[Equest-users] Electric duct heaters
via Equest-users
equest-users at lists.onebuilding.org
Wed Apr 27 11:11:51 PDT 2016
Hi Manuela,
Following is a plot of OADB (red) vs. interior space temperature (blue)
for the C-DUMMY zone, for the first ~500 hours of the simulation:
Note a few things:
- Every hour your space temperature is bouncing like a pinball
around your setpoint (40F), oftentimes beyond the throttling range
(registering an unmet hour).
- While the hourly need for heating remains constant (OA
remains below 40F), the "pendulum" swing hour by hour gradually lessens
over time.
- When OA crosses the setpoint line (>40F), the resulting swing
after it drops again can be either mitigated or exacerbated depending on
whether the system was trying to heat in that hour. Kinda like jumping
on a trampoline out of sync with a friend - depending on timing, you may
lose all momentum or you may rocket into the sky!
Now let's look at the "whole year" picture for the same plot:
- You can more clearly see the above behavior repeating through
the winter months
- The pendulum swinging is actually worst in the spring/autumn
(when OA more frequently crosses the 40F line), but the general hourly
behavior (hot/cold/hot/cold) doesn't really ever go away...
These behaviors led me to guess (correctly) that you have little (zero)
thermal mass in your dummy zones. Freezing air comes in one hour so the
system turns on, the space gets overheated next hour the next so system
turns off, rinse and repeat.
I tried making this tracked dummy zone bigger (100ft x 100ft), adding a
single interior wall to the bottom (of type INTERNAL so no thermal
connections elsewhere in your model), and assigning a library LAYERS
construction (any of the ASHRAE Concrete walls should do) to effectively
give it some thermal mass to store heat hour by hour. The result is
zero unmet hours (for that zone) and this "bouncing" behavior goes away:
After fixing the flailing dummy zone temps by adding thermal mass, you
may wish to re-evaluate whether the heatpumps are receiving tempered OA.
I'm not aware of a particularly simple/direct way to do so short of
crunching a bunch of custom hourly SYSTEM and ZONE outputs to work it
backwards from the coil leaving temperature (any tips from the crowd
that uses dummy zone workarounds for DOAS more regularly?)... however if
I more simply set you're the above "DUCT" system to SUM & remove the
associated OA-FROM-SYSTEM inputs (effectively pushing all required OA
conditioning back onto the "receiving" heatpump systems - whose heating
capacity inputs are fixed/specified), I observe the associated spaces
shift from "many" annual unmet heating hours to "many more..." This
suggests (A) the contributions of OA-FROM-SYSTEM tempering exist and are
at least mitigating the situation, and (B) you probably have issues
beyond OA temperatures to deal with.
A few other things caught my eye along the way:
* Like the dummy zone discussed above, ALL of your assigned
envelope constructions & floors are "massless" (defined with the U-value
specification method in lieu of layers). I expect if we plot more zone
temperatures we would find more pinball behavior! Switching to
constructions with thermal mass is probably going to help reduce unmet
hours altogether.
* I notice substantial unmet hours outside of the dummy zones,
and that you have chosen to group zones with likely dissimilar load
profiles under PVVT systems - bear in mind the typical relationship for
the control zone vs. the "slave" zones when evaluating whether those
unmet hours are meaningful... you might want to re-evaluate your system
quantities and/or whether a few thermostat schedules should be removed
where they don't exist in the actual design.
I hope that helps you and others!
~Nick
------------------------------------------------------------------------
------------------------------------------------------
Nick Caton, P.E.
Senior Energy Engineer
Energy and Sustainability Services
North America Operations
Schneider Electric
D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
<mailto:nicholas.caton at schneider-electric.com>
F 913.564.6380
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States
From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org]
On Behalf Of Manuela Mengue via Equest-users
Sent: Wednesday, April 27, 2016 7:51 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Electric duct heaters
Dear All,
Water source heat pumps are serving spaces in the cellar and first floor
that are of concern for this model. Please review the eQuest energy
model attached. I am trying to find the appropriate way to model two
electric duct heaters and outside air fans on the cellar & first floors
mainly utilized to provide outside air to the WSHP units on these
floors. The electric duct heaters heat the outside air from 0F to 40F
(cellar) or 60F (first floor). The temperature of entering outside air
should be 40F for units "C EXERCISE ROOM SYS", "C AC SYS" AND "C SCREEN
ROOM SYS" on cellar floor, and 60F for units "1 LOUNGE", "1 LOBBY SYS"
and "1 CORR BIKE MAIL SYS" on first floor. I created two heating and
ventilation systems "C DUCT HEATER" and "1 DUCT HEATER", and fixed the
heating set point temperatures to 40F and 60F respectively. I also
created and assigned dummy zones with no internal gains to these
systems. After simulation, I am getting extremely high heating unmet
hours for the dummy zones, plus the zones served by the above systems do
not seem to receive outside air at the appropriate temperature. I am
wondering if some inputs in my model are incorrect. If possible, can
anyone please indicate how this design strategy should be modeled and
the resulting issues addressed ?
Thank you,
Manuela
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