Heat Transfer Through WindowsAn Energy and Cost Analysis

Mike CallGreg Rasmussen31 March 2010

http://corporateportal.ppg.com/NR/rdonlyres/30CA79BD-584C-410F-9068-0BABE8BF8EEA/0/radiant_heat_flow.gif

Introduction• Heat transfer through

windows greatly contributes to the energy loss from a house

• There are many varieties of windows; from single paned, to double paned with aluminum, to double paned with injected Argon, PVC lining, and composite glass connectors

http://www.shadesshuttersblinds.com/blog/wp-content/uploads/double-pane.jpg

Introduction

• With the focus on energy, we are consultants for home owners regarding their fictitious house.

• They want us to recommend whether they should replace their aluminum windows with PVC lined windows.

• We have the theory, training, and software to do so, and are ready for the challenge.

http://www4.uwm.edu/pps/Sustainability/CampusInit/images/EnergyStarLogo.jpg

Objective1. Perform energy and cost comparison to the

following:– Double paned windows with aluminum framing– Double paned windows with PVC framing

2. Calculate the total energy loss difference between them

3. Use that difference to see how much the PVC windows would save each year.

4. Calculate how many years would be needed to recuperate the costs of installation.

5. Make recommendations whether to change the windows or not.

Assumptions

1. The industry average delta T was used: 39 C

2. The house has 10 windows 3. Each window is 4 ft by 4 ft, or 1.3

m^2 per window 4. Only 215 days of heat use with

average temperature difference of 39 C (conservative for Utah since 20 F and 71 F difference is 27.7 C) http://www.servitokss.com/wp-content/

uploads/2009/07/man_question_mark.jpg

Theory

http://www.energystar.gov/ia/business/Web_art/EPA-BUM-SupLoads_7-2.gif

• This diagram illustrates the different modes of heat transfer seen by windows

• “Therm” software has the capability to calculate the U value for each window based on geometry, material and certain standardized assumptions.

Modeling Method• Measure the dimensions of the window to be modeled • Download and install “Therm” software, available for download at:

http://windows.lbl.gov/software/ • Build the window geometry• Choose the materials for each component• Set the inside and outside boundary conditions• Enter the assumed temperature difference between the inside and

outside panes• Run the calculation to determine the window’s U value• Calculate the total heat loss difference between the PVC and

Aluminum windows using the calculated U values• Calculate the heat loss for a year, and determine annual cost

differences

Software Calculations/Results

•Above is the cross section of a window with Aluminum trimming the finite element mesh and the heat flux are shown graphically.

•Above is the cross section of a window with PVC trimming. The

heat flux is noticeably smaller than the Aluminum version to the left.

Hand Calculations

• Once the software gave us the U-factor, we could calculate:– Flux– Heat– Heat/year– $$/year

savings

Excel Calculations• First, we have:

– The bill date– The decatherms used during

the bill period– A correction for water

heater use– The total cost for that many

decatherms– The cost per decatherm

Address DateActual

Decatherms Used

Decatherms - Water Heater Total Cost Cost per

Decatherm

115 N 2770 W, Provo, UT

84602

3/2/2010 10.827288 9.327288 $99.89 9.225763642/2/2010 11.594301 10.094301 $104.77 9.036336041/5/2010 12.43253 10.93253 $106.81 8.59117171

12/2/2009 10.51932 9.01932 $95.71 9.0984968611/3/2009 7.761675 6.261675 $62.85 8.09747896

*assuming water heater = 1.5 (subtract)

Average Cost Decatherm/ Year Savings / Year Cost of New

Windows

# of years to

recuperate cost

$8.81 2.44086 $21.50 $3,000 139.51$8.81 2.44086 $21.50 $10,000 465.04

• As seen to the right, the amount of time required to recuperate the cost of installing new windows is incredible

$3,000-$10,000 is the estimate cost range to replace 10 windows in an average sized home www.costhelper.com

Conclusion

Though there is extra heat loss through the aluminum seals, the cost of replacing the

windows is much too high to justify based on the current price of natural gas.

It could be a different story if the windows were single paned, but with the already double

paned windows, we recommend not to replace them!