Package VAV w/reheat Versus CHW VAV w/reheat

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It might be that your CHW loop is on "Standby" mode
which might require for your loop to be always loaded.
Try setting it to "Demand" and see if that makes any


Gerald Pde's picture
Joined: 2011-10-01
Reputation: 0


I am assuming your packaged system is DX. If not, the next few comments
might be a little presumptuous, especially if you are already doing

Is it possible you are letting DOE2 over-specify the chilled water and
condenser water pumps? I found that it was necessary to go through some
careful design steps to accurately specify pumps (flows, heads,
efficiencies) in my water-cooled chiller models to obtain valid results.
Also, you need to be careful in specifying the right two-way chilled
water coil valve(s) in the secondary chilled water loop if you are using
variable capacity pumping. The same design rigor may be needed for the
cooling tower specification.

If the CHW system is running at 2/3 of summer kW usage during the time
the package system is practically nil, then it must be due to auxiliary
loads, and not the chiller, assuming the system schedules and control
strategies for both systems are achieving the same effect within the
cooled spaces.

If DOE2 is allowed to default on most specifications, it can sometimes
be difficult to trace the real cause of an obvious flaw in the results
back to what you actually specified within the model. It's probably
best to nail down everything if you need to accurately compare such a
simple system to one so complex.

Good luck with this issue. Please let us know if and when you identify
the real problem. Could be educational for some of us.

Glenn Haynes, PE

Glenn Haynes's picture
Joined: 2011-09-30
Reputation: 0

You might also double check that your chilled water loop control is set
to demand instead of standby. standby operation can cause lots of
parasitic energy consumption during no load periods.

Peter Alspach's picture
Joined: 2011-10-01
Reputation: 0

John, you might run hourly reports on the compressor power inputs to see
if their unloading curves are reasonable.

Here's a step through for those not familiar with the hourly reports:
1. Under the "Mode" menu, switch to "Detailed Data Edit".
2. Select the "Project & Site" button.
3. Right-click on the Project name and select, "Create Hourly Report
4. Name your block and "Create from scratch".
5. Under Variable Type, select one of your chillers (something like
Chiller1a if using the default name).
6. Under the Variable List, pick your variables, like "Assigned load
(already selected", "Operating capacity at current conditions" and
"Electrical power consumed".
7. Then, under the Hourly Results Selection dialog, pick "New Report" at
the bottom of the dialog.
8. Name your report, create from scratch.
9. For Report Schedule, select "Hourly Report Schedule". For First
Report Block, select your newly created Report Block.
10. Create any other required Report Blocks and then highlight your new
Report and select all the applicable Report Blocks.
11. Select Done and run your model.
12. With the current version of eQUEST on the website, you can
now return to the Input Building Description and select "File" on the
menu and "Export Hourly Results...". Select the appropriate run and
click Export. It will create a .csv file and asks you to select the
report and select Open.
13. You'll likely see the default reports in the first 30 columns, but
your new custom reports will be to the right of that default output
14. You can then do a little post production work on the numbers to see
if the part loading is reasonable. I did a simple test building with
chilled water and got between 0.3 kW/ton and 3.4 kW/ton for the chiller
operation. You can then compare this kind of info with your chiller
manufacturer data to see if it makes sense. Often, water cooled chillers
can only unload to 30% (lower with VFD's), depending on the type. Also,
don't forget that there is also cooling tower energy in addition to the
pumps. You may want to create hourly reports for those too. You might
find some surprises (depending on your inputs, of course) with the way
the CT and pumps are operating. Reports can also be made for the CHW and
CW temperatures to verify reset.

Mark Nieman, PE, CEM

Mark Nieman's picture
Joined: 2011-09-30
Reputation: 0

Hi all:

I swear this continued comparison will send me to an early grave.

ASHRAE 90.1 requires a building less than 5 stories and between 75K -
150K floor area to have a package VAV/reheat as its system. There are
those designers that like the CHW VAV system better. There is a penalty
of having a running CHW pump in that case.

However, I just did an eQuest comparison between the two systems, all
things equal (I hope). The CHW system space cooling used 60% MORE energy
than the package system. In the dead of summer, the CHW beats the
package. But in the winter, there seems to be a false load the chiller
is seeing, as the package cooling is almost nil, but the CHW system is
still has 2/3 of the summer usage.

I THINK I have CHW reset correctly programmed for the CHW system.


John R. Aulbach, PE, CEM

Aulbach, John's picture
Joined: 2011-09-30
Reputation: 0