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11-01-2011, 09:43 PM   #1
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## Can someone look at this and help me??

Hello,
I am at a college taking HVAC as an introductory for the course I am in, Energy System Design Program.

We were given a pay back period calculation formula, which gives us the pay back in years. SEE ATTACHED

We were given a model of a condo and we had to find the information for R and U values and calculate the heat loss. From my form you will see OLD and NEW heat loss values meaning the heat loss with the OLD windows and the NEW windows heat loss..............with a difference of 3370.

SEE ATTACHED to show you what I have done. Because this teacher is not very helpful or he gives you cryptic answers and you still have to figure it out and have no clue how I AM HERE TO ASK.

The question is Assuming an annual heating cost of \$ 1500 with the less costly windows, how much will be saved each year with the better windows. AND should the change be made and why???

I asked him the question and he would not answer at all, so I have no clue if I did it right to begin with BUT more importantly WHAT DOES \$ 1203 mean? We would save that much with the new windows each year??? SO if this is the case, then it would be wise to change to the new windows if were saving \$ 1203 when we were paying out \$ 1500 with the old???

The hand out we were given does not explain it AT ALL what \$ 1203 means after the calculation.

Any help would be grateful thanks alot.
Jo
Attached Thumbnails

11-01-2011, 10:27 PM   #2
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Doesn't seem reasonable that your heating cost would be reduced by 82% for the whole structure by replacing just the windows, does it?

I'm not familiar with the formula you are using, but I think it would be much more reasonable to deduct that your 1203 would be your new annual heating cost (providing you have done the calculations right of course). Therefore your payback would be slightly over 5 years (which would seem to be more likely, and perhaps feasible)

 The Following User Says Thank You to HVACDave For This Useful Post: bradycat (11-02-2011)
 11-02-2011, 06:40 AM #3 Member   Join Date: Oct 2009 Location: Nebraska Posts: 2,362 Rewards Points: 1,000 Agreed. Looks like you're saving \$270 a year,slightly more then a 5 year payback. Payback is shorter then both the life of windows and the 7 years an average home owner stays in one place so yes it's a good idea.
 The Following User Says Thank You to Marty S. For This Useful Post: bradycat (11-02-2011)

 11-02-2011, 08:56 AM #4 Newbie   Join Date: Nov 2011 Posts: 3 Rewards Points: 10 Thank you both, how did you figure it was over 5 years to pay back?? can you show me the calculation please? I am not getting that part. So your saying it's worth while to change to the new windows in my lab then??? Thanks Jo
 11-02-2011, 06:07 PM #5 Member   Join Date: Oct 2009 Location: Nebraska Posts: 2,362 Rewards Points: 1,000 Sorry jo, thought I read the windows were \$1500. Not knowing the cost of the windows we can't say how long payback will take. Cost of windows/savings per year= payback time in years.
 The Following User Says Thank You to Marty S. For This Useful Post: bradycat (11-02-2011)
 11-02-2011, 06:55 PM #6 Member   Join Date: Aug 2011 Posts: 729 Rewards Points: 500 Your teacher is not giving you an answer beacuse this formula makes no sense.
 The Following User Says Thank You to Master of Cold For This Useful Post: bradycat (11-02-2011)
 11-02-2011, 07:49 PM #7 Newbie   Join Date: Nov 2011 Posts: 3 Rewards Points: 10 Thank you Marty and Master of Cold. I appreciate the replies. Well alot of ppl in class think he doesn't know either, why he talks cryptic to us. We believe he likes to hear himself talk actually. I am sure you have encountered someone like that in your travels. Thanks
11-02-2011, 08:51 PM   #8
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There are other factors that would have to be considered also. That being efficiency and design temperature, and climate. And also number of windows.

I calculated the difference in my home. It would take 22 years to pay off the windows, due to the north/south orientation of the home, and the Florida climate.

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