also keep in kind that when autosizing the result is generally that the
system autosizes based on the sum of all coil loads without accounting
for diversity, resulting in what would be an oversized system from a
design perspective. depending on your equipment curves this can result
in low part load efficiency which will affect your results, sometimes in
a significant manner. One way to check is to look at your equipment
operation and see the PLR ranges it is operating in as well as comparing
equipment capacity to actual loop loads. After a prelim run you can then
hard size the primary equipment to a reasonable value based on the
actual loop loads if you don't have equipment selected at your point in
the design process.
Peter
Rohini,
Is it possible that your system is allowed to autosize? If so, you may
be reducing outside air when the fan CFM is reduced due to the cooling
load reduction. This could explain the decrease in heating energy.
BTW, if you are designing a new building or sizing a new system, it is
OK to allow the system to autosize, and the surprising interactive
results of other changes may actually be valid.
But if you have a system (such as an existing system) that is not
actually going to change with some other change, you should not forget
to fix that system capacity (and, possibly other things) and run your
fixed baseline before you add the desired change and rerun the model.
With everything else fixed, all you will see are the effects of the
desired change.
Glenn
I have also seen similar results. If I remember correctly, I was autosizing the system - and the sizing of the fan cfm is small with shades - which seemed like the reason for lower heating. Also, reduction in reheating is a likely cause - any report that just shows the reheating energy? If you look at the LS-D report, you should find that the heating has increased and the cooling has decreased.
Having said that, the shading results always tend to surprise me.
- Rohini
What do you mean by Horizontal Shading? Like fins (the device fin are vertical, but cut down light horizontally) ??
I just ran a typical office (250,000 sf) in Fresno, CA in eQuest. When I put 2.5 ft fins on all the windows, I got a slight decrease in heating as well (but not to your % level).
I traded the 2.5 ft fins with 2.5 overhangs. I got a 15% decrease in cooling, and a 13% decrease in heating. This building is dominated by VAV/reheat. What I can surmise is that when the cooling load decreased, the need to reheat decreased as well. That the building is not skin dominated for energy.
Anyone else have a clue?
John R. Aulbach, PE, CEM
What do you mean by Horizontal Shading? Like fins (the device fin are
vertical, but cut down light horizontally) ??
I just ran a typical office (250,000 sf) in Fresno, CA in eQuest. When
I put 2.5 ft fins on all the windows, I got a slight decrease in heating
as well (but not to your % level).
I traded the 2.5 ft fins with 2.5 overhangs. I got a 15% decrease in
cooling, and a 13% decrease in heating. This building is dominated by
VAV/reheat. What I can surmise is that when the cooling load decreased,
the need to reheat decreased as well. That the building is not skin
dominated for energy.
Anyone else have a clue?
John R. Aulbach, PE, CEM
I am examining several options for Horizontal shading for an office building using EQUEST.
A synopsis of the answers is as follows:
Total Annual Energy MBTU Space Cooling MBTU Space Heating MBTU Unshaded Option 8380 2411.9 308.1 2.5 ft Horizontal shading on East West and South
7936.8 2178.8 284.2 5 ft Horizontal shading on East West and South
7835
2158
269.7
The Questions:
1. Its very baffling to see the heating consumption go down too. I wanted to see an increase in heating on implementing the shading devices. How can I justify that decrease?
2. The building has a total area of 210273 sqft. with 15 ft deep perimeter zones. The shading reduces the energy consumption by 5 to 6 %. Is that acceptable?
Any help is greatly appreciated.
Thank you
Jaya Mukhopadhyay