Window Comparison

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I am doing a model for a two story office building with the windows
making up approximately 30% of the building's exterior. My goal is to
find what type of energy savings the owner can expect by replacing all
the existing single pane windows with double pane, low e windows. When I
change the window type to a double pane low-e window I am seeing savings
that are very small, around 3% of the total building kWh.

For reference, the buildings HVAC system consists of two air handlers
(one per floor) ducted to a VAV system with hot water reheat.

Has anyone else tried to do this type of comparison and if so what type
of energy savings did you find? I have attached my model pd2 and inp
files for reference. Thanks in advance for any information.

Brett West

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I couldn't get it to open to check, but since there's a total of 3% change, you might check the difference between space cooling and heating. The difference might be counteracting each other enough to reduce the benefits that you thought you'd see. The location and internal loads might have some affect as well.

BRIAN J. WOLFE, CDT, LEED AP BD+ C

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Take a peek at the LS-C report in the *.sim output file, this may help give
you an idea what percentage of the building heating and cooling loads are
due to the windows.

David

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David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, HBDP

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*From:* equest-users-bounces at lists.onebuilding.org [mailto:
equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Brett M. West
*Sent:* Monday, November 08, 2010 3:38 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] Window Comparison

I am doing a model for a two story office building with the windows making
up approximately 30% of the building?s exterior. My goal is to find what
type of energy savings the owner can expect by replacing all the existing
single pane windows with double pane, low e windows. When I change the
window type to a double pane low-e window I am seeing savings that are very
small, around 3% of the total building kWh.

For reference, the buildings HVAC system consists of two air handlers (one
per floor) ducted to a VAV system with hot water reheat.

Has anyone else tried to do this type of comparison and if so what type of
energy savings did you find? I have attached my model pd2 and inp files for
reference. Thanks in advance for any information.

Brett West

David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

Brian,

I believe the wrong .inp file was sent the first time. This one should
work.

The 3% change is in the total kWh of the building. There is an
approximately 10% change in the kWh for space cooling only. However, the
heating is all in therms since the system uses hot water reheat provide
by a natural gas boiler throughout. This actually increases with the
improved windows, which makes sense since during the heating periods
there will not be as much solar gain from the windows. But, it increases
enough to the point where it outweighs the gains in cooling to where the
total cost for the year actually goes up by about $1000.00 overall.

The building is located in Columbia, SC so I would think the systems
would be in cooling most of the time and I would see a savings over the
course of a year.

Brett West

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I agree with Brian. I have done similar studies and found that in certain
climates the savings offset. However, 3% sounds a little low, especially
with 30% glass.

There are other issues that come into play as well with VAV systems. What
probably is happening is that the airflows are different and the VAV
minimums are affected (or perhaps they are identical which could also cause
a lack of savings).

You may want to compare using the sim file, the number of hours running at
VAV minimum, since the better glass may actually be eliminating "free
reheat" during certain times of the year. I have seen this often. Make sure
all of your design airflows are the same by locking them into place in both
runs (since I'm guessing you are not changing the fans).

Bob Fassbender
Cosmo Logic LLC
Energy-Models.com

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I looked through the file (novice user myself) but did notice some things that seem odd:

Are both your 1st floor plenum and 2nd floor plenum exposed to the outside? Are both of your "floor" elements designated as touching the ground? That's what it looked like when I opened up your model... That alone would double the envelope loads on the ground floor and roof (which means that windows as a percentage will be less effective).

I would check that before going onto determine the effectiveness of windows.

Cheers,
Alex

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Brett,
Just a cursory glance shows you have a lot of unmet loads - mostly on heating. Try enabling night cycling - that should reduce that and show better savings on the high performance glass.

Vikram Sami, LEED AP

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Brett:

I took a quick look at your baseline model. Your EUI (KBTU/FT2/Year in the BEPS
report is very high at 202.9. If you go to Project & Site you can enter the QC
Reporting Climate Zone for CBECS Comparison. I chose the under 2000 CCD and
fewer than 4000 HDD. I do not know if this is correct.

After you run the simulation go to Tools then Quality Control Reporting. The
end of the report states your model's EUI as compared to the CBECS. Your model
consumes about double the energy use. Your unmet hours are a little high.

I would try setting the throttling range from 3 to 6 degrees and change the
thermostats from 2 Position to Proportional in the Zone Parameters Basic
Specifications. Also try entering a Control Zone for each air handling unit in
the Airside Basics tab.

Paul Diglio

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Hi Brett,

As I was going through a research document, I found another answer to your question.

http://gaia.lbl.gov/btech/papers/59190.pdf

Figure 5 on page 12 shows how different performing windows impact energy use. Although this graph is for residential buildings, it shows that improving U-factor only saves energy when reducing SHGC. I would guess that for commercial buildings (typically higher internal loads than residential) this effect will be even greater. Once the model is working, try looking into a low-SHGC solution.

Hope this helps.

Cheers,
Alex

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