Cycling fans

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I've got an eQUEST model where my designed building has PIUs. I was
getting a number of hours of unmet loads till I turned on Night cycle
control. With PIUs you can set it to Zone fans only - which makes sense.
The loads were met and the energy usage went up marginally.

The base building is a standard VAV system (system 7). Since the VAV
system doesn't have zone fans, I have to set it to cycle on any. The
energy use goes up considerably in this case. Is this normal? Should I
be meeting the loads in another way?

Also - is it acceptable to have night cycling for a LEED/Appendix G
submittal? The building is just not meeting its heating needs if I don't
do this. It's a historic building with a lot of perimeter glass (single
pane) and so the perimeter zones get pretty cold at night (one of the
reasons we have PIUs)

Am I correct in taking credit for this?

Vikram Sami, LEED AP

Vikram Sami's picture
Joined: 2011-09-30
Reputation: 0

Re night cycling of fans. See Table 3G.1.4

>> Christopher Jones, P.Eng.

Chris Jones's picture
Joined: 2011-09-30
Reputation: 0

Same model - different fan question. Table G3.1.3.15 Gives you a part
load performance table for VAV fan systems. Based on that, I created a
performance curve (linear) for my VAV fan system that looks like this:

Problem is when I use this as opposed to the default curve that came
out of the wizard, the fan energy doubles. The fan energy is still only
8% of the overall building load, so its not alarming. Does this mean
that the default curve was way to low? Is the method above the way to
model this performance curve?

Thanks for your help

Vikram Sami, LEED AP

Vikram Sami's picture
Joined: 2011-09-30
Reputation: 0


Reid Hart of PECI did a presentation on this issue for the building
simulation user's group a few months ago, and discussed a spreadsheet tool
to construct accurate DOE2 performance curves for variable flow air and
water systems. I believe the powerpoint presentation is still archived on
the BSUG yahoo site, and the spreadsheet is downloadable from PECI's
website. I haven't tried using this tool yet, so I cannot vouch for its
accuracy or useability.

Steven Savich, LEED AP

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Joined: 2011-09-30
Reputation: 0

Steven, Vikram, et. al.,

Reid communictated with me in developing that presentation, since I authored
the spreadsheet tool. As the author of the tool, I will vouch for the
analytical approach, and the tool was peer reviewed since it was funded by
the California Commissioning Collaborative.

As for tool useability, well, it is pretty useable but not where I would
like it to be. The tool (and others) is available at:
It was developed in Excel 2003; I am uncertain whether there are any issues
with other versions.

Yes, I believe the default DOE-2/eQuest VAV curves are quite optimistic.
Even the tool that comes out of my spreadsheet tool is likely somewhat
optimistic, even though it attempts to include fan, VFD, and motor
efficiencies at the various operating points. Pressure drops relative to
flow could be slightly better modeled for the portion of the system with
filters and coils, and that would further increase off-design power.

How well a particular curve fits a particular system is most significantly a
function of
(1) whether and how the static pressure setpoint (to which the VFD controls
the fan speed) is reset.
(2) if the static pressure setpoint is not reset, where the sensor is in the
duct system, and how high the setpoint is relative to the fan pressure rise.

This figure is related to what Reid showed in his presentation, and relates
some of these issues.

William E. Koran, P.E.

Bill2's picture
Joined: 2011-09-30
Reputation: 0


Does anyone have the link that Steven mentioned?

Thank you,

Paul Diglio

Paul Diglio's picture
Joined: 2011-09-30
Reputation: 400

I owe you all an apology. I sent an email to Vikram using the reply vs
reply-all button telling him that I thought his problem might be in
specifying the curve as linear. He changed it to quadratic and got much more
reasonable results.

cmg750's picture
Joined: 2010-10-05
Reputation: 0

The fan and pump system toolkits are here:

The BSUG presentation is here -- you may need to be a member of the BSUG
Yahoo group to access the file. It was the Sept 2009 presentation.

bfountain's picture
Joined: 2011-09-30
Reputation: 201