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Biology Coursework Heat Loss Through Windows

As you may recall from Chapter 7, heat loss is calculated using this formula:

Using this same formula, you can calculate the heat loss for windows.

Example 1

A house in State College, PA has 380 ft 2 of windows (R = 1.1), 2750 ft 2 of walls and 1920 ft 2 of roof (R = 30). The composite R-Value of the walls is 19. Calculate the heating requirement for the house for the heating season. What is the percentage of heat that is lost through the windows?

Solution:

Heat loss in a heating season is given by

Heat Loss through windows =

Heat loss through walls =

Heat loss through roof =

Total heat loss = 79,803,560 BTUS

Percentage of heat loss through the windows =

Example 2

Windows in the house described in Example 1 are upgraded at a cost of \$1,550. The upgraded windows have an R-value of 4.0.

  • What is the percent savings in the energy and the heating bill if the energy cost is 11.15/MMBTUs.
  • What is the pay back period for this modification?

Solution:

a) New heat loss for the same window size with the new R-value is

Annual energy savings = 49.745 MMBTUs -13.680 MMBTUs = 36.06 MMBTUs

The percent savings is

The old heating bill would be

The new heating bill would be

The monetary savings = \$402.06 per year.

The Pay Back Period =

The table shows the cost effectiveness of replacing old windows with new and improved windows. The costs are calculated using a computer program called RESFEN developed by US Department of Energy.

PerformanceBase ModelRecommended LevelBest Available
Window DescriptionDouble-paned, clear glass, aluminum frameDouble-paned, low-e coating, wood or vinyl frameTriple-paned, tinted, two spectrally selective low-e coatings, krypton-filled, wood or vinyl frame
SHGCa0.610.550.20
U-factor b0.870.400.15
Annual Heating Energy Use547 therms429 therms426 therms
Annual Cooling Energy Use1,134 kWh1,103 kWh588 kWh
Annual Energy Cost\$290\$240\$210
Lifetime Energy
Cost c
\$4,700\$3,900\$3,400
Lifetime Energy Cost Savings-\$800\$1,300

a SHGC, or Solar Heat Gain Coefficient, is a measure of the solar radiation admitted through a window. SHGC ranges between 0 and 1; the lower the number, the lower the transmission of solar heat. SHGC has replaced shading coefficient (SC) as the standard indicator of a window's shading ability. SHGC is approximately equal to the SC multiplied by 0.87.

b U-factor is a measure of the rate of heat flow through a window. The U-factor is the inverse of the R-value, or resistance, the common measure of insulation.

c Lifetime energy cost savings is the sum of the discounted value of annual energy cost savings, based on average usage and an assumed window life of 25 years. Future energy price trends and a discount rate of 3.4 percent are based on Federal guidelines (effective from April 2000 to March 2001). Assumed electricity price: \$0.06/kWh, the Federal average electricity price in the U.S.Assumed gas price: \$0.40/therm, the Federal average gas price in the U.S.

Cost-Effectiveness Assumptions: The model shown above is the result of a simulation using a residential windows modeling program called RESFEN. Calculations are based on a prototype house: 1,540 sq. ft., two stories, a standard efficiency gas furnace and central air conditioner, and window area covering 15 percent of the exterior wall surface area.

Windows typically occupy about 15 to 20 percent of the surface area of the walls. Windows not only add aesthetic looks and often a very important aspect of a home, but also a very significant component of home heating and cooling costs. Windows lose more heat per square foot of area in winter and gain more heat in summer than any other surface in the home.

We already discussed in Lesson 5 that simple glass (1/8th inch) has a very low R-value (0.03). So even if the walls are well insulated to an R-value of about 13 to 19 and the windows have poor R-value, most of the heat escapes through the windows and the purpose of having a well insulated wall is lost.

It is estimated that in 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States \$20 billion (one-fourth of all the energy used for space heating and cooling). However, when properly selected and installed, windows can help minimize a home's heating, cooling, and lighting costs.


Heat Loss Through Windows

Instructions: Click the play button to see how heat loss occurs through a window.

Heat Loss Through Windows

Click here to open a text description of the Heat Loss Through Windows activity

Heat Loss through Windows

Warm inside air gives up energy through the window and becomes cold, sinking to the bottom of the room. Cold outside air hits the window pane, picks up energy, and then warm air rises. This is how heat loss occurs through a window.

Although energy is spent heating the air in the room, windows can make the temperatures uncomfortable. However, by making the windows efficient, a significant amount of the energy and money can be saved.

Here is a graph that compares heating costs for different types of windows. The figures are based on a typical home in Boston, MA, a relatively heating intensive place.

Heating Costs for Different Window Types

Click here to open a text description of the Heating Costs for Different Window Types graph

Heating Costs for Different Window Types

Heating Costs for Different Types of Windows. Single pane clear glass with aluminum frame has highest heating costs at about \$875. Single pane tinted glass with aluminum frame costs about \$650. Double pane clear glass with wood or vinyl frames cost about \$600. Double pane clear glass, low SHGC, low-e coating, wood/vinyl frame has the lowest heating costs at about \$575.

Similarly, poor windows allow the solar energy to penetrate through the windows and heat the space. The incoming solar radiation consists of infrared (IR), ultraviolet (UV), and visible waves.

The IR radiation, which is also called heat radiation, heats the space excessively and adds to the air conditioning in the summer time. Therefore, energy efficient windows are critical in summer time or even in places where the cooling requirement is high.

Instructions: Click the “play” button to observe the effects of solar energy on windows.

Effects of Solar Energy on Windows

Click here to open a text description of the effects of solar energy on windows activity

Effects of Solar Energy on Windows

A cross-sectional view of a room has a window on the left wall and an air conditioner on the right wall. The sunlight coming through the window heats the room until the temperature around the air conditioner rises. When it does, the air conditioner turns on and cools the room by moving the heat back outside. The process repeats itself as long as the sun heats the room through the window.

Here is a graph that compares cooling costs for different types of windows. The figures are based on a typical home in Phoenix, AZ.

Cooling Costs for Different Window Types

Click here to open a text description of the Cooling Costs for Different Window Types graph

Cooling Costs for Different Window Types

Annual Cooling Costs for Different Types of Windows. Single pane clear glass with aluminum frame has highest heating costs at about \$800. Single pane tinted glass with aluminum frame costs about \$750. Double pane clear glass with wood or vinyl frames cost about \$675. Double pane clear glass, low SHGC, low-e coating, wood/vinyl frame has the lowest heating costs at about \$550.