Facts About Winter HVAC Comfort

As the heating season is upon us, we wanted to review some of the common questions that homeowners have this time of year surrounding humidity levels and comfort in residential homes.

The cooling air is coupled with a sharp decline in humidity levels. Homes will be “drying out” for the next few weeks. You’ll likely see condensation forming on your windows or maybe see a crack open up as wood floors adjust.

For most people, most of the time, the range of conditions that we’re comfortable in is pretty narrow. If the indoor temperature is between 65 and 75 degrees Fahrenheit and the relative humidity (RH) between 30% and 60%, we’re more or less comfortable.

Of course, the closer the humidity is to 70%, the lower the temperature needs to be for us to be comfortable. At 70% RH, most of us will want the temperature to be below 70 degrees. By keeping the RH at 50% or below, we can tolerate a slightly higher temperature.

(These two factors, temperature and humidity, are intertwined in the psychrometric chart. Developed by Willis Carrier, the inventor of modern air conditioning, the psychrometric chart is fundamental to building science)psychometric chart humidity

So, temperature and humidity are two important factors of comfort. Let’s look at all four factors of comfort now:

  • Temperature – How hot or cold is it? People like to be in the range of 65 to 75 degrees, more or less. This is the most obvious of the 4 factors of comfort and gets the most attention.
  • Humidity – How much moisture is in the air? Forty to sixty percent relative humidity is the ideal range. In addition to decreasing comfort, a relative humidity that’s too low or too high increases the likelihood of things like bacteria, mold, dust mites. (See the diagram below.)
relative humidity chart
  • Air Movement – Is the air moving across your skin? In the summer, one way that our bodies keep cool is by the evaporation of sweat. A nice breeze or a ceiling fan can keep us comfortable even when the the temperature and humidity are at or above the narrow range we find most comfortable. In the winter, that same air movement will make us uncomfortable.
  • Mean Radiant Temperature – How hot or cold are the surfaces around you? Of the four factors of comfort, this one’s the least recognized or understood, but let me give you a couple of examples. If you’ve ever been in an old uninsulated house with single pane windows on a cold night, you’ve probably experienced discomfort due to a low mean radiant temperature. The cold inner wall surfaces and glass draw heat out of your body because you’re radiating heat to them but they’re not radiating much back. Another mean radiant temperature problem occurs in bonus rooms, where you have walls with hot attic on the other side. Most of the time those attic kneewalls are poorly insulated and thus get very hot. Even if the air temperature in that room is 70 degrees, you may be uncomfortable because the walls keep blasting you with heat.

The two things that most affect our comfort in buildings are the building envelope (insulation and air barrier) and the heating and cooling system. Good insulation, properly installed and in contact with the air barrier, and a tight house go a long way to helping. Good HVAC design takes us the rest of the way. When it all works harmoniously together, we’re comfortable, and the house is also efficient, durable, and healthful all winter and summer long.

 

ABOUT THE AUTHORS

Clint Pharo is a local HVAC Contractor in Madison, WI. He and his team of HVAC professionals at Pharo Heating & Cooling cover a broad range of services in commercial buildings, new home construction, and residential homes and locally represent Bryant Heating and Cooling Products. He regularly posts on his company’s blog at www.PharoHeating.com.

Ben Lindberg is partner in a marketing and design house in Madison, WI called Lion Tree Group. He regularly blogs at the Bark and Roar blog at www.LionTreeGroup.com.

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