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"...as your time averaged outside air requirements also come into play and may not allow the VAV system to put enough flow into a zone due to OA requirements of other zones."

Very good point.

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----- Reply message -----
From: "Bruce Easterbrook" To: "Carol Gardner" Cc: "equest-users at lists.onebuilding.org" Subject: [Equest-users] Heat Pump Heating Undersized
Date: Sat, Apr 9, 2011 10:46 am
I agree with Carol and Joe.? In almost all cases you
shouldn't need to have the cfm/SF 400% higher than the eQuest set
point.? If you do, it is likely going to be a process load that
causes this.? I have found the 0.5 cfm/SF eQuest default too low
in most cases but leave it for my first run because what you
really want are the heat/cool loads of the zones.? Joe's tip, CFM*(Tsupply-Troom)*(1.085) =
BTU/h provides your tool.? Your equipment provides
the delta T of your air heating and cooling flows, the only
variable is CFM.? Use your loads required by the building to
determine your CFM.? The old rule of thumb was around 1 CFM/SF but
in most cases this is too generous.? I think the reason eQuest
uses a low default is to help you keep the fan energy to a minimum
and maximize the efficiency of your system.? You want just enough
CFM to do the job, more is a waste, too little gives lots of unmet
load hours.? This is where I start when tweaking a system.? Your
cooling flow normally sets the flow as it has the smallest delta
T.? Heat pumps have a lower heating delta T than a furnace.? VAV
further complicates as your time averaged outside air requirements
also come into play and may not allow the VAV system to put enough
flow into a zone due to OA requirements of other zones.? OA is
also quite dominate when it comes to energy consumption.? One
other area to consider it where your controlling zone is for your
temperature control.? Selecting the proper zone can have a large
effect on unmet load hours.

eQuest is a tool and will never give you the perfect system with
the first press of the sim button.? If you are just starting with
eQuest consider yourself lucky if it even runs through.? Your
detailed reports give you the clues you need to refine your
model.? Start with the coarse variables, CFM, delta T's, note I
didn't mention throttling range.? Allowing your system to run
loose too early will hide problems in your design.? A 2 degree
throttling range is tight in my opinion but again eQuest is trying
to help with getting an efficient system.? Look for errors in
input, incorrect assumptions and numbers which don't seem right.?
Slow and plodding, one change at a time, check the effect and
ALWAYS have a backup file.

Bruce Easterbrook P.Eng.

Joe Fleming's picture
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Brian,

A few additional thoughts.

1. If you haven't already, set NIGHT-CYCLE-CTRL = CYCLE-ON-ANY. If you don't do this or schedule a warm up, you will likely have unlet load hours in the morning. You can check this in the SS-O.
2. I've found that perimeter zones are usually the most troublesome and have employed specifying cfm/ft2 at zone level in the past, though I agree with others that 2cfm/ft2 seems high.
3. Consider setting your Tstat control to REVERSE ACTION if you have low min flow ratio (i.e. < 0.4). This will allow the boxes to open beyond MIN-DES-FLOW during heating.
4. Don't know anything about your project, but if this is a LEED ASHRAE90.1 baseline and you are modelling one system floor, take a look at your peak loads by zone. If your problem zone loads differ by more than what is specified in Section G3.1.1 b. of the Performance Rating Method, you may be able to create a separate system. DOE2 has difficulty with systems where loads vary a lot between zones. I doubt any energy model can handle this that well.

Good luck,

Shuichi Hendrickson

Shuichi Hendrickson's picture
Joined: 2011-09-30
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