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Building Operating Management

Operable Windows Can Save Energy





An issue that facility managers will face when considering replacement windows is whether to make them operable. The problem is that building occupants want operable windows, while those who are responsible for the building’s operation do not. The reasoning on both sides is straightforward. Occupants want the natural ventilation and control over temperature operable windows allow. Facility managers usually do not want operable windows as they believe that operable windows waste energy. Open windows allow conditioned air to escape the building and allow unfiltered air to enter.

It is possible, however, to have operable windows without wasting energy by integrating the HVAC system with a window switch. Whenever the system senses that the window is open, the heating and cooling air supply to that zone is turned off, allowing the open window to provide ventilation air while allowing the temperature to float. Using computer simulations, engineers have shown that natural ventilation coupled with a disabling of the HVAC system results in reduced energy use. Those same computer simulations showed that if the HVAC system is not disabled, energy use for the area with operable windows increases by as much as 30 percent.

To gain the full benefit of operable windows, the building must be set up with multiple zones for the HVAC system so that when windows are opened, HVAC systems can be switched off in such a way that they affect only the area with the open windows. This unfortunately increases HVAC system costs and places limitations on space configurations.

Another issue related to both occupant satisfaction and energy use is daylighting. In the typical system that is designed to harvest daylight, windows and skylights are combined with lighting controls that automatically dim the lighting system output when daylight is sufficient. A side benefit to proper daylighting is reduced costs for cooling as the amount of heat generated by the lighting system is also reduced.

Daylighting is most successful when it is incorporated into the building design early. That allows designers to properly configure windows and skylights to provide the maximum amount of daylight. It also allows designers to select the most suitable glazing to allow visible light to enter the space while limiting solar heat gain.

One of the environmental problems that facility managers face when it comes to maintaining or replacing windows in older buildings is the existence of lead paint. If the original windows are more than 30 years old, it is safe to assume, particularly if there are multiple layers of paint, that there is at least some lead-containing paint present. The only way to know for certain is to have the paint tested.

Because windows typically have a service life of 30 to 35 years, the best strategy in almost all cases is to replace the windows rather than remove the lead paint. This means that facility managers have the issue of safe disposal. Debris containing old lead paint is considered a hazardous waste and must be disposed of through a government permitted treatment/ disposal facility.


posted on 12/1/2008

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