Wednesday, January 11, 2012

When the Wind Blows - its Cold: The indoor wind chill factor

By Doug Hargrave
Mid-Atlantic Masonry Heat
Everyone has experienced the cooling effect of a strong wind or breeze while engaging in some outdoor activity. The extent of the cooling effect is determined by the speed of the wind and the temperature of the air. For example, a warm breeze has to be significantly stronger than a cool breeze to produce a cooling effect. The cooling effect of wind can be moderated by the use of insulation and/or a wind breaker. Someone is more comfortable wearing a wind breaker in a strong breeze. In a strong winter breeze one would have to add a sweater (insulation) under the wind breaker in order to achieve the same relative comfort.

When weathermen talk about outdoor temperature condition they often refer to the wind chill factor. The wind chill temperature is always lower than the air temperature. The movement of air indoors is not generally referred to as "wind", however, the effect of air movement indoors is the same as outdoors - it has a cooling effect - it does not make you warmer.

Air movement is often introduced into indoor living areas in a number of different ways; a few of the prime examples are as follows:

  1. Outdoor air infiltration that causes noticeable drafts through leaky doors, windows and other openings.
  2. Forced air systems and fans that mechanically move the air
  3. Natural convection of air from a hot radiator surfaces in the primary living areas.
The most uncomfortable type of indoor air movement (draft) is outdoor air infiltration that causes noticeable drafts. Just as you would wear a wind breaker for comfort in windy outdoor conditions you want the shell of your home to act as a wind breaker for indoor comfort. No amount of insulation will help if you have drafts from the outside blowing in around it. The only way to counteract the effect of this type of indoor wind chill is massive amounts of hot air which will mask the effect of the infiltration. However this is accomplished at a high energy cost and only marginal improvement in personal comfort. The first line of defense in any home heating plan is reducing the air infiltration so that drafts from the outside are not noticeable. Only after this problem is fixed should someone turn their attention to other issues in the home heating plan.

In the United States the use of forced air heating systems is so pervasive that it is difficult for most people to imagine any other way of heating their home. The fact that these systems produce indoor wind chill is accepted as a necessary evil.
These systems typically force heated air into a room at the outside walls (usually under the windows) and then extract return air from locations high on interior walls. This forced air ducting arrangement results in relatively strong drafts at the ceiling level while minimizing drafty conditions at floor level. In a room with standard eight foot ceilings the movement of the heated air at ceiling level mixes fairly well with the cooler air lower in the room but there is always a marked temperature difference between the warm ceiling and the cold floor. In rooms with higher ceilings (especially vaulted ceilings) the mixing results of warm air near the ceiling and cooler air near the floor is compromised by the greater separation and larger volume. In order to compensate for this, more heated air is required and more mixing of air is required. This results in more air movement and more indoor wind chill. It is not unusual for someone seated in a vaulted ceiling room during cold weather to wrap a blanket around them self as a shield from this intensified indoor wind chill.

Hot radiators cause air movement through natural convection which is then felt as indoor wind chill. The best known example of this is the wood stove where surface temperatures often run between 400 - 600 °F. At these temperatures when the air in the room makes direct contact with the stoves surface it expands dramatically and quickly rises to the ceiling. Other air follows behind creating a draft at floor level in the direction of the stove. This draft is quite cool because it comes off the coolest surfaces in the room usually the least insulated window areas. The wind chill effect from the combination of hot stove surfaces combined with cold window surfaces is very noticeable. Less noticeable wind chill is felt from electric resistance or hot water radiators placed on outside walls (usually under windows). These radiator heat systems send heated air up along the cooler surfaces in the room to the ceiling level. Cooler air to replace the heated air is drawn along the floor toward the radiator but it comes from warmer areas of the room resulting in less indoor wind chill than with a wood stove or other centrally located radiators that would tend to draw air from the outside walls and windows.

The question is often asked, "Wouldn't it be a good idea to use a ceiling fan to blow the hot air near the ceiling down to the floor or reverse the fan and draw the cool air up to the ceiling?" On close examination, this solution, could come right out of the pages of Alice in Wonderland. When the ceiling fan is used in the winter time to "blow" the warm air down you almost always can see the slowly turning fan blades, which means it is not really blowing the warm air down but rather just stirring it up at the ceiling level. If the fan were actually run at a high enough speed to blow the air down (or draw the air up), the wind chill factor in the room would increase substantially and your comfort would decrease.
On the other hand, in the summer time, when you have hot air at the ceiling and relatively cooler air at the floor, you can turn on the ceiling fan, force the hot air down on you and the indoor wind chill will make you feel cooler and more comfortable. The fan cools in winter and it cools in summer - period.

1 comment:

  1. Your post about the indoor wind chill factor is very interesting. Thank you very much for providing such a helpful article.


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