Unusual Result

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The project I am working on is an office building that operates 20 hrs per day
with most of the loads occuring at night and low occupancy on daytime.?It is a
cooled by a DX roof unit and has a very high internal load. No heating equipment
is required because the building is in Climate 1. I was surprised that when I
changed my window U-value from 0.3 to 0.7 I got less cooling consumption. I
always thought that getting low?u-value will give me less coling consumption.
Can somebody tell me why I am getting less cooling when I use?a higher window



Jaigath Chandraprakash's picture
Joined: 2011-10-02
Reputation: 0

If most of your cooling loads come from internal loads as you said, then
having windows with a higher U-value allows the building to lose heat to the
outside any time it is cooler outside than inside. If you look at ASHRAE
90.1, the requirements for windows changes dramatically as you move from
colder to warmer zones. In warmer zones, the allowed U-value is higher, but
the SHGC is lower.

Also, it is usually good to verify that this is actually the case, often
installed equipment runs only a fraction of the day, or at a fraction of the
peak energy use. If your internal loads are not actually as high as you are
modeling, this could change things.

Lastly, does the building have an economizer? This may make sense for this
particular project.


No Username provide's picture
Joined: 2011-09-30
Reputation: 200


Karen is right on. Lower window U-values and additional insulation in walls and roofs can cause a net energy increase in buildings with high internal heat gains. If you look at the month-by-month consumption, you may see that the "better" envelope helps out in winter and summer, but hurts in spring and fall. The overall net effect over the year is sometimes negative.

When the air outside is cooler than inside, then some of the internal heat can get out of the building through the envelope. A more insulated envelope slows the heat escaping through the envelope, so more of the heat has to get out of the building through the HVAC system, resulting in more cooling energy. If there is an economizer ("free cooling"), the effect is not as severe the net effect of more insulation could be positive.

Keith Swartz, P.E., LEED AP

Keith Swartz's picture
Joined: 2011-09-30
Reputation: 0

Hi Jaigath,

With such high internal loads, your building may be losing more heat through
the windows with the higher U-value of 0.7 and thus cause for less cooling
as less heat is built up within the building.

Glenn Mercurio

energyefficiency's picture
Joined: 2011-01-14
Reputation: 0

You might also check 100% outside air during late night, this might even help more

Abaza Hussein's picture
Joined: 2011-10-02
Reputation: 200

Thank you Karen, Joe,?and everyone?for?your advice.?

To answer your question if the building has an economizer,?no it does not have
an?economizer. I have also asked the mechanical designer regarding the
electrical load in his design criteria and he did confirmed that his assumptions
are typical for this type of building. I am really tempted to adjust the
electrical load because it is too high compared to typical offices and I agree
with you Karen that?equipment do runs?only at a fraction of the day.

Thanks again,


Jaigath Chandraprakash's picture
Joined: 2011-10-02
Reputation: 0