things that you can do:

  • Unplug or disconnect power to the humidifier.
  • Close the water valve that supplies water to the humidifier.
  • Open the humidifier and clear any standing water.
  • Remove the media and discard.
  • Have new media on hand for the cooler months.
  • Remove any scale or mineral deposits from inside the unit.