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Why Does Your Furnace Lower Relative Humidity?

Posted on March 5, 2014
by Appleby Systems

You are stumbling across the bedroom carpet one early winter morning, still in your pajamas and socks, headed for the kitchen to start the coffee maker. As you reach for the doorknob, your fingers are zapped with 20,000 volts of static electricity. Now you are awake! Does that sound familiar? If so, your home might be suffering from low relative humidity.
Low Air Humidity

Besides static electricity shocks, you may have noticed your family is suffering from chapped lips, dry eyes and more colds than usual. The latter may be due to dry mucous membranes. Moist mucous membranes are one of our body’s natural defenses against viruses and bacteria. Dry winter air also has adverse effects on your home’s furniture, pets and houseplants.

The culprit in this case is low air humidity in your home. It may seem like a paradox that during the winter the air in your home is drier than in the summer, but there is a simple explanation for this.

Relative Humidity

Humidity refers to the amount of water vapor in the air around us. The warmer air is the more water vapor it can hold. Cold air holds less water vapor. The actual amount of water vapor in the air is the absolute humidity. Relative humidity refers to the actual amount of water vapor in the air relative to the amount it could hold at a given temperature. A relative humidity of 100 percent would mean the air is holding the maximum amount of water vapor possible, whereas a 50 percent relative humidity indicates the air is only holding half the moisture of which it is capable.


How Your Furnace Affects the Relative Humidity in Your Home

Relative Humidity

So, if your home is warm inside and it is cold outside, should not your indoor humidity be higher since warm air holds more moisture? It is not quite that simple. The outdoor cold air can have a much higher relative humidity than indoors, but that does not mean it contains more moisture per cubic meter than the air inside your home. In fact, it probably contains far less.

When your furnace draws in outside air, heats it and sends it indoors, the air has the same moisture content, but the relative humidity drops dramatically. For instance, winter air at -18C and a relative humidity of 75 percent heated to a comfortable indoor temperature will have relative humidity of just 5 percent! Even the Sahara desert has an average relative humidity of 20 percent.

How to Improve Your Home’s Humidity Level

Ideally, your home’s relative humidity should be in the range of 30 to 50 percent. Even in the harshest winters, 20 percent relative humidity is the minimum you should aim for.

Consult an HVAC contractor about various options to improve the humidity in your home. Often, humidification units can be added to your home’s heating and cooling system including controls that monitor for the correct humidity level.

There are three types of whole house humidifiers to consider:

  • Steam units that add water vapor via electrodes
  • Bypass units that passively add moisture via evaporative pads
  • Fan-powered units that blow air through evaporative pads

In order to reduce illness during the winter months, you may want to add an air cleaner that removes bacteria and viruses from your home’s air. Your HVAC contractor can outline all the options and help you choose the best solution for your home and family. If you’re looking for advice, contact us now.

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